05-15-2012 City Council Study Session Packet
City of Grand Island
Tuesday, May 15, 2012
Study Session Packet
City Council:
Larry Carney
Linna Dee Donaldson
Scott Dugan
John Gericke
Peg Gilbert
Chuck Haase
Vaughn Minton
Mitchell Nickerson
Bob Niemann
Kirk Ramsey
Mayor:
Jay Vavricek
City Administrator:
Mary Lou Brown
City Clerk:
RaNae Edwards
7:00 PM
Council Chambers - City Hall
100 East 1st Street
Grand Island Study Session - 5/15/2012 Page 1 / 124
City of Grand Island Tuesday, May 15, 2012
Call to Order
This is an open meeting of the Grand Island City Council. The City of Grand Island abides by the Open
Meetings Act in conducting business. A copy of the Open Meetings Act is displayed in the back of this room
as required by state law.
The City Council may vote to go into Closed Session on any agenda item as allowed by state law.
Invocation
Pledge of Allegiance
Roll Call
A - SUBMITTAL OF REQUESTS FOR FUTURE ITEMS
Individuals who have appropriate items for City Council consideration should complete the Request for
Future Agenda Items form located at the Information Booth. If the issue can be handled administratively
without Council action, notification will be provided. If the item is scheduled for a meeting or study
session, notification of the date will be given.
B - RESERVE TIME TO SPEAK ON AGENDA ITEMS
This is an opportunity for individuals wishing to provide input on any of tonight's agenda items to reserve
time to speak. Please come forward, state your name and address, and the Agenda topic on which you will
be speaking.
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City of Grand Island
Tuesday, May 15, 2012
Study Session
Item -1
Grand Island Dewatering Study 2012 Update
Staff Contact: John Collins, Public Works Director
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Council Agenda Memo
From:Terry Brown, P.E., Manager of Engineering Services
Meeting:May 15, 2012
Subject:Grand Island Dewatering Study 2012 Update
Item #’s:1
Presenter(s):John Collins, P.E., Public Works Director
Background
On December 21, 1998 the City entered into an agreement with the Central Platte Natural
Resources District (CPNRD), which provided for the installation of test and monitoring
wells to study lowering groundwater levels. The study concluded September 2000 with a
recommendation to implement a dewatering program.
On June 28, 2011, by Resolution 2011-162, the City Council was presented with
information on the costs and construction on a dewatering system for the City of Grand
Island.
On July 26, 2011 City Council approved an Interlocal Agreement with the Central Platte
Natural Resources District (CPNRD) to update the September 2000 Groundwater Study.
The study and cost are being shared equally between the City and the CPNRD.
Discussion
Results of the updated study are being presented in draft form to both the City and the
CPNRD. A final study will be presented to City Council upon collection of input from
both the City and CPNRD.
Conclusion
This item is presented to the City Council in a Study Session to allow for any questions to
be answered and to create a greater understanding of the issue at hand.
It is the intent of City Administration to bring this issue to a future council meeting for
direction on how to proceed with a dewatering project for the City of Grand Island.
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Grand Island Dewatering Study
2012 Update
Grand Island City Council Study Session
May 15, 2012
Grand Island Study Session - 5/15/2012 Page 5 / 124
Grand Island Study Session Outline
•Project Background
•Dewatering Areas of Concern
•Project Scope
•Previous Dewatering Options
•2012 Groundwater Model
•Comparison to 2000 Model
•Model Development
•Model Results
•Conveyance and Discharge Options
•Preliminary Opinion of Probable Cost
•Financing Options
•Implementation Recommendations
Grand Island Dewatering Study – 2012 Update
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Project Background
•1998 Record High Groundwater Levels
•Olsson Associates Completed Dewatering System
Study to lower Groundwater Levels
•Engineers Opinion of Costs $9,943,000
•Annual Project Costs w/Operations $1,574,100
(20 yr., int.7%)
•2008 High Groundwater Levels
•Indexed Dewatering System Costs to 2007 Dollars
•Engineers Opinion of Costs $17,802,000
•Annual Project Costs w/Operations $1,895,602
(20yr., Int. 5%)
•2012 Groundwater Model update – Dewatering
System Layout and Opinion of Costs Update
Grand Island Dewatering Study – 2012 Update
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Grand Island Dewatering Study – 2012 Update
Dewatering Areas of Concern
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2012 Update – Project Scope
•Evaluate Previous Options and Current GW Conditions
•Assess Current Groundwater Contamination Plumes
•Develop updated groundwater model
-to evaluate dewatering well layouts
•Prepare conceptual layout of conveyance piping
•Develop preliminary opinion of probable cost
•Identify project financing options
Grand Island Dewatering Study – 2012 Update
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Dewatering Well
Influence
Image sources: USGS.gov and fayettecountygroundwater.com,
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2000 Dewatering Option
Grand Island Dewatering Study – 2012 Update
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Groundwater Contamination Plumes
Grand Island Dewatering Study – 2012 Update
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Comparison to 2000 Model
Grand Island Dewatering Study – 2012 Update
•Higher resolution topographic survey data
•Higher resolution aquifer data including
Hydraulic conductivity
Specific yield
•Refined Recharge areas with four areas defined
Irrigated land, riparian, urban open and urban developed
•Detailed Platte River and Wood River flow data input
•Model includes all industrial and irrigation wells
•Model period simulates 1999-2011
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Model Development
Grand Island Dewatering Study – 2012 Update
Steady-State Flow Model
METHOD: MF2K/GWV, PEST
GOAL: Estimate aquifer permeability, establish initial
water table conditions representing the late 1990s,
set initial conditions for transient period.
Transient Flow Model
METHOD: MF2K/GWV, forward calibration
GOAL: Development of a model that simulates
changing water levels witnessed from 1999-2011, and
is capable of accurately determining water level
changes resulting from new dewatering wells.
Design & Test Wellfield Configurations
METHOD: MF2K/GWV, calibrated transient model testing well
configurations vs. a 15 ft. bgl elevation.
GOAL: Use of transient model to determine number of wells,
appropriate pumping rates and times to reduce water table below a
critical depth in the areas of concern underlying Grand Island. Assess
timing of water table recovery with wells idle.
MT3DS Test
METHOD: MF2K/GWV, MT3DS
GOAL: Test influence of new wells in south area of
concern on hypothetical contaminate plumes in areas
where mapped plumes exist under Grand Island.
Capture Zone Analysis
METHOD: MF2K/GWV, MODPATH
GOAL: Determine spatial area of influence of each
new dewatering well over 12 year period, ensure no
interference with contaminant plumes by capture
zones in south area of concern.
Final Product
Configuration of dewatering wells in the north and south areas of concern,
showing necessary rates and pumping durations to reduce water table elevations
below a 15-ft bgl "critical surface", with reasonable confidence that minimal
influence on contaminant plumes will occur in south area of concern.
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Groundwater Model Animations
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2012 Dewatering System Layout
Grand Island Dewatering Study – 2012 Update
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Grand Island Dewatering Study – 2012 Update
Area 1
•Eleven dewatering
wells
•Two discharge points
•Five new and Five
existing monitoring
wells
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Area 1
•Simulated 12 year
well capture zones
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Area 2
Grand Island Dewatering Study – 2012 Update
•Sixteen dewatering
wells
•Four discharge points
•Five new and Five
existing monitoring
wells
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Area 3
Grand Island Dewatering Study – 2012 Update
•Six dewatering
wells
•Three discharge
points
•One new
monitoring well
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Area 3
•Simulated
12 year
well
capture
zones
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Preliminary Opinion of Probable Cost
Grand Island Dewatering Study – 2012 Update
Area 1 Area 2 Area 3 Combined Total
Transmission Line $7,085,000 $1,617,000 $477,000 $9,179,000
Utility Conflicts $31,000 $28,000 $4,000 $63,000
Dewatering Wells $939,000 $1,347,000 $498,000 $2,784,000
Control System $118,000 $135,000 $62,000 $315,000
Construction Cost Subtotal $8,173,000 $3,127,000 $1,041,000 $12,341,000
Contingency $818,000 $313,000 $105,000 $1,236,000
Overhead, Legal, Fiscal,
Engr.
$981,000 $376,000 $125,000 $1,482,000
ROW Acquisition $1,877,000 $1,496,000 $496,000 $3,869,000
Total Project Costs $11,849,000 $5,312,000 $1,767,000 $18,928,000
Annual Costs
(20 years, 7%, A/P)
$1,118,462 $501,415 $166,792 $1,786,669
Annual Costs
(20 years, 3%, A/P)
$796,439 $357,050 $118,770 $1,272,259
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Annual Operational Cost
Grand Island Dewatering Study – 2012 Update
Item Area 1 Area 2 Area 3 Combined
Total
Labor $18,000 $18,000 $18,000 $54,000
Power $120,000 $94,000 $30,000 $244,000
Miscellaneous Repairs &
Supplies $47,700 $68,900 $26,500 $143,100
Total O & M (Per Year) $185,700 $180,900 $74,500 $441,100
TOTAL Annual Cost $982,139 $537,950 $193,270 $1,713,359
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Monthly Operational Cost
Grand Island Dewatering Study – 2012 Update
Item Area 1 Area 2 Area 3 Combined
Total
Labor $1,500 $1,500 $1,500 $4,500
Power $10,000 $7,833 $2,500 $20,333
Miscellaneous Repairs &
Supplies $3,975 $5,742 $2,208 $11,925
Total O & M (Per Month) $15,475 $15,075 $6,208 $36,758
TOTAL Monthly Cost $81,845 $44,829 $16,106 $142,780
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Financing Options
•Dewatering Districts
•User Fees
•Revenue and Various Purpose Bonds
•Water Banking
Grand Island Dewatering Study – 2012 Update
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Implementation Recommendations
•Conduct neighborhood meetings for education and public
outreach
•Work with CPNRD for funding options
•Develop Dewatering Districts
•Determine financing schedule
•Complete final design including plans and specifications
•Solicit bids for construction
•Conduct final public hearings on assessment fees
Dewatering Districts
•Finalize project financing
Grand Island Dewatering Study – 2012 Update
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Questions
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2012 UPDATE
DRAFT
GRAND ISLAND DEWATERING SYSTEM STUDY
GRAND ISLAND,NEBRASKA
PREPARED FOR
CITY OF GRAND ISLAND, NEBRASKA
AND
CENTRAL PLATTE NATURAL RESOURCES DISTRICT
MAV7,2012
OLSSON PROJECT No.011-2231
O\.OLSSON
ASSOCIATES
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Olsson Project No. 011-2231
TABLE OF CONTENTS
EXECUTIVE SUMMARY ................................................................................................................................................... 1
1.0 INTRODUCTION ................................................................................................................................................ 3
1.1 Background .................................................................................................................................................. 3
1.2 Study Area and Areas of Concern ................................................................................................................ 3
1.3 Document Purpose, Scope, and Organization ............................................................................................. 4
2.0 GEOLOGY, HYDROGEOLOGY AND GROUNDWATER PLUMES .......................................................................... 4
2.1 Geology ........................................................................................................................................................ 4
2.2 Hydrogeology ............................................................................................................................................... 4
2.3 Groundwater Plumes ................................................................................................................................... 5
3.0 PREVIOUS DEWATERING OPTIONS aND PROPOSED ANALYSIS ........................................................................ 6
4.0 GROUNDWATER MODELING ............................................................................................................................ 7
4.1 Comparison to 2000 Model ......................................................................................................................... 7
4.2 Model Development .................................................................................................................................... 8
4.3 Modeling Results ........................................................................................................................................ 10
5.0 CONVEYANCE AND DISCHARGE OPTIONS ...................................................................................................... 10
5.1 Well and Pump Locations ........................................................................................................................... 10
5.2 Conveyance System Routing and Discharge Points ................................................................................... 11
5.3 Dewatering systems operation options ..................................................................................................... 12
6.0 PRELIMINARY OPINION OF PROBABLE COST .................................................................................................. 12
7.0 FINANCING OPTIONS ...................................................................................................................................... 14
7.1 Dewatering Districts ................................................................................................................................... 14
7.2 User Fees .................................................................................................................................................... 15
7.3 Outside Funding ......................................................................................................................................... 15
7.4 Revenue and Various Purpose Bonds ........................................................................................................ 15
7.5 Water Banking ............................................................................................................................................ 15
8.0 IMPLEMENTATION RECOMMENDATIONS ...................................................................................................... 16
LIST OF FIGURES
Figure 1 Study Area Location Map
Figure 2 Water Table versus Annual Precipitation (Incorporated in page 5)
Figure 3 Groundwater Contamination Plume Map
Figure 4 2000 Dewatering Well Layout
Figure 5 2012 Groundwater Model Extent and Boundary Conditions
Figure 6 2012 Groundwater Model Large Capacity Well Locations
Figure 7 Updated Dewatering System Layout
LIST OF TABLES
Table 1 Comparison of the 2000 and 2012 Groundwater Models
Table 2 Updated Dewatering System- Probable Opinion of Cost
Table 3 Updated Dewatering System – Estimated Annual Cost
Table 4 Updated Dewatering System – Estimated Monthly Cost
LIST OF APPENDICES
Appendix A Groundwater Model Report
Appendix B Detailed Layout of the Updated Dewatering System
Appendix C Detailed Breakdown of Probable Opinion of Cost
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Olsson Project No. 011-2231
EXECUTIVE SUMMARY
In 2000, as part of a comprehensive study to evaluate options for dewatering high water table
conditions in Grand Island, Olsson Associates constructed a numerical groundwater model of
the aquifer underlying Grand Island to use as a tool to design a dewatering system. The 2000
study used state-of-the-art groundwater modeling and analytical tools, however, in the past
twelve years, computer modeling capabilities and computer computation speeds have
dramatically increased. Additionally, the issues of high water table conditions in certain areas of
Grand Island have remained and even expanded in some parts of town. For these reasons, the
City of Grand Island and Central Platte NRD authorized an update to the 2000 study using the
latest groundwater modeling tools.
The primary areas within the Grand Island city limits that have recurring high water table
conditions are in the west, northwest and southern part of town. The areas have been
subdivided into three areas where Area 1 encompasses the northwest part of the City and is
located primarily between Highway 2 and Highway 30 and west of Highway 281. Area 2
encompasses the southern part of the City located south of Oklahoma Avenue to the Wood
River, west from Highway 281 to the east along South Stuhr Road. Area 3 is located east of
South Stuhr Road to the eastern edge of the City just east of North Shady Bend Road.
An updated groundwater model was developed, calibrated and executed to evaluate the optimal
locations for a system of dewatering wells in Areas 1, 2 and 3. The Updated Dewatering
System includes a total of 33 dewatering wells (11 in Area 1, 16 in Area 2 and Area 3 has 6).
The system is designed to reach the optimal groundwater target depth of 10-15 feet below
ground surface through the network of dewatering wells without significant impacts to the
existing groundwater contamination plumes that are known to exist within the City limits.
The major findings of this study are as follows:
1. The City of Grand Island and the CPNRD should continue to develop this project. The
plan proposed in this study consists of a series of vertical wells placed throughout the
project area. The wells will be connected together with a system of pipes to transfer the
water to the Platte River. The City will be able to monitor and control the system with a
centrally located control system.
2. The groundwater levels continue to plague local residents, not knowing the next time the
groundwater will enter their basements. The property values in the affected areas would
return to the current market values of the City when the projects are completed.
3. The findings of the updated study are similar to that found in the 2000 report, but the
modeling is more refined to better understand the groundwater movement. Construction
costs have increased but the interest rates have decreased, resulting in only minor
annual cost increases from the 2000 study. The project remains affordable to the City of
Grand Island.
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Olsson Project No. 011-2231
Implementation of a project of this magnitude is impossible without the support of the public.
The following recommendations will provide guidance in the implementation of a comprehensive
dewatering system for the City of Grand Island.
Conduct neighborhood meetings within the three project areas. Determine the local
support and discuss with the residents the Dewatering District concept, and how it would
be used to fund the proposed capital improvements and OM&R.
The City should work closely with the Central Platte Natural Resources District (CPNRD)
to review the possible local, state and federal funding sources. Determine if any outside
funds could be made available to help finance the projects. Use the CPNRD resources
to create a water banking program for future income options.
The City should proceed to create the Dewatering Districts. Three Districts should be
created.
Determine the financing and revenue sources and schedule, to meet the necessary
obligations. Obtain temporary financing to begin the development of the Projects.
Authorize Olsson Associates to complete project plans and specifications and obtain all
necessary permits and right-of-way and /or easements in cooperation with the City, as
necessary, for the project construction, operations and maintenance.
Solicit construction bids for the project from local contractors.
Construct projects per plans and specifications
Determine Project Costs and Assessments. Conduct hearing for to make the final
assessments to each resident within the benefited area.
Finalize financing based on the amount of bonds required to pay off the temporary
financing and pay for the project over a 20 year period.
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1.0 INTRODUCTION
This Dewatering System Study Update was prepared to identify the best option available to
reduce high water table conditions in three specific areas within the City of Grand Island,
Nebraska. This document was prepared by Olsson Associates under contract with the City of
Grand Island and Central Platte Natural Resources District (Central Platte NRD). The
document was written for the sole use of the City and Central Platte NRD.
1.1 BACKGROUND
In 2000, as part of a comprehensive study to evaluate options for dewatering high water table
conditions in Grand Island, Olsson Associates constructed a numerical groundwater model of
the aquifer underlying Grand Island to use as a tool to design a dewatering system. The 2000
study used state-of-the-art groundwater modeling and analytical tools, however, in the past
twelve years, computer modeling capabilities and computer computation speeds have
dramatically increased. Additionally, the issues of high water table conditions in certain areas of
Grand Island have remained and even expanded in some parts of town. For these reasons, the
City of Grand Island and Central Platte NRD authorized an update to the 2000 study using the
latest groundwater modeling tools. Using the results of this updated study, the City will be able
to make an informed decision on the best options available to reduce the high water table
conditions. This study also includes updated project cost estimates and construction phasing
options that will help in the development of dewatering districts to provide funding for the
project.
1.2 STUDY AREA AND AREAS OF CONCERN
The City of Grand Island is located in an area where the depth to groundwater varies from less
than 10 to 30 feet below the ground surface. The City lies within the relatively flat sandy river
deposits of the Platte River Valley. There is very little topographic relief across the City with one
prominent exception. There is a broad topographic plateau that elevates the center of the City.
The plateau lies roughly parallel to the railroad tracks that cut through the center of the City from
the southwest to the northeast. This broad topographic feature elevates the center of town
approximately 12 feet above the rest of the City. Because of the relief that this plateau
provides, the issues of high water table conditions in the central portion of the City are not as
significant. Conversely, as the City has grown westward and to the south, there are areas of the
City that now directly overlie the sandy alluvial river deposits with recurring high water table
conditions.
The three primary areas within the Grand Island city limits that have recurring high water table
conditions are illustrated in Figure 1. Area 1 encompasses the northwest part of the City and is
located primarily between Highway 2 and Highway 30 and west of Highway 281. Area 2
encompasses the southern part of the City located south of Oklahoma Avenue to the Wood
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Olsson Project No. 011-2231
River, west from Highway 281 to the east along South Stuhr Road. Area 3 is located east of
South Stuhr Road to the eastern edge of the City just east of North Shady Bend Road.
In these three area, the citizens of Grand Island have had repeated incidents of groundwater
inundation into their basements. This causes significant property damage, health issues related
to mold and mildew, and for many home owners, the annual problem directly affects property
values since it is difficult to sell homes with recurring water problems.
1.3 DOCUMENT PURPOSE, SCOPE, AND ORGANIZATION
The purpose of this project is to provide the City with the most cost effective and efficient option
to reduce the recurring high water table conditions in Areas 1, 2, and 3. As stated above, this is
an update to the study completed by Olsson Associates in 2000. The scope of the current
project included:
Evaluating the current groundwater hydrogeologic conditions in Grand Island (Section 2)
Reviewing the previously proposed dewatering options (Section 3)
Preparing a transient numerical groundwater model that simulates the aquifer conditions
in and around Grand Island (Summary in Section 4 with the full analysis presented in
Appendix A)
Preparing an optimized dewatering system layout (Section 4)
Describing system conveyance and discharge options and a preliminary opinion of
probable cost (Section 5)
Describing the City’s financing options (Section 6)
Recommendations for project implementation (Sections 7)
2.0 GEOLOGY, HYDROGEOLOGY AND GROUNDWATER PLUMES
2.1 GEOLOGY
Grand Island is underlain by sands and gravels deposited by the braided Platte River as it
shifted across what is now Hall County. These sands and gravels were deposited on top of the
Seward Formation which in contrast is composed of silts and clays. In the Grand Island area,
the Platte River sand and gravel deposits are from 70-175 feet thick with some isolated
interbedded silts and clays within the thick sequence of sand and gravel.
2.2 HYDROGEOLOGY
In Grand Island, as in much of central Nebraska, the movement of groundwater occurs through
the pore spaces between uncemented grains of clay, silt, sand, and gravel. The depth to the
water table ranges from less than 10 feet to greater than 30 feet below ground surface
depending on the location within the study area and the prevailing climate conditions. The sand
and gravel aquifer beneath Grand Island ranges in saturated thickness from 50 to 150 feet. The
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coarse sand and gravel deposits provide for excellent water bearing capacity and as such,
Grand Island has some highly productive wells that can generate over 2,000 gallons per minute.
Because of the relatively shallow water table conditions combined with the coarse sand
deposits, groundwater levels rise and fall in response to precipitation events. Figure 2 illustrates
the water table changes over time in comparison with annual precipitation. Differences between
the two plots are mostly due to a slightly delayed response time in the aquifer conditions and as
well, the affects of pumping from nearby irrigation wells. What this means for the citizens of
Grand Island living in Areas 1, 2 and 3 is that after significant rain events, there is an almost
immediate rise the water table such that basements are inundated until the high water table
conditions subside.
Figure 2 Groundwater levels versus annual precipitation at USGS monitoring well 405318098252202
2.3 GROUNDWATER PLUMES
The high water table conditions in Grand Island also cause other problems across the city.
There are numerous plumes of contaminants that occur in the aquifer due to the fact that any
spills or leaks of hazardous liquids at the surface migrate directly to the aquifer with little to no
natural attenuation of the contaminants. As shown in Figure 3, there are three Groundwater
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Control Areas (GCA) and two primary groundwater plumes that have been mapped beneath
Grand Island. In the center of the City is groundwater control area No. 1. The control areas
were designated where there has been significant deterioration of groundwater quality and to
ensure the protection of human health and the environment, regulations were put in place to
restrict the installation of wells and use of the groundwater. All three of the GCAs are
undergoing groundwater remediation and cleanup. GCA No. 1, Cleburn Street Site and GCA
No. 2, Cornhusker Army Ammunition Plant, are not within the Areas of Concern specified for
this project. Furthermore, the plume that had previously emanated from the Cornhusker Army
Ammunition Plant has been remediated to the required groundwater cleanup standard.
Therefore, these two areas of groundwater contamination will not be discussed in further detail
in this report.
GCA No. 3, Parkview Well Site, north and south plumes are on the western edge of Area 2 and
the Former NSC plume cuts across Area 3. These plumes have significant impact on the
placement of dewatering wells since any dewatering within these areas must be accomplished
without impacting the direction or flow regime of the groundwater plumes. The City of Grand
Island must avoid withdrawals of water from within these plumes, otherwise, the City will have to
treat the water prior to discharge.
Both the GCA No. 3 Parkview and the Former NSC plumes contain levels of chlorinated
solvents that are above the US EPA Drinking Water Standards. These types of contaminants
are typically more dense that water and therefore sink to the bottom of the aquifer. For this
reason, the dewatering scenarios described in this report include a series of smaller capacity
wells that withdraw water from the top of the water column to ensure that the deeper
contaminants are not mobilized toward the dewatering pumps.
3.0 PREVIOUS DEWATERING OPTIONS AND PROPOSED ANALYSIS
In 2000, the selected alternative for dewatering the northwest and southeast portions of the City
included the well configurations illustrated in Figure 4. In the northwest area, 11 dewatering
wells pumping at 500 gallons per minute (GPM) were proposed in a north south configuration
along Independence Avenue. In the southeast area, a total of 17 new wells pumping at 300
GPM and 1 existing City well pumping at 1100 GPM were proposed to dewater the southeast
part of the City. Although this was the best option at the time, the following items have changed
since that time rendering the recommendations invalid:
The northwest area of high water table conditions has expanded to include parts of the
City that have new residential and commercial development. In order to meet achieve
dewatering across this expanded area, the wells need to be strategically placed across
the expanded area.
The US EPA has implemented a cleanup program for the GCA No. 2 at the Parkview
Well Site and several extraction wells have been installed. If the previously suggested
11 wells were installed as described in the 2000 report, they would interfere with the US
EPAs pump and treat system that was installed since the time of the initial report.
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In 2000, the NSC plume was not defined. If the five wells were installed as originally
designed, they would intercept the plume and the water pumped from the wells would
require treatment prior to discharge.
Due to these issues with the 2000 dewater system design, a new analysis of the best alternative
for dewatering the high water table conditions in Grand Island was proposed. The proposed
analysis had the same objectives as the original study in that the intent was to lower the static
water levels within Areas 1, 2, and 3 1-2 feet below the levels of residential basements.
Therefore target level set for dewatering in Areas 1, 2, and 3 was 10 to 15 feet below the ground
surface.
4.0 GROUNDWATER MODELING
The following section provides a summary of the methodology and results of the groundwater
modeling analysis performed for this dewatering study update. A detailed description of the
modeling techniques, all hydrologic parameters and calibration statistics is included in Appendix
A. This summary is intended for a general audience. Those that would like to review a detailed
description of model development, calibration and implementation are directed to Appendix A.
4.1 COMPARISON TO 2000 MODEL
Before delving into a summary of the model development and results, it is important to consider
the differences between the 2000 and current groundwater models. The results of the two
analyses are different and reflect the dissimilarities listed in Table 1. It is also important to note
that the primary reasons for these dissimilarities is because of the increased computational and
storage capacity of computers currently available for engineering work. Additionally, there have
been several technological advances in the science of groundwater modeling including the use
of parameter estimation simulation techniques (PEST) that test various input parameters and
optimize selection of appropriate values during the calibration phase of model development. The
list in Table 1 provides a summary of the differences between the two groundwater models.
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Table 1 Grand Island Dewatering Model Differences – 2012 versus 2000
2012 Groundwater Model 2000 Groundwater Model
Land surface based on recent high resolution
LiDAR topographic information
Land surface based on topographic contours from
the City of Grand Island
Lower mode boundary, base of the aquifer,
replicates natural conditions
Lower model boundary set to a uniform elevation
across the entire model
Variable hydraulic conductivity across modeled
area were based on inverse calibration of steady
state model using Parameter Estimate Simulation
Technique (PEST)
Uniform hydraulic conductivity across the entire
model
Four recharge zones were used in the model –
irrigated land, riparian, urban open and urban
developed areas.
Two recharge zones were used – undeveloped
and urban
The Platte River and Wood River were simulated
using the stream package which allows for
simulation of actual recorded flow rates including
no flow during drought conditions.
The Platte River was simulated using the River
Package and the Wood River as a drain. The
simulations were based on water table contours
from regional maps.
Transient model incorporated all registered
industrial and irrigation (high capacity) wells and
including the Grand Island municipal wells whose
pumping rates were calculated based on
electrical usage.
No industrial or irrigation wells incorporated into
the model.
Transient model uses a specific yield term that is
very similar to values reported by CPNRD from
pump tests in the study area.
Steady state model calibration to high water
table conditions observed in 1994.
Transient model simulates 12-year period from
1999 to spring of 2011. This time period
witnessed excessive fluctuations due to drought
in the first half of the decade and water table
recoveries in the second half.
Transient simulation calibrated to observed
drawdowns at one point in time (144 days). Thus
the model did not replicate temporal variations in
the water table across the model area.
4.2 MODEL DEVELOPMENT
The new groundwater model was developed in a series of eight separate steps. Table 2 lists the
steps and description of the types if information gathered and simulated during each step of the
way. Groundwater modeling is done in a step-wise fashion that goes from simple data
gathering and analysis to additional complexity in the model simulations. Specifically, the model
goes from simulating one point in time to a series of simulations that represent a twelve year
modeling period with the proposed extraction wells and groundwater plumes. Detailed
information on each step is provided in the Groundwater Model Report along with a description
of the steps presents a flow diagram illustrating the process in graphical format (Figure 3.1-1 in
Appendix A).
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Table 2 The Eight Step Process of Groundwater Model Development
Model Development Step Description/Comments
1. Data gathering and site conceptualization Data such as high resolution survey topography,
water level measurements, registered well
locations, pumping rates, and geologic
/hydrogeologic regime defined.
2. Development and calibration of a steady
state model to conditions observed in the late
1990s
An initial groundwater model was developed to
simulate conditions at one point in time. This was
done to evaluate the initial model input
parameters and their ability to simulate natural
conditions
3. Development of a transient model that
simulates the period from 1999 to 2011
Include water pumping data from irrigation and
municipal wells as well as changes in recharge
across the model to simulate changes in climate
and pumping
4. Test and evaluate dewatering well
configurations in the Northwest area of
concern
Assess the optimal location of dewatering wells
and minimize piping lengths where possible.
5. Test and evaluate dewatering well
configurations in the South area of concern
Assess the optimal location of dewatering wells
and minimize piping lengths and avoid impacts to
the Parkview plume.
6. Assess the time of recovery of the water table
when wellfield is idle following initial
dewatering
This was completed at the request of the City in
order to assess operational needs of the system
7. Develop capture zones for new dewatering
wells over a 12 year period
An initial check on the impact to groundwater
plumes in Areas 2 and 3
8. Assess influence of dewatering well pumping
on simulated contaminant plumes on the
west and east sides of the South area of
concern where known plumes have been
mapped and are the subject of federal and
state mitigation programs and operations
Further evaluation of the impact of the proposed
dewatering wells on the groundwater plumes in
Areas 2 and 3.
Figures 5 and 6 present the important groundwater, surface water and pumping well
components built into the groundwater model. The model grid is illustrated in Figure 5 and it
represents the area covered by the groundwater model. The horizontal and vertical grid lines
indicate the size of the grid across the model. The grid is 250 by 250 feet in the center of the
City and is larger at the edges of the model boundary. This is to ensure that the accuracy of the
model is focused on the central portion where Area 1, 2, and 3 are located. Figure 5 also
illustrated the location of the boundary conditions that define the edges of the model, specifically
the constant head, stream and drain boundaries built into the model to represent active
groundwater flow and surface water influences.
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Figure 6 illustrates the location of active high capacity pumping wells. The pumping records for
all municipal wells were incorporated into the groundwater model. There were no pumping
records available for industrial or irrigation wells and therefore the pumping rates introduced into
the model were based on the registered capacity of industrial wells. For irrigation wells, the
pumping rates were based on the registered pumping rate on the well registration records along
with the number of acres irrigated and climatic conditions for each year of the simulation period.
4.3 MODELING RESULTS
The objective the groundwater model was to optimize the well configurations in the 2000
dewatering well study for Areas 1, 2, and 3. Figure 7 illustrates the updated well configuration
based on numerous simulations that included variations on pumping rates, well locations and
pumping durations. The results are summarized as follows:
Dewatering Area 1 required 11 dewatering wells, each pumping at 500 gpm for 6.5 to 7
months.
Dewatering Area 2 required 16 dewatering wells, each pumping at 400 to 500 gpm for 9
to 10 months.
The Area 2 wells are strategically located to minimize interaction with the Parkview
plume and extraction wells.
Dewatering Area 3 required 6 dewatering wells, each pumping at 400 to 500 gpm
Dewatering Areas 2 and 3 initially takes approximately 9-10 months.
The Area 3 wells are located outside the area to minimize the effect on the NSC plume.
For Areas 1, 2 and 3, water table recovery approaches the critical surface (10-15 feet
below ground surface) in 1.5 to 2 months after the dewatering wells are turned off.
5.0 CONVEYANCE AND DISCHARGE OPTIONS
There were several conveyance and discharge options evaluated as part of this analysis. This
section provides information how the different options were analyzed, evaluated and selected.
5.1 WELL AND PUMP LOCATIONS
The first part of the conveyance and discharge analysis was to evaluate the well and pump
locations. After the modeling analysis was complete and illustrated the optimal well
configuration for the system, the well locations were checked in the field to ensure that the site
identified for wells and pumps were accessible for installation, maintenance, and repair. Where
possible, sites on City property or Right-of-Way were selected. In Appendix B, detailed maps of
the proposed well sites are presented. During final design of the system, these locations will be
further checked for other issues related to constructability such as underground utilities etc.
Minor changes in location (less than 75 feet) will not require rerunning the groundwater model,
however, if significant relocation is required due to access or other constructability issues, a
model run will be required to verify that the target dewatering depths are achieved.
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5.2 CONVEYANCE SYSTEM ROUTING AND DISCHARGE POINTS
As described in the 2000 Dewatering Study Report, there are two discharge options on the
south end of the City:
1. Discharge to the Platte River via the Wood River Diversion Channel
2. Discharge to Wood River
For this updated study, a third option was evaluated:
3. Discharge to Moores Creek Drain that flows into Boy Scout Lake
Looking at hydrogeologic factors alone, the best option would be Number 1: discharge to the
Platte River via the Wood River Diversion Channel. This is the best option because the
discharge water is routed three miles away from the areas of the City with high water table
conditions. As will be discussed in more detail in the Opinion of Probable Cost (Section 6), this
option is the most costly. Option Number 3: Discharge to Morris Creek Drain would route the
discharge water away from the Areas 1, 2 and 3, however, the drainage ditch does not have a
large enough capacity to handle the entire discharge stream from the proposed system.
Furthermore, the added surface discharge from the entire system may cause problems in the
northeast part of the City.
For these reasons, Option Number 2: discharge to the Wood River was evaluated as an option
for the updated dewatering system. The designed discharge would be on the order of 25-35
cubic feet per second. This represents only 15% of the flow in the Wood River and the added
volume of water should not be a problem for the area. During the study analysis phase,
questions arose about the additional flow that would need to be accommodated in the Wood
River such as, ‘would the additional flow cause the proposed dewatering system to fail along the
western margin of the Wood River in Area 2? The groundwater model indicated that the system
was able to achieve the target dewatering levels even when flow was increased. For these
reasons, the conveyance for the dewatering system was designed with several discharge points
along the Wood River and one to the north into Morris Creek drain. Details of the conveyance
and discharge layout are as illustrated in Figure 7, Appendix B and described as follows:
Area 1 has a total of eleven wells and two discharge points.
o For ten of eleven dewatering wells in Area 1, the discharge will be routed south
either directly to the Wood River or to the Platte River via the Wood River
Diversion Channel.
o The difference between the two options is primarily a cost decision because the
volume of flow that is scheduled for discharge from the dewatering system is 25-
35 cubic feet per second which is only 15% of the maximum capacity in the
Wood River. The difference in cost between a direct discharge to the Wood River
and a discharge to the Wood River Diversion Channel is nearly $1.7 million. This
cost differential is due to extra pipe and to provide a low-flow liner in the bypass
channel.
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o The final discharge point for Area 1 is for well #11 on north side of Highway 30.
The cost of piping the discharge from well #11 south beneath the highway is too
high to justify tying the conveyance piping to the rest of the system. For this
reason, the system includes one discharge point at the Moores Creek drain
which flows to Boy Scout Lake. The cost to line the discharge channel with
concrete is included in the cost estimate for the conveyance system.
For Areas 2 and 3, there are 22 wells that are subdivided into seven different discharge
points.
o Four discharge points are piped directly to the Wood River and three discharge
to drainage ditches that flow to the Wood River.
o The three discharge points that flow into ditches are for wells #15 and #16 in
Area 2 and #20 in Area 3. The locations were designed to minimize piping since
installation of piping in heavily developed areas is costly.
5.3 DEWATERING SYSTEMS OPERATION OPTIONS
In the 2000 Dewatering System study, there were two options proposed for operating the
system, one was manual operation where an operator would both physically monitor water
levels and operate the dewater system pumps based on the water table readings. The second
included a computer-controlled operating system where a centrally located computer would
operate the system based on remotely monitored water level probes. The cost of the computer
operated system monitoring systems has dramatically decreased in the last ten years and so
whereas in the initial study, the cost of the computer automated monitoring system may have
been prohibitive, in the current analysis, the cost for manual water level monitoring and pump
operation is actually more. For this reason, the updated dewatering system has been designed
with pumps that are controlled remotely. Additionally, a network of eleven new observation
wells are proposed along with four existing observation wells with water level transducers to
remotely assess water level readings to evaluate dewatering system performance. The
locations of the proposed and existing observation wells are illustrated on Figure 7.
6.0 PRELIMINARY OPINION OF PROBABLE COST
An updated Preliminary Opinion of Probable Cost was developed for the Updated Dewatering
System. The costs are summarized in Table 3 with additional backup included in Appendix C.
Installation of the system could be completed as a whole, or alternatively, the cost has been
subdivided into three phases that correspond to Areas 1, 2 and 3. One advantage to phased
construction would be to assist with project financing. Section 7.0 provides information on the
financing options for the project and by phasing the project, the capital improvement loan could
be divided by Phase which make the loan easier to secure as well as repay.
Another reason that the project has been subdivided into three phases is that due to the
ongoing groundwater remediation systems in both Areas 2 and 3, coordination will be required
when final design and construction begin. With EPA oversight on the Parkview Well Site project
and the NDEQ involvement with the NSC remediation, Olsson anticipates that there will be
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additional Regulatory Agency review and approval required in these two areas which may cause
project implementation delays.
The capital improvement loan that is part of the Probably Opinion of Cost is based on a 20 year
loan with two different interest rates, 7% and 3%. The 7% value was included so that the
overall project costs could be compared easily with the 2000 estimates. Currently interest rates
are at record low values and a 3% interest rate is achievable in the near future since, the
Federal Reserve Bank plans to keep interest rates low until 2014 (Federal Reserve Update,
April 25, 2012, http://www.money-rates.com/fed.htm). Other items of note in Table 3 include
the importance of minimizing the transmission line length due to the cost of the materials and
construction of underground transmission lines in the City.
Construction rates were based on cost estimates developed by local contractors on projects in
the Grand Island area. No escalation rates were applied to the project costs since the Phasing
of project construction change the escalation factors and may skew the costs if the timing of the
project is modified.
Table 3 Updated Dewatering System - Probable Opinion of Cost
Area 1 Area 2 Area 3 Combined
Total
Transmission Line $7,085,000 $1,617,000 $477,000 $9,179,000
Utility Conflicts $31,000 $28,000 $4,000 $63,000
Dewatering Wells $939,000 $1,347,000 $498,000 $2,784,000
Control System $118,000 $135,000 $62,000 $315,000
Construction Cost Subtotal $8,173,000 $3,127,000 $1,041,000 $12,341,000
Contingency $818,000 $313,000 $105,000 $1,236,000
Overhead, Legal, Fiscal, Engr. $981,000 $376,000 $125,000 $1,482,000
ROW Acquisition $1,877,000 $1,496,000 $496,000 $3,869,000
Total Project Costs $11,849,000 $5,312,000 $1,767,000 $18,928,000
Annual Costs
(20 years, 7%, A/P)
$1,118,462 $501,415 $166,792 $1,786,669
Annual Costs
(20 years, 3%, A/P)
$796,439 $357,050 $118,770 $1,272,259
Table 4 and 5 include Estimated Annual and Monthly Costs for the updated dewatering system.
In each of these tables, the largest expense for system operation is power usage. The rates
included in the cost analysis were based on current electrical rates provided to Olsson by the
City. Miscellaneous system repairs and maintenance are the second largest annual/monthly
expenditure. This line item includes pump, pump control and observation well monitoring
equipment replacement.
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Table 4 Updated Dewatering System – Estimated Annual Cost
Item Area 1 Area 2 Area 3 Combined
Total
Labor $18,000 $18,000 $18,000 $54,000
Power $120,000 $94,000 $30,000 $244,000
Miscellaneous Repairs & Supplies $47,700 $68,900 $26,500 $143,100
Total O & M (Per Year) $185,700 $180,900 $74,500 $441,100
TOTAL Annual Cost $982,139 $537,950 $193,270 $1,713,359
Table 5 Updated Dewatering System – Estimated Monthly Cost
Item Area 1 Area 2 Area 3 Combined
Total
Labor $1,500 $1,500 $1,500 $4,500
Power $10,000 $7,833 $2,500 $20,333
Miscellaneous Repairs & Supplies $3,975 $5,742 $2,208 $11,925
Total O & M (Per Month) $15,475 $15,075 $6,208 $36,758
TOTAL Monthly Cost $81,845 $44,829 $16,106 $142,780
7.0 FINANCING OPTIONS
Significant capital improvements like the Updated Dewatering System presented for the City of
Grand Island in this report are typically paid for by municipalities using a combination of different
financing options. For this project, although the specific financing options have not changed
since the 2000 study, more detail is available about each option and as such, each are
described in more detail below.
7.1 DEWATERING DISTRICTS
As presented in the 2000 report, drainage districts for surface water drainage improvements
have been used successfully across the State; however, dewatering districts are new in
Nebraska. Grand Island’s City Attorney has reviewed Nebraska State Statues and has
determined that dewatering districts are within the City’s jurisdiction to implement. Accordingly,
Areas 1, 2 and 3 could be designated as dewatering districts by the City with additional
assessments applied to the residents that benefit from the project. It is recommended that a
multiple level assessment be developed since some residents may receive more benefit from
the project than others. A final rate determination would be developed based on additional
study including issues such as the need to account for the size and/or location of the assessed
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property. The City will determine the appropriate level of assessment to meet the needs of the
project, yet remain affordable for its residents.
7.2 USER FEES
Another option for revenue generation presented in the 2000 report was to apply additional
charges to the City Utilities user fees. This option is not recommended due to the fact that the
user fees would be applied at a flat rate across the City with no adjustment afforded to the
portions of the City that are not benefiting from the project.
7.3 OUTSIDE FUNDING
Grants and loans are available through a variety of organizations and in the 2000 Study Report;
the Nebraska Department of Natural Resources Development Fund was identified as a potential
source of outside funding. This option is no longer available since the Fund is no longer viable.
7.4 REVENUE AND VARIOUS PURPOSE BONDS
The City of Grand Island has the authority to secure bonds to complete capital improvements
projects. There are two types of bonds available to the City including Revenue Bonds and
Various Purpose Bonds. Revenue bonds are retired with revenue such as from the sale of
discharge water for beneficial uses. Alternatively, the Various Purpose bonds are retired with
revenue generated by the assessments, operating income or a combination of both. As with the
initial study, the Various Purpose bonds are proposed as the best option for the City because of
the flexibility in repayment opportunities.
7.5 WATER BANKING
Water banking is one way that the project may be able to generate revenue. According to the
Central Platte NRD website (http://www.cpnrd.org/Water_Bank.html), the NRD’s Water Banking
Program began in January 2007 to try to reduce the need to regulate irrigators within the
District. As part of the program, the NRD purchases water rights as a solution to balance water
that is being used with water that is available. Two major programs required the NRD to find a
solution– the Platte River Recover Implementation Program (PRRIP) and Legislative Bill
962. The NRD must stay in compliance with both of these programs. Currently, the majority of
the NRD is at its limit for water use, known as fully appropriated. The western most part of the
District, above Elm Creek, is designated as over-appropriated; which requires the NRD to bring
water back to a fully appropriated status.
As water demands on the Lower Platte River increase, depletions will need to be offset. Using
Central Platte NRDs innovative system of water banking, there may be some opportunity for
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revenue generation through offsets depletions to the Platte River downstream of Grand Island.
This type of activity would be administered through the Central Platte NRD in cooperation with
the City of Grand Island.
8.0 IMPLEMENTATION RECOMMENDATIONS
Implementation of a project of this magnitude is impossible without the support of the public.
The City continues to experience high groundwater levels based on the frequency and amounts
of the rainfall events. Since the 2000 report was completed, the City has experienced periods of
wet and dry cycles and that will continue. The following recommendations will provide guidance
in the implementation of a comprehensive dewatering system for the City of Grand Island.
Conduct neighborhood meetings within the three project areas. Determine the local
support and discuss with the residents the Dewatering District concept, and how it would
be used to fund the proposed capital improvements and OM&R.
The City should work closely with the Central Platte Natural Resources District (CPNRD)
to review the possible local, state and federal funding sources. Determine if any outside
funds could be made available to help finance the projects. Use the CPNRD resources
to create a water banking program for future income options.
The City should proceed to create the Dewatering Districts. Three Districts should be
created.
Determine the financing and revenue sources and schedule, to meet the necessary
obligations. Obtain temporary financing to begin the development of the Projects.
Authorize Olsson Associates to complete project plans and specifications and obtain all
necessary permits and right-of-way and /or easements in cooperation with the City, as
necessary, for the project construction, operations and maintenance.
Solicit construction bids for the project from local contractors.
Construct projects per plans and specifications
Determine Project Costs and Assessments. Conduct hearing for to make the final
assessments to each resident within the benefited area.
Finalize financing based on the amount of bonds required to pay off the temporary
financing and pay for the project over a 20 year period.
The City of Grand Island and the CPNRD should continue to develop this project. The plan
proposed in this study consists of a series of vertical wells placed throughout the project area.
The wells will be connected together with a system of pipes to transfer the water to the Platte
River. The City will be able to monitor and control the system with a centrally located control
system.
The groundwater levels continue to plague local residents, not knowing the next time the
groundwater will enter into their basements. The property values in the affected would return to
the current market values of the City when the projects are completed.
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The findings of the updated study are similar to that found in the 2000 report, but the modeling
is more refined to better understand the groundwater movement. Construction costs have
increased but the interest rates have decreased, resulting in only minor annual cost increases
from the 2000 study. The project remains affordable to the City of Grand Island.
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FIGURES
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APPENDIX A
Groundwater Model Report
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DRAFT Groundwater Model Report
for the Grand Island 2012 Dewatering Study Update
Prepared by Olsson Associates for the City of Grand Island
and Central Platte Natural Resources District
Contents
1.0 INTRODUCTION ....................................................................................................... 3
2.0 SITE CHARACTERIZATION/CONCEPTUAL MODEL .............................................. 4
2.1 Topography and Land Use .................................................................................................. 4
2.2 Climate Conditions .............................................................................................................. 4
2.3 Geology/Hydrostratigraphy .................................................................................................. 4
2.4.1 Surface ......................................................................................................................... 6
2.4.2 Groundwater Flow System ........................................................................................... 7
3.0 MODEL DEVELOPMENT ......................................................................................... 9
3.1 Model Code and Applications .............................................................................................. 9
3.2 Model Structure ................................................................................................................... 9
3.3 Boundary Conditions ......................................................................................................... 12
4.0 STEADY-STATE MODEL ....................................................................................... 14
4.1 Parameter Inputs ............................................................................................................... 15
4.2 Calibration ......................................................................................................................... 15
4.3 Steady State Results ......................................................................................................... 17
5.0 TRANSIENT MODEL .............................................................................................. 21
5.1 Transient Model Development .......................................................................................... 21
5.1.1 Calibration Targets ..................................................................................................... 22
5.1.2 Specific Yield .............................................................................................................. 22
5.1.3 Pumpage and Recharge Inputs .................................................................................. 23
5.2 Transient Model Results .................................................................................................... 23
6.0 DEWATERING ANALYSIS ..................................................................................... 26
6.1 Methodology ...................................................................................................................... 27
6.2 Dewatering Results ........................................................................................................... 28
6.2.1 Northwest Area ........................................................................................................... 28
6.2.2 South Area .................................................................................................................. 28
6.3 Capture Zone Analysis ...................................................................................................... 32
6.4 Transport Model Assessment ............................................................................................ 34
6.5 Water Table Recovery ....................................................................................................... 37
7.0 CONCLUSIONS AND RECOMMENDATIONS ....................................................... 38
7.1 Key Findings ...................................................................................................................... 38
7.2 Recommendations ............................................................................................................ 38
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APPENDIX A - Additional calibration hydrographs for the transient model.
LIST OF TABLES
1.0-1 A comparison of the 2000 and 2012 Grand Island dewatering
models prepared by Olsson.................................................................................... 2
4.3-1 Final statistics from calibrated steady-state model................................................. 21
4.3-2 Calibrated steady-state model budget........... ....................................................... 21
5.2-1 Final transient calibration statistics........................................................................ 26
5.2-2 Transient model budget showing cumulative volumes at the
final time step of stress period 72.......................................................................... 26
LIST OF FIGURES
2.3-1 Groundwater levels vs. annual precipitation at USGS monitoring
well 405318098252202........................................................................................... 6
2.4-1 Flow rates in Wood River near west boundary of the model
domain from 1984 to 1994....................................................................................... 7
2.4-2 UNL-CSD 1995 regional water table contours across the model study area.......... 8
3.1-1 Flowchart of model development and application process...................................... 10
3.2-2 Study area model grid for the Grand Island area.................................................... 11
3.3-1 Configuration of active grid area and model boundary features..............................14
4.2-1 Hydraulic conductivity field in steady state model resulting
from calibration with PEST....................................................................................... 17
4.3-1 Observed vs. simulated groundwater level residuals for
calibrated steady-state model at 37 observation locations...................................... 18
4.3-2 Observed water levels vs. simulated vs. observed water level residuals................ 19
4.3-3 Simulated vs. observed water levels for the calibrated steady state model............ 19
4.3-4 Simulated steady-state water table contours.......................................................... 20
5.1-1 Locations of wells and water-level change observations
in the 1999-2011 transient model period................................................................. 24
5.2-1 Hydrographs of simulated vs. observed water
level changes during the transient model period..................................................... 25
6.0-1 Northwest and south areas of concern for high water table
conditions in Grand Island...................................................................................... 27
6.2-1 NWA dewatering patterns of 9 month period......................................................... 29
6.2-2 Locations of 11 dewatering wells in the NWA......................................................... 30
6.2-3 Locations of 22 dewatering wells in the SA............................................................ 30
6.2-4 SA dewatering patterns of a 10 month period........................................................ 31
6.3-1 12 year capture zones for dewatering wells in the NWA........................................ 33
6.3-2 12 year capture zones for dewatering wells in the SA............................................34
6.4-1 Results of a hypothetical contaminant transport analysis
at the Parkview plume in the SA............................................................................. 36
6.4-2 Results of a hypothetical contaminant transport analysis
at the VCP plume in the SA.................................................................................... 37
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1.0 INTRODUCTION
In 2000, as part of a comprehensive study of dewatering high water table conditions for the City
of Grand Island, Nebraska (City) and the Central Platte Natural Resources District (CPNRD),
Olsson Associates (Olsson) constructed a numerical groundwater model of the aquifer
underlying Grand Island to use as a tool for designing a system of dewatering wells to reduce
high water table conditions impacting several areas of the city. At the time of this study, funding
options were not available for the City to follow through with designs presented in the findings of
the study. In 2011, Olsson again partnered with the City to update the original study conducted
a decade earlier. This report presents the development process, results, and recommendations
of an updated groundwater model for the Grand Island area.
The groundwater model (model) presented in this report has many general similarities to the
model developed in 2000, including the areal extent, two-dimensional (single layer) aquifer
representation, and analogous model boundaries. However, with the luxury of nearly a decade
of elapsed time, availability of observation data and advancements in model technology and an
overall understanding of accepted groundwater modeling practices, Olsson is afforded the
ability to improve on the 2000 model version, an enhancement that will provide greater
confidence in engineering designs for dewatering high water table conditions in Grand Island.
The most significant difference with the updated model is the application of inverse model
calibration methods, specifically using the PEST code (Doherty, 1994) with application of the
pilot point method. Implementation of pilot points automates the estimation of input parameters
such hydraulic conductivity, and the resulting parameter fields are smoothed over the model
domain instead of using zones with discreet boundaries between input values. The second
major difference in the two models is the more rigorous calibration of both the steady state and
transient conditions, especially in regards to the ability of the model to simulate observed water
level changes that occurred during the drought of Table 1 presents a comparison of the 2000
and 2012 models prepared by Olsson.
Groundwater model development and application for dewatering analysis was completed in the
following sequence of tasks- 1) Data gathering and site conceptualization, 2) Development and
calibration of a steady state model to conditions observed in the late 1990s, 3) Development of
a transient model that simulates the period from 1999 to 2011, 4) Test and evaluate dewatering
well configurations in the Northwest area of concern, 5) Test and evaluate dewatering well
configurations in the South area of concern, 6) assess the time of recovery of the water table
when wellfield is idle following initial dewatering 7) Develop capture zones for new dewatering
wells over a 10 year period, and 8) Assess influence of dewatering well pumping on simulated
contaminant plumes on the west and east sides of the South area of concern where known
plumes have been mapped and are the subject of federal and state mitigation programs and
operations.
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2.0 SITE CHARACTERIZATION/CONCEPTUAL MODEL
2.1 Topography and Land Use
The City of Grand Island is situated in the Valleys topographic region of Nebraska as
designated by the University of Nebraska-Lincoln's Conservation and Survey Division (UNL-
CSD, 1973). Valley regions are defined as flat-lying areas along major streams that are
underlain by mixes of stream-deposited silt, clay, sand and gravel. Specifically, Grand Island is
situated within the broad Platte River valley, which extends over 14 miles in width near the City.
The landscape within the model study area slopes from a high of approximately 1,903 feet
above sea level (asl) in the west-northwest corner of the study area to a low point of about
1,820 feet asl just north of the study area's southeast corner at a slope of 0.0011. In the
southern half of Grand Island, a pronounced southwest to northeast trending "ridge" in the
surface topography separates the land to the north by as much as 10 feet from the landscape
to the south and is clearly evident in area topographic maps.
Land use within the model study area is primarily mixed urban with surrounding agricultural land
and riparian areas that are crossed by numerous highways and secondary roads. Grand
Island's population in 2010 is 48,520 (U.S. Census, 2010) and covers a large portion of the
study area. Within the city proper, various land uses exist, including residential, industrial, retail,
open spaces/parks, roadways, parking lots, schools, and undeveloped land.
2.2 Climate Conditions
Annual precipitation at Grand Island averages about 25.5 in/yr, with approximately 80 percent of
this amount received during the growing season months of May through September. During the
transient period simulated in this study, precipitation extremes range from a high of nearly 39
inches in 2007 to a low of just over 17 inches in 2002. The period from 2000 to 2010 witnessed
one of the driest periods on record in east-central Nebraska, with below average precipitation in
each of the first five years of the decade, followed by four of the last six years of the decade with
above average precipitation. Temperatures average 49.8° F in the model study area, with an
average annual high of 61.1° F and an average low of 38.6° F (www.usclimatedata.com). Pan
evaporation is approximately 65 inches annually in the Grand Island area, which with a cited
conversion to lake evaporation rate for larger bodies of water of 0.7, equates to about 46 inches
annually of lake evaporation (Penman, 1948, Gutentag and others, 1984).
2.3 Geology/Hydrostratigraphy
Movement of groundwater occurs through the pore spaces between unconsolidated (non-
cemented) grains of clay, silt, sand and gravel. Grand Island is underlain by Quaternary-age
alluvial sands and gravels deposited by the braided Platte River as it shifted across what is now
Hall County during recent geologic time. The Quaternary deposits range in thickness from 70-
175 feet across the study area, with a saturated thickness ranging from 50 to 150 feet in
thickness depending on location. The Quaternary sand and gravel deposits that form the
primary aquifer under the Grand Island area overlie the Pleistocene-age Seward Formation.
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This formation, composed primarily of fine-grained silts and clays, forms the lower base of the
aquifer, as its permeability is several orders of magnitude lower than the overlying sand and
gravel deposits (UNL-CSD, 1998). A 10 to 20 ft thick silty clay bed is found under the northwest
part of Grand Island at a depth of around 60 ft bgl (Dreezen, 1999).
The Ogallala Group is not mapped under Grand Island and most of northeast Hall County,
although the formation is encountered both west and north of the study area boundary (Cannia
and others, 2006).
Hydraulic conductivity, a numeric representation of aquifer permeability, varies across the site
but generally is high enough to allow for development of highly productive wells for irrigation,
municipal and industrial use. Various data sources from studies conducted over the last several
decades provide a basis of magnitude and spatial variability of the Quaternary deposits in east-
central Nebraska's Platte Valley. A U.S. Bureau of Reclamation study of the Platte River study
(Johnson, 1970) has mapped hydraulic conductivities (converted from transmissivity) ranging
from 174 to 468 feet per day (ft/d) across the study area. The Platte River Cooperative
Hydrology Study indicates a range of hydraulic conductivity from 50 to greater than 125 ft/d for
that study's "Hydrodstratigraphic Unit 2" which is comprised of Quaternary sand and gravel
(Cannia and others, 2006). The Central Platte Natural Resources District (CPNRD) provided
Olsson a summary of aquifer (pump) tests conducted by various federal, state, and local entities
within its boundaries over the last two decades. Within the vicinity of Grand Island, aquifer tests
yield hydraulic conductivities ranging from 133 to 373 ft/d. These results provide a basis for
suitable and acceptable ranges of simulated hydraulic conductivities in the groundwater model.
The specific yield of an aquifer is an expression of the volume of drainable water per volume of
total aquifer material. Data for this aquifer characteristic is more limited than for hydraulic
conductivity. Near Grand Island, specific yield ranges from 0.049 to 0.19. The test location
closest to the city was calculated at 0.165. Further downstream in the Platte Valley, an aquifer
test yielded a specific yield of 0.11. These are all typical values for alluvial aquifer material
(Fetter, 1994), and overall the parameter will show a much smaller range of possible values
than that of hydraulic conductivity. The range of 0.05 to 0.2 was used as a basis for values to
utilize in calibration of the transient groundwater model.
The water table depth ranges from less than 10 feet to greater than 30 feet depending on
location within the study area and the prevailing climate conditions. Because of the relatively
shallow water table conditions combined with the coarse deposits in the shallow subsurface,
groundwater levels respond closely to precipitation patterns. Figure 2.3-1 shows a comparison
of the water table at USGS monitoring well no. 405318098252202 (near the western city limits
of Grand Island) and annual precipitation. As shown in this hydrograph, the water table trends
in an analogous pattern to precipitation. Deviations between the two plots are most likely
attributed to operations of nearby wells. The changes in water levels observed in monitoring
wells around the site provided guidance in assigning recharge rates in the model during the
transient simulation.
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Figure 2.3-1. - Groundwater levels vs. annual precipitation at USGS monitoring well 405318098252202.
2.4 Hydrology
2.4.1 Surface
The Platte River flows in a northeasterly direction within the model study area. The braided
river system contains at least four channels, with shorter interconnecting braids between the
main channels in the Grand Island area. Average daily flow at the USGS streamflow gage
(06770500) near the US Highway 34 bridge southeast of the city is 1,877 cubic feet per second
(cfs). The river has been considered to be either neutral in exchange of water with the aquifer
or losing to the aquifer near the City. The river/aquifer relationship can vary seasonally
depending on climatic conditions both locally and within the overall river basin.
Wood River parallels the Platte River and crosses the model study area on the south side of the
City. Wood River is not considered perennial in the Grand Island area, although it does carry
discharge from the Grand Island municipal wastewater system on the east side of the city.
However, hydrographs from streamflow gage observations near the west boundary of the model
study area do not indicate perennial flow historically, although runoff from summer precipitation
events and irrigation runoff are quite common. Figure 2.4-1 shows a sample of these flow
patterns for a 10-year period from 1984-1994.
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Figure 2.4-1 - Flow rates in Wood River near west boundary of the model domain from 1984-1994.
North of the city, three drainages, Silver Creek, Prairie Creek and Moores Creek cross the
model study area. USGS topographic maps indicate ephemeral conditions in these creeks, but
conditions do occur during wet periods when these streams will carry perennial flow (personal
comm. with D. Woodward, CPNRD, Nov. 13, 2011). East of Grand Island, the ephemeral
flowing Warm Slough drains eastward and carries excess runoff from the city.
Over 50 small ponds and lakes, many of which were former gravel pits, are found across the
southern half of the city. Water levels in these lakes are a surface expression of the water table
and are in direct connection with the shallow aquifer underlying Grand Island. Although the
largest lake is approximately 75 acres in size and not a significant body of water, collectively,
these surface water bodies cover over 600 acres are capable of evaporating upwards of 2,360
acre-feet annually at an annual lake evaporation rate of 46 in/yr.
2.4.2 Groundwater Flow System
Groundwater movement in the alluvial aquifer underlying the Grand Island area is generally
southwest to northeast at a gradient of 0.0013 and roughly parallels the Platte River. The
aquifer is considered unconfined, in that the water table freely interacts with and responds
atmospheric pressure. Although the presence of a fine-grained silt/clay interval is present within
the aquifer under the northwest part of Grand Island, its influence on creating local semi-
confined or confined conditions is unknown. Figure 2.4-2 shows the UNL-CSD 1995 regional
water table contours across the model study area. This map shows the general orientation of
the flow regime, however, it does not highlight local gradients that arise from pumping of the
0
200
400
600
800
1000
1200
9/30/19841/31/19855/31/19859/30/19851/31/19865/31/19869/30/19861/31/19875/31/19879/30/19871/31/19885/31/19889/30/19881/31/19895/31/19899/30/19891/31/19905/31/19909/30/19901/31/19915/31/19919/30/19911/31/19925/31/19929/30/19921/31/19935/31/19939/30/19931/31/19945/31/19949/30/1994Daily Flow (CFS)Wood River near Alda, NE (USGS Gage 677200) Daily Flow
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municipal well fields in Grand Island. Historically, depressions in the local water table have
occurred as early as the 1930s, when Wenzel (1940) mapped over 20 feet of drawdown near
municipal well locations. With the re-location of a majority of Grand Island's municipal wells
south of the city, and the typically rapid flux of recharge to the water table, these depressed
conditions have since recovered.
Recharge to the aquifer is primarily derived from precipitation, seepage from surface water
sources, and irrigation (both agricultural and urban) return flow. As displayed in figure 2, the
typically shallow water table responds rapidly to precipitation events, which would be expected
considering the coarse surface soils and materials found in the unsaturated zones. The quantity
of recharge reaching the water table varies by land use and other topographic characteristics.
Because of the relatively flat landscape in the Grand Island area, land use is the dominant factor
that dictates the downward movement of water beyond the root zone. Szilyagi and others
(2005) estimated a "long term mean annual recharge" range in eastern Hall County of 1.9 to 2.4
inches per year based on a statewide water balance model. These rates equate to 7.5 to 9.4
percent of annual precipitation in the area. Dugan and Zelt (2000) applied a soil moisture
balance model of the Northern High Plains and estimated recharge rates on irrigated lands in
east-central Nebraska. Depending on soil type, recharge potential on irrigated land for row
crops ranged from 4 to 5 inches annually in eastern Hall County, which equates to 15.7 to 20
percent of annual precipitation at Grand Island. The Platte River Cooperative Hydrology Study
Figure 2.4-2 - UNL-CSD 1995
regional water table contours across
the model study area. Orange outline
delineates the city of Grand Island.
Contour interval = 10 ft.
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recharge application for a simulation period of 1950-1997 averaged a recharge rate on irrigated
land of over 6.5 inches or 27 percent of annual precipitation (Peterson, 2007). Recharge rates
from these models assume irrigated areas are subject to excess application of water that
eventually moves beyond the root zone (deep percolation), or runoff of excess applications
accumulate in low areas (ditches) and provide concentrated areas of aquifer recharge.
Seepage from the Platte River and to a lesser extent Wood River can also contribute water to
the subsurface. Historical maps of the water table from the 1940s through the 1970s indicated
the Platte River being in a neutral state with the aquifer with the river neither gaining or losing.
However, the 1979 water table map produced by the UNL CSD (1980) indicates potential losing
conditions in the river at that time. This is further substantiated by a USGS investigation
(Stanton, 1999) that indicates potential losing conditions southwest of Grand Island and near
neutral conditions just south of the city.
3.0 MODEL DEVELOPMENT
3.1 Model Code and Applications
Olsson utilized an array of groundwater model codes and tools to evaluate appropriate wellfield
designs with the goal of reducing high water table conditions in Grand Island's two areas of
concern. The groundwater model was constructed using MODFLOW_2000 (MF2K), one of the
industry's most commonly used groundwater modeling codes. MF2K simulates groundwater
flow by dividing the flow regime into a grid where water levels and flows are computed for each
individual block within the grid. Groundwater Vistas (version 6.14, Build 20) was used as a
graphical interface platform to construct the model grid, enter and modify data inputs, execute
and evaluate calibration, and run predictive analyses.
Steady state model calibration was performed using the inverse parameter estimation code
PEST (Doherty, 1994). Calibration of the transient model was done by forward (trial-and-error)
adjustment of the specific yield, recharge, and estimated pumping rates for area wells
(domestic, industrial, municipal) in the model area.
The capture zone analysis, which determines the spatial area of the aquifer contributing flow to
a pumping well over specified periods of time, was completed using the particle tracking code
MODPATH (Pollock, 1989). This analysis was performed to determine the areas of contribution
to each new dewatering well and to assess potential impacts of pumping on established
contaminant plumes in the south area of concern. Finally, the three-dimensional solute
transport code MT3DS (Zheng and Wang, 1999) was utilized to assess the potential impacts of
south area dewatering wells on two hypothetical plumes in locations where two mapped
contaminant plumes persist in the aquifer. Figure 3.1-1 shows a flowchart that summarizes the
modeling process step-by-step with each individual method and goal.
3.2 Model Structure
The overall rectangular model grid used in this study covers over 263 square miles (Figure 3.2-
2). However, the active model area covers approximately 169 square miles. The discrepancy
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Figure 3.1-1 - Flowchart of model development and application process.
between these two sizes is a result of applying a traditional grid that is rectangular to an active
model domain that is oriented along the axes of the Platte River valley, which angles from the
southwest to the northeast in the Grand Island area. This approach was determined the most
useful in terms of project time as well as representing the flow regime in relation to the layout of
the city and demonstrating results in maps. The active grid area has lengths of 13 miles in both
the southwest- northeast and southeast-northwest directions, originating at UTM coordinates x
= 2048503.9 and y = 350533.32. The grid is discretized into 162 rows and 224 columns with a
cell-size arrangement that telescopes inward toward the city proper where model cells are 250 x
250 ft (1.4 acres). The grid contains a single layer, which represents the entire thickness of
Quaternary materials from the land surface downward to the top of the silt/clay deposits of the
Steady‐State Flow Model
METHOD: MF2K/GWV, PEST
GOAL: Estimate aquifer permeability, establish initial
water table conditions representing the late 1990s,
set initial conditions for transient period.
Transient Flow Model
METHOD: MF2K/GWV, forward calibration
GOAL: Development of a model that simulates
changing water levels witnessed from 1999‐2011,
and is capable of accurately determining water level
changes resulting from new dewatering wells.
Design & Test Wellfield Configurations
METHOD: MF2K/GWV, calibrated transient model
testing well configurations vs. a 15 ft. bgl elevation.
GOAL: Use of transient model to determine number
of wells, appropriate pumping rates and times to
reduce water table below a critical depth in the areas
of concern underlying Grand Island. Assess timing of
water table recovery with wells idle.
MT3DS Test
METHOD: MF2K/GWV, MT3DS
GOAL: Test influence of new wells in south area of
concern on hypothetical contaminate plumes in
areas where mapped plumes exist under Grand
Island.
Capture Zone Analysis
METHOD: MF2K/GWV, MODPATH
GOAL: Determine spatial area of influence of each
new dewatering well over 12 year period, ensure no
interference with contaminant plumes by capture
zones in south area of concern.
Final Product
Configuration of dewatering wells in the north and
south areas of concern, showing necessary rates and
pumping durations to reduce water table elevations
below a 15‐ft bgl "critical surface", with reasonable
confidence that minimal influence on contaminant
plumes will occur in south area of concern.
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Seward Formation. Although some vertical heterogeneity likely exists in the model domain,
particularly near the clay/silt lens in the northwest part of the city, it was decided that not enough
laterally continuous deposits of fine materials are present to warrant multiple grid layers. It was
assumed that lateral heterogeneity in the flow regime would dominate in regards to the
groundwater flow regime, and that calibration of horizontal hydraulic conductivity would suffice
in the reaching a solution of simulated flow conditions.
Figure 3.2-2. - Study area model grid for the Grand Island area. The developed area for the city of Grand Island
in relation to the telescoped grid is delineated by the yellow outline
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The top of the grid, which represents the land surface, is based on interpolation to the grid of a
highly dense set of GIS points with elevation data from recent LIDAR (Light Detection and
Ranging) studies of land surface elevations in eastern Nebraska. LiDAR land surface data is
highly detailed and refined, which is advantageous for this study when considering the margin of
error needed when modeling water table elevations in relation to basement depths. The bottom
grid surface elevations were set by interpolating GIS point elevation data that represent the
base of the coarse and gravel deposits in the area depicted in the UNL-CSD borehole data
(Dreezen, 1999).
3.3 Boundary Conditions
The governing equations that represent groundwater flow require definition of boundaries for the
calculation of water levels to reach a sound solution. Typically in groundwater models, surface
water features (if in connection with the aquifer) or geologic structures/materials that inhibit
groundwater flow in the flow regime are used as boundaries. When such conditions are not
available, water table elevations and/or groundwater fluxes can be used to set fix water level
("head") or flow ("flux") boundaries. Within the model study area, the west and east boundaries
are set as fixed (constant) head boundaries. Water level conditions at these boundaries remain
constant during the entirety of stress periods, but change between stress periods. This feature
of allowing these boundaries to change between stress periods, or "transient constant heads," is
an important feature when simulating the drought that persisted from 2001-2004, a time when
water levels declined by several feet in the Grand Island area.
The north model border is specified as a "no-flow" boundary, a condition where no flow crosses
perpendicular to an assumed flow line that is oriented in the aquifer from west to east. Since
the aquifer is unconfined, the water table is considered the upper model boundary, and the base
of the Quaternary alluvium is the base of the model domain.
The south model boundary is set as the Platte River with the MF2K Streamflow Routing (SFR)
Package. This package allows for explicit simulation of flows observed in the river, and routes
the volume of water through cells that are defined as streams. Movement of water across the
streambed is determined by the difference in river stage and the adjacent water levels in the
aquifer. This exchange of water is controlled by the permeability of the streambed. Streambed
permeability was assigned to the SFR package as a vertical hydraulic conductivity, with a value
of 139 ft/d based on the average of 21 streambed permeameter tests conducted by UNL
researchers (Chen, 2005). Wood River, an ephemeral stream as it enters the model area, was
also simulated with the SFR package. Flows do occur at times in the river, and the SFR
package simulates each of these flows as they happen in time and space in the model domain.
Four north-northeast oriented drainages that typically have ephemeral flow were simulated with
the MF2K Drain Package. The Drain Package allows for remove of water from the aquifer when
the water table elevation exceeds the base of the drain elevation in each cell. Water is not
added to the aquifer from this boundary, however. Figure 3.3.1 shows the active model grid
domain with all of the major boundary conditions and the outline of the developed portion of the
Grand Island for location reference. Note that significant portions of the model grid are inactive
(black) as a method to fix the orientation of the flow regime without tilting the grid.
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Inflow to the aquifer from precipitation is represented through MF2K's Recharge Package.
Rates were assigned initially as the average rates for four land use types, including irrigated and
non-irrigated agricultural land, urban area (mixed paved/lawn), and open urban areas (parks,
large fields). Initial values were assigned based on recharge rates estimated by Szilagyi (2005).
It was assumed that irrigated agricultural lands would receive the most recharge, and would be
at the high end of the range or potential recharge from this study, 2.4 inches, which is about 9
percent of annual precipitation in the Grand Island area. The other three land uses applied
slightly less initial recharge, as it was assumed areas with more chance for runoff (paved areas,
high roof density) would receive less recharge to the aquifer. Urban open areas and non-
irrigated agricultural land were each assigned initial (pre-calibration) rates of 7.5 and 6 percent
of annual precipitation respectively, with urban areas receiving 3 percent of annual precipitation
at the onset of model calibration. These initial base rates were assumed to deviate during the
transient model period of 1999 to 2011, since precipitation patterns fluctuated greatly with some
of the lowest annual precipitation rates ever recorded as well as near-record years in
precipitation. Adjustment of precipitation-based recharge on a stress-period by stress-period
played a key role in calibrating the transient model, especially in the early 2000s during the
historic drought period afflicting the region.
Evapotranspiration (ET) from the saturated zone via deep plant roots was simulated via through
the MF2K Evapotranspiration Package. Land types that were delineated into unique zones for
ET include agricultural land, urban land, riparian zones and open water. Rates applied for each
of these zones were based on the 1950-98 COHYST crop irrigation requirement (CIR) rates.
Extinction depths, which are the level above which water is extracted through plant roots from
the water table, were set at 3 and 7 feet in non-riparian and riparian zones respectively. It was
assumed that higher water table conditions in the riparian zones near the Platte River likely
have higher water table with vegetative roots that reach deeper than the other land use areas in
the model domain. The numerous small gravel pit lakes/ponds were also simulated with the
Evapotranspiration Package. Model cells containing these lakes were assigned as ET cells with
a deeper extinction depth (10 ft) and an evapotranspiration rate equivalent to a daily rate
required to evaporate the annual mapped lake evaporation rate for the Grand Island area (46
in/yr).
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.
4.0 STEADY-STATE MODEL
The groundwater flow system underlying Grand Island was first simulated to steady-state
conditions to establish simulated water levels that represent development period conditions
witnessed in the 1990s after development and long-term utilization of both irrigation wells and
municipal wells had occurred. The last half of the 1990s showed groundwater levels around 5
to 10 feet above minimum levels observed in the 2000-2005 timeframe. Although the higher
Wood River
Platte River
Prairie
Creek
Silver Creek
Moores Creek
Warm
Slough
N
Figure 3.3-1. - Configuration of active grid area and major model boundaries. Names of individual surface water features
shown near location within grid. The outline of the developed portion of Grand Island is shown in black for
location reference
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water levels in the 1990s are likely attributed to several years of above average precipitation
(especially 1993 and 1998) and a corresponding decrease in irrigation and municipal pumpage,
the resulting water levels likely represented conditions closer to pre-development conditions and
are thus considered appropriate for target levels in the steady state calibration. This process
involved calibration of hydraulic conductivity using the parameter estimation code PEST
(Dougherty, 1994), with the goal of attaining simulated water levels for use as initial conditions
in the transient model. Groundwater levels were calibration to 37 individual observation well
locations across the model domain that represent water table conditions in the late 1990s when
water levels were considered to be at levels close to predevelopment conditions. These wells
include those maintained and monitored by the City of Grand Island, the US Geological Survey,
and the Central Platte NRD. Locations (and observed vs. simulated residuals) of the
observation locations are shown in figure 4.2-2.
4.1 Parameter Inputs
Both fixed and adjusted parameters were applied in the steady-state model. Hydraulic
conductivity, a characteristic of the aquifer materials that dictates both rate of travel in the
aquifer and groundwater levels, was initially set (but allowed to change during calibration) at an
initial value of 200 ft/d across the model grid. This value was set as it represented typical
average hydraulic conductivities observed in Platte River alluvium. Calibration of this parameter
is discussed in the following section. Recharge was fixed at the base rates for the four recharge
zones described in section 3.3. Vertical hydraulic conductivity terms in the MF2K Stream
Package was fixed at 140 and 10 ft/d, respectively, for the Platte River and Wood River. The
model interface automatically calculated conductance terms from these values with
incorporation of each model cell's length and width. Drain conductance in the MF2K Drain
Package for the four northeast sloping ephemeral drainages in the model domain was set at 1
ft/d. These values were assigned with the assumption that fine-grained material and
decomposed organics are likely present in the beds of these drainages. Elevations for each of
these surface features were obtained from contours in the USGS Grand Island 7.5' quadrangle
map as well as elevations from Google Earth. Groundwater pumpage was simulated during the
steady state period by using well pumping capacity records from the NDNR and associated crop
irrigation requirements (for irrigation wells) as well as City of Grand Island pumping records for
municipal wells.
4.2 Calibration
The steady state model was calibrated by using a fixed recharge input value and estimating
hydraulic conductivity with the automated PEST code as described previously in this report.
PEST estimates parameters by continual adjustment through a series of model runs with the
goal of minimizing an objective function. The objective function expresses the amount of error
between all of the observations from the real world system and their simulated equivalents in
the model. A unique feature within PEST is the option to use "pilot points" in estimating
parameters. Pilot points are essentially "anchors" where the parameter value is defined in the
model domain and allowed to adjust during the calibration process. During each calibration
iteration, the parameter values at each pilot point are re-interpolated to the grid before the next
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run. This approach essentially makes each pilot point a parameter. The advantage this
approach has over traditional application of assumed geologic zones is that smoother
transitions of geologic characteristics, and thus parameter values, are more easily obtained if an
appropriate number and placement of pilot points are applied. This approach is considered
superior to arbitrarily drawn zone boundaries, since changes in fluvial-deposited settings are
more likely gradational than abrupt, and it is often difficult to defend the placement of a zone
boundary where little is known of the subsurface characteristics. The process of parameter
adjustment adheres to what the observations indicate the parameters should be instead of pre-
defined estimates by modelers of what the parameters should be (Environmental Systems, Inc,
2007). For calibration of hydraulic conductivity, 589 pilot points were used in the steady-state
calibration process, and were evenly spaced every 0.5 miles using an automated feature in the
graphical user interface. This density of points was considered appropriate for the model area
to capture variations in hydraulic conductivity yet allow model computation to complete in a
reasonable timeframe.
It should be noted that in groundwater systems without flow observations, high correlation can
exist between hydraulic conductivity and recharge parameters. Highly correlated parameters
can lead to non-unique models, i.e. models where more than one set of parameter value
combinations could lead to the same model solution. Because of a lack of consistent data
between two gages on the Platte River not allowing for a true estimate of actual river flow
gains/losses, a reliable flow observation was not attainable in the model domain. The same
conditions exist for Wood River as well since non-perennial conditions typically persist in that
drainage. To bypass this potential problem, OA decided to fix the steady state recharge rate and
focus exclusively on the hydraulic conductivity. This was decided after initial trial-and-error tests
of these parameters indicated that the hydraulic conductivity values and distribution have much
greater influence on the distribution of heads than the range of recharge values expected for
pre-irrigation conditions in east-central Nebraska. It should be noted however, that the
COHYST project completed an estimate of pre-development gains and losses in numerous
Nebraska rivers and tributary streams in the Platte and Republican River basins, including the
reach from Odessa, NE to Grand Island (Peterson and Carney, 2002). This report included a
normalized mean estimate of exchange of water between the aquifer and the Platte River of -3
cfs per mile (losing conditions). Across 13 miles of river simulated in the model, this equates to
a loss of about 40 cfs from the Platte River. With the fixed conductance and flow terms
assigned to the SFR Package, the model simulated a loss along the Platte River in the model of
about 41 cfs. Although not formally used as an observation, this resulting seepage provides
greater confidence in the steady state simulation results even with fixed input parameters and
observed flows in the Platte River. The resulting hydraulic conductivity field from the final,
calibrated steady state model, along with the pilot point locations used in the calibration
process, are shown in figure 4.2-1.
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Figure 4.2-1. - Hydraulic conductivity field in steady state model resulting from calibration with PEST (pilot point
locations show by cross marks) .
4.3 Steady State Results
Figure 4.3-1 shows final simulated water level contours and the differences (residuals) of
simulated and observed heads in the calibrated steady state model. The residuals tend to have
no dominant pattern, although the simulated water levels trend slightly above observed levels
near the center of Grand Island, and an east to west line of points north of the Platte River show
N
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the largest absolute value of residuals (0.16 to 0.62 ft) over the entire grid area. A plot of
simulated residuals versus observations (fig. 4.3-2) allows for a graphical demonstration of the
randomness or residuals across the model domain, which indicates if bias is present in the
simulation. Ideally, an even distribution of points around the zero line should exist. Values in
this plot show a reasonable scatter of points, indicating that little to no bias in the hydraulic
conductivity parameter field exists and that an evenly distributed, constant recharge rate is
acceptable for the steady state simulation. Another demonstration of the quality of calibration is
a basic simulated vs. observed water level plot (fig. 4.3-3). Ideally, a perfect model would place
N
Figure 4.3-1. - Observed vs. simulated groundwater level residuals for calibrated the steady state model at 37
observation locations. Negative values (red) indicate simulated water levels greater than observed,
positive values (blue) indicate simulated water levels below observed levels.
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all the points on the 1:1 line. The points plot relatively evenly on the 1:1 line and does not reflect
bias in any direction.
Figure 4.3-2 - Observed water levels vs. simulated vs. observed water level residuals.
Figure 4.3-3 - Simulated vs. observed water levels for the calibrated steady state model.
Figure 4.3.4 shows the simulated water table contours plotted across the model domain at a 10
ft interval. Accompanying the simulated contours are the 1995 regional water table contours as
defined by UNL-CSD. The contours match closely with the exception of the east part of Grand
Island, where the simulated 1830 and 1840 ft contours show a depressed water table in the
vicinity of where most of the city's public water supply extraction wells operated prior to the
2000s. Another area of deviation between the two contour sets is at the Platte River where the
simulated contours deviate eastward are controlled by the simulated river stage and adjacent
-1.0
0.0
1.0
1821.0 1833.8 1846.6 1859.4 1872.2 1885.0Residual
Observed Value
1820.8
1833.6
1846.5
1859.4
1872.2
1885.1
1820.8 1833.6 1846.5 1859.4 1872.2 1885Model ValueObserved Value
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groundwater levels. The resolution of the 1995 regional water table contours, which cross the
entire state, was not detailed enough to capture this local area of water table depression or
conditions immediately adjacent to the Platte River.
Table 4.3-1 shows the final model calibration statistics, including the objective function produced
from PEST. The total for the objective function, which is the sum of the squared residuals at
each of the observation points, is 1.34. The steady state model budget is shown in Table 4.3-2.
N
Figure 4.3-4. - Simulated steady state water table contours (blue contours). The 1995 mapped regional
water table contours from UNL-CSD are shown in thin black lines.
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Table 4.3-1 Final Statistics from Calibrated Steady State Groundwater Model.
Table 4.3-2 Calibrated Steady-State Groundwater Model MF2K Budget.
5.0 TRANSIENT MODEL
5.1 Transient Model Development
The transient model for the study area utilized the steady state model as a base framework for
modification of inputs needed to simulate conditions that change over time. The transient
Steady State Calibration Statistics
Residual Mean 0.05
Residual Standard Deviation 0.18
Absolute Residual Mean 0.13
Residual Sum of Squares 1.34
Minimum Residual -0.4
Maximum Residual 0.62
Range of Residuals 1.02
In (ft3/d) Percent of In/Out
Component Total
Storage 0 0
Constant Head 1,400,393 18.8
Wells 0 0
Drains 0 0
Evapotranspiration 0 0
Stream Leakage 4,964,665 14.4
Recharge 1,074,494 66.7
TOTAL IN 7,439,521
Out (ft3/d)
Storage 0 0
Constant Head 1,235,975 16.6
Wells 1,779,309 23.9
Drains 20,718 0.29
Evapotranspiration 2,921,597 39.3
Stream Leakage 1,481,551 0
Recharge 0 19.9
TOTAL OUT 7,439,151
IN - OUT 370.5
PERCENT DISCREPANCY 0.0
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version of the model was developed to 1) calibrate the specific yield and recharge to
demonstrate that the model can reasonably track changes in the water table conditions which
change over time and 2) conduct the various engineering assessments for designing wellfields
intended to provide long-term reduction of the high water table conditions under the City. The
transient model simulates the period from May 1999 through to April 30, 2011, a period that
includes one of the most severe High Plains droughts on record from 2001-2004, as well as
several years with above average precipitation. This period was considered ideal because of its
relatively recent occurrence and the abundance of associated groundwater level records
available. As is the case in many locations worldwide, data gaps and lack of records become
more prevalent the further back in time information is sought from. Along with the abundance of
observation data, the time period was considered idea to calibrate transient recharge conditions
and specific yield. The 12 year transient period was simulated with 72 stress periods and based
approximately on an agricultural irrigation schedule that is common for central Nebraska.
Individual stress periods were defined for the months of May through September each year, and
one stress period representing the non-irrigation season period from October through April (212
days). Pumping rates for municipal, industrial and irrigation wells were adjusted during this
timeframe.
5.1.1 Calibration Targets
The transient model was calibrated to water level changes observed at select wells that
contained records over the entire transient period. Calibrating to water level changes in
transient models is common industry practice, as one of the primary goals of transient
calibration is to ensure that the model is capable of replicating temporal changes in the water
table elevation. This approach reduces the focus on the tedious calibration to water levels
measured at a point in time, which in many cases will have an offset between simulated and
observed conditions. For each observation location, a base water level datum was selected in
the spring of 1999, and water level changes were calculated from the elevation observed at this
point in time. Considering the precipitation patterns and water table elevations in the late 1990s,
this point in time was considered an appropriate datum to base water level changes on as the
water table in the Grand Island was relatively high in comparison to its overall period of record.
5.1.2 Specific Yield
Specific yield is the ratio of the volume of water drainable by the influence of gravity to the entire
volume of saturated material. This parameter is applied to transient models only, since water
levels do not change over time in steady state simulations. Specific yield plays an important
role in calibration of water levels that change over time, and in particular dictates the rate of
drawdown and recovery (as demonstrated by the slope in drawdown/recovery curves). During
calibration of the transient model, a specific yield value of 0.13 was ultimately arrived at through
a trial-and-error process and was fixed for the engineering analyses performed with the model.
The High Plains Regional Aquifer Systems Analysis (Gutentag, 1984) reported an average
specific yield in the High Plains aquifer of 0.15, with the area near Grand Island in the 0.10-0.20
range. As previously mentioned, aquifer tests reported by the CPNRD in the Grand Island area
also reported specific yield values of 0.11 to 0.16.
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5.1.3 Pumpage and Recharge Inputs
Along with adjustment of recharge and specific yield, groundwater pumpage from industrial,
municipal and irrigation wells was also modified during the transient timeframe. Because the
city covers a substantial portion of the model study area, particular focus was placed on
adjusting pumping rates within the city limits where many of the transient water level
observations are located. The city of Grand Island provided Olsson pumpage volumes by
month for many of its municipal wells. For these wells with available data, total volumes were
converted to daily rates and only required slight modification during calibration. For other
municipal wells and industrial wells without records, initial rates were set at the registered
pumping rate listed in the NDNR registered well database for each well, pumping for full 24 hour
periods. Initial transient runs indicated excessive simulated drawdown, and subsequent runs
incorporated pumping rates calculated with a shorter assumed pumping duration each day (6-12
hours). The city of Grand Island also provided pumping volumes for dewatering wells located in
both the northwest and south areas of concern.
Pumpage rates assigned to area irrigation wells was based on a rate determined by the net
irrigation requirement (crop irrigation requirement less growing season precipitation) for corn
and soybeans (based on 2005 CALMIT land use map). The volume of water required for each
well was simulated with an extraction rate that pumped this total volume over the entire growing
season (May-September). This approach approximates the total volume needed over a growing
season, but likely deviates from actual pumping patterns as irrigators likely turn wells on and off
throughout the growing season depending on crop growth stages and precipitation patterns.
Simulated wells and transient water level change observation points are displayed in figure 5.1-
1. It should be noted that in creating the transient pumpage dataset, all wells that were
registered with the NDNR at a rate of less than 50 gallons per minute (gpm) were excluded from
the simulated wells. These wells were typically domestic or low-capacity commercial use wells.
Recharge in the transient simulation was adjusted with multipliers of the base recharge rates
applied in the steady state model (Section 3.3). Recharge ranged from 1.5 percent of the base
recharge rates during the worst periods of the drought (2002-04) to a multiplier of 3 times the
base rate in 2008 when annual precipitation was about 60 percent above the average amount of
25.5 in/yr.
5.2 Transient Model Results
Matches of simulated versus observed water level changes are displayed for four monitoring
well locations in Figure 5.2.-1. The wells displayed are from different locations (west, central,
and east) across the city of Grand Island to show the spatial variability in how water levels
changed across the city during the transient model period. Appendix A displays the remaining
hydrographs for the wells in the transient model calibration (all of which have incomplete
observation periods of record). Because of the intended use of the model (dewatering),
calibration effort focused on ensuring that the simulation is capable of attaining the maximum
water level increases or decreases observed at the monitoring wells, and that the simulated
changes trends mimic the observed changes. From the hydrographs displayed in figure 5.2.-1,
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the model is capable of reaching these maximum and minimum water level changes observed
across the model area, although at two locations, the simulated changes recover at a slower
rate than the observed data. This condition could be due to excess simulated pumpage in
nearby wells where simulated pumpage rates are estimated using limited information. Further
calibration of individual pumping rates could bring these matches closer, but for the intended
uses of the model, this was considered unnecessary with the fac that the model appears to
satisfactorily mimic the large swing in water levels witnessed between 1999 and 2011.
G‐100378A
(CPNRD)
4053‐5202 (USGS)
4056‐0401 (USGS)
G‐100377
(CPNRD)
G‐100378D
(CPNRD)
4054‐3001
(USGS)
G‐104419
(CPNRD)
Figure 5.1-1. - Locations of wells (circles) and water-level change observations simulated in the 1999-2011
transient period. Monitoring wells are labeled by USGS or NDNR registration number.
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Tables 5.2-1 and 5.2-2 list the model calibration statistics and final cumulative model budget,
respectively. The mean absolute residual between simulated and observed water level changes
is 1.8 feet across the monitoring well's 5,163 water level change observations between 1999
and 2011. The mean water level change residual is -0.87, indicating that the simulated water
level changes are slight greater than the observed changes on average, by slightly less than
one foot. The maximum and minimum residuals were 6.7 and -9.3 feet respectively. These
residuals likely originate from monitoring well 100378-D (Appendix A), a well that appears to
show rapid responses to pumping that the model could not account for, likely due to either a
well near the observation location not simulated or from semi-confined conditions in the area of
the wells.
The cumulative model budget for the transient model reveals that inflow of water from the Platte
River is a substantial component of flow in the groundwater system in the Grand Island area,
nearly three times the volume of precipitation based recharge. This condition would be expected
considering that Grand Island has been pumping from a municipal wellfield adjacent to the river
over much of the transient model time period and from this stress induces seepage from the
river channels into the aquifer. Pumpage from wells and evapotranspiration account for nearly
‐4
‐3
‐2
‐1
0
1
2
3
4
5
6
7
8
3464084595005567467958388869931,1281,1681,2191,2591,4611,5041,5541,5991,6561,8461,8931,9401,9882,1552,2302,2812,3252,3742,5672,6152,6592,7102,8002,9542,9983,0483,0913,2923,3393,3863,4353,5093,6763,7233,7763,8174,0194,0634,1144,1614,227Water Level Changes (ft)Time
USGS Well 4053‐5202 (East Alda)
obs
sim
‐3
‐2
‐1
0
1
2
3
4
5
6
3534284915697698318921,1041,1651,2271,3231,5041,5681,6321,8521,9191,9812,1952,2572,3222,4132,5952,6592,7222,9373,0043,0693,2883,3503,4143,4923,6893,7543,8174,0334,1004,168Water Level Change (ft)Time
USGS Well 4054‐3001
obs
sim
‐4
‐3
‐2
‐1
0
1
2
3
4
5
6
3464264885407528098669661,1341,1921,2511,4611,5221,5811,6381,8521,9111,9702,1122,2382,2992,3572,5672,6242,6872,7612,9573,0183,0783,2903,3503,4113,4703,6763,7373,7983,9514,0694,1314,189Water Level Changes (ft)Time
USGS Well 4056‐0401
obs
sim
‐2.5
‐2
‐1.5
‐1
‐0.5
0
0.5
1
1.5
2
2.5
4575157367978589101,1291,1891,2521,4661,5261,5871,6961,8671,9271,9842,2012,2612,3222,4132,5952,6552,7132,9282,9883,0493,1173,3233,3833,4433,6573,7173,7783,8414,0524,1124,173Water Level Change (ft)Time
G‐104419(CPNRD)
sim
obs
Figure 5.2-1 - Hydrographs of simulated vs. observed water level changes during the transient model period. Note
the x-axis lists time in model units.
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75 percent of the outflow component of the model budget. The percent discrepancy between
the in- and outflow components is 0.04 percent.
Table 5.2-1 Final transient calibration statistics.
Transient Calibration Statistics
Number of Observations 5,163
Residual Mean -0.87
Residual Standard Deviation 2.2
Absolute Residual Mean 1.8
Residual Sum of Squares 28,900
Minimum Residual -9.3
Maximum Residual 6.7
Range of Residuals 15.4
Table 5.2-2 Transient model budget showing cumulative volumes at the final time step of stress period 72. Note that
the units are in acre-feet.
6.0 DEWATERING ANALYSIS
The calibrated transient model was utilized to test various configurations of dewatering wells in
both the northwest area of concern (NWA) and the south area of concern (SA) in Grand Island
In (acre-feet) Percent of In/Out
Component Total
Storage 130,501 15.0
Constant Head 104,084 12.0
Wells 0 0
Drains 0 0
Evapotranspiration 0 0
Stream Leakage 155,690 55.2
Recharge 479,915 17.9
TOTAL IN 870,190
Out (acre-feet)
Storage 116,774 13.4
Constant Head 80,055 9.2
Wells 397,112 45.7
Drains 4,714 0.5
Evapotranspiration 243,902 28.0
Stream Leakage 27,285 3.1
Recharge 0 0
TOTAL OUT 869,842
IN - OUT 348
PERCENT DISCREPANCY 0.04
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(Fig. 6.0-1). The objective of this phase of the study focused on optimizing a configuration of
wells in each area that could achieve long-term reduction of the water table to levels
consistently below residential basements.
Figure 6.0-1: Northwest (NWA) and South (SA) areas of concern for high water table
conditions in Grand Island.
6.1 Methodology
The calibrated transient model was applied to this exercise without altering the time structure of
the model. In many situations where models are used to assess future conditions or test
modifications in system stresses, additional time is added to a model to represent some future
scenario. However, in this case, it was decided that this time-consuming process would be
unnecessary since the anticipated timeframe of dewatering would be on the order of months
instead of years. With this approach, it was determined that in the last two years of the transient
simulation period (May 2009 - May 2011), average to above average precipitation patterns
helped return water table conditions to pre-drought conditions at levels that would be considered
"normal" or possibly "wet" as some hydrographs (fig. 5.2-1) showed water levels recovering
above 1999 observations. The two year window of normal to wet conditions during the transient
period was considered opportune for testing well configurations in areas of concern.
Dewatering the water table to at least 15 feet below the land surface was defined as the target
goal for the wellfield simulations in the NWA and SA. This criteria was set with the assumption
that the base of a typical residential basement in Grand Island did not exceed 10 feet below the
NWA
SA
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land surface. An additional five feet was added to this depth as a conservative precaution with
model error and deviations/inaccuracies in the land surface elevations taken into consideration.
This surface was created by subtracting 15 feet from the LiDAR-based land surface dataset (top
of the model grid) at each model cell. This new "critical level" at 15 feet below the interpolated
land surface in the model was then used to compare water levels simulated with the various
wellfield test designs.
Wellfield design testing involved placement of new wells in the NWA and SA one well at a time
to determine the effectiveness one new could induce on the water table and to determine what
areas required higher density of extraction wells. The simulated water table was compared to
the critical surface after each model run to assess where placement of the next well would be
most effective. This process was repeated until the water table was below the critical surface
across each of the areas of concern. Well capacities were tested with rates of 300 to 500
gallons per minute (gpm).
6.2 Dewatering Results
6.2.1 Northwest Area
A total of 11 wells across the NWA, each pumping at 500 gpm, reduced the water table below
the critical level after about 9 months of pumping (Fig. 6.2-1). It should be noted that a majority
of the area becomes dewatered below the critical level in the 6-7 month timeframe, but a few
isolated zones on the fringes of the NWA remain until the 9 month mark. Figure 6.2-2 shows
the locations of the NWA dewatering wells. It should be noted that many of the lingering areas
above the critical level are very small in magnitude, typically on the order of less than one foot
and with this condition, dewatering below most basements should have already occurred well
before nine months have elapsed.
6.2.2 South Area
The SA required 22 wells to lower the water table below the critical level (Fig. 6.2-3). The
timeframe for dewatering to occur across most of the SA takes about 2-3 months longer than for
the NWA, as initial water table conditions in the SA are typically higher in relation to the critical
level than areas in northwest Grand Island. By the 10 month point, the water table was below
the critical level across the entire SA with the exception of small areas in the northeast corner.
Two contaminant plumes are present in the west (Parkview - managed by US EPA) and
northeast (VCP - managed by the Nebraska Dept. of Environmental Qualtiy) areas of the SA.
Wells that are closer to the plumes (within 0.5 miles) are simulated at 400 gpm and wells
beyond this distance from the plumes are simulated at 500 gpm. The dewatering wells in the
northeast corner of the SA, some of which are situated beyond the SA boundary, were not
placed in the area of remaining critical level exceedence in order to not disturb the contaminant
plume crossing the area. When cleanup operations are complete and contaminant levels fall
below the NDEQ requirements, additional wells could be placed in this area to further lower the
water table below the critical level. The progress of dewatering over time is displayed in Figure
6.2-4.
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Conditions at onset of pumping Conditions at 6.5 months
Conditions at 7 months Conditions at 9 months
Figure 6.2-1- NWA dewatering over 9 month period. Negative values on scale
(right) indicate level (in feet) water table exceeds critical level (value is
negative due to calculation of critical surface - water table).
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Figure 6.2-2 - Locations of 11 dewatering wells in the NWA.
Figure 6.2-3 - Locations of 22 dewatering wells in the SA.
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Conditions at onset of pumping Conditions after 6 months of pumping Conditions after 7 months of pumping Conditions after 10 months of pumping Figure 6.2-4 - SA dewatering over a 10 month period. Negative values on scale (above) indicate level (in feet) water table exceeds critical level (value is negative due to calculation of critical surface - water table). Grand IslandStudy Session - 5/15/2012Page 86 / 124
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6.3 Capture Zone Analysis
Following completion of a preliminary layout of wells across the NWA and SA, a capture zone
analysis was performed to assess the areas of influence (capture zone) for each new
dewatering well over the transient model period. The overall impetus for this test was to ensure
that no capture zones for dewatering wells contain or have particles that terminate in either of
the Parkview or VCP plumes that are within the SA. Additionally, capture zone analysis for the
NWA wells provides an exploration of how hydraulic conductivity heterogeneities vary across
the model domain and influence the characteristics of each well's capture zone and to ensure
no boundary influences occur on the wells in the NWA.
MODPATH (Pollock, 1989) was utilized to perform the capture zone analysis. MODPATH is a
particle tracking code that couples with MF2K and communication between the two model
applications is enhanced by the GUI (Groundwater Vistas) used in this project. The code
assigns "particles" in the model grid cell containing the pumping well and tracks them
backwards in time for the length of the transient period in the model, until the particles either
encounter a model boundary or cease at the end of the 12 year time period. The code uses
velocities and gradients from the transient MF2K output to direct the track of particles over time.
Inputs required from the user include porosity and the elevation within the model cell to assign
the particles that corresponds to elevation of the well screen. A porosity value of 0.3 was
assigned over the entire model domain (Fetter, 1994), and the particles were assigned to an
elevation corresponding to the bottom 40 feet of the cell containing each pumping well.
The 12-year capture zones for the NWA wells vary in length from 0.75 miles to about 1.5 miles
in length and are oriented slightly northeastward (fig. 6.3-1). Capture zones tend to extend
further westward in the northern half of the NWA, which indicative of the higher hydraulic
conductivities in this area of the model domain. No capture zone particles approach the west
model boundary, which indicates that no little to no boundary influences occur during the
dewatering analysis. Particle tracks overlap for several of the wells in the northern half of the
NWA, which indicates that well-to-well interference, which indicates potential for further
drawdown than what a single well can induce can occur.
Figure 6.3-2 displays the capture zones for each of the dewatering wells in the SA. Although
the well locations in this figure display the final locations for dewatering wells in this area, it
should be noted that determination of the final well locations in the SA was a trial-and-error
process that involved testing well capture zones locations in relation to the contaminant plumes
in the SA along with guidance from the OA engineering design group regarding the most
appropriate locations for sighting wells, pipeline, and discharge areas within the city.
Development of the final well configuration in the SA required significantly more time and testing
than the final well field design in the NWA. Like the NWA, the capture zone sizes vary across
the well field area, but unlike the NWA, the orientations of the SA capture zones show more
variability in shape, length and orientation. In addition to variability in aquifer permeability within
the SA boundary, a significantly greater number of high capacity wells operate in this area and
induce local changes in gradient which could influence the orientation of the dewatering well
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capture zones. In general, capture zones for wells on the west side of the SA are longer than
the rest of the well field, with some particle traces approaching 2 miles. The greatest amount of
particle overlap occurs in the area of the west wells, thus indicating that greater potential for
well-to-well interference (and more drawdown) could occur. Capture zones for wells in the
center and northeast side of the SA tend to be shorter in length and have more variability in the
orientation of the particle traces.
Figure 6.3-2 displays the mapped boundaries of the Parkview and VCP plumes. No particle
traces cross or terminate in these plume areas over a 12 year period. Dewatering wells on the
south side of the northeast plume have particles that terminate near the south plume boundary.
Figure 6.3-1 - 12-year capture zones for dewatering wells in the NWA.
Grand Island Study Session - 5/15/2012 Page 88 / 124
Grand Island Dewatering Study DRAFT - Groundwater Model Report
Olsson Project No. 011-2231 Page 34 of 43
It is possible that in the future, contribution of water to these dewatering wells could occur from
within the plume area. However, it is assumed that over time, remediation efforts for this plume
will reduce contaminant concentrations below regulatory limits and thus reduce and eventually
eliminate the question of the capture zone areas for the dewatering wells on the northeast
fringes of the SA.
Figure 6.3-2 - 12-year capture zones for dewatering wells in the SA. Note the location of the contaminant
plume boundaries (magenta lines) on the west and east sides of the SA.
6.4 Transport Model Assessment
A contaminant transport analysis was conducted as a final test of the potential influence that
dewatering wells in the SA could induce on the Parkview and VCP plumes. Although particles
from each dewatering wells capture zones do not cross the mapped plume areas, gradients
induced from pumping can still influence lateral migration of contaminants. The goal of this test
was to use hypothetical plumes located in the areas of the mapped plumes to assess if
latitudinal and/or longitudinal changes occur from dewatering well operations over the time
period tested for dewatering the water table below the critical level in the SA. This exercise was
not an attempt to explicitly recreate the west and east plumes in terms of exact shapes,
concentrations, and historic migration pathway. OA utilized the transport code MT3DS (Zheng
Parkview Plume
VCP Plume
Grand Island Study Session - 5/15/2012 Page 89 / 124
Grand Island Dewatering Study DRAFT - Groundwater Model Report
Olsson Project No. 011-2231 Page 35 of 43
& Wang, 1999) to conduct this analysis. This code interfaces with MF2K under the control of
Groundwater Vistas.
Like MODPATH, MT3DS uses flow velocities and gradient from MF2K to compute contaminant
movement in the saturated portion of the flow system. MT3DS required more parameter inputs
than MODPATH, as well as trial-and-error adjustments in input parameters with the goal of
approximating the shape of the mapped contaminant plumes with hypothetical simulated
plumes. This exercise required adjustment of a parameter called hydrodynamic dispersion, or
simply dispersion. This term describes the movement of water at varying velocities through
pore spaces and the diffusion of solutes within the groundwater and is expressed by:
DL = hydrodynamic dispersion (L indicating longitudinal direction, the same equation applies for both transverse and vertical flow),
unitless
aL = dynamic dispersivity (ft), from equation 0.0175L1.46, with L = flow path length (Neuman, 1990)
vx = average linear groundwater velocity (ft/d)
D* = molecular diffusion
Initial concentrations similar to the those measured in a transect of monitoring wells near where
the Parkview plume enters the SA were set in grid cells along the west border of the SA instead
of the original source of the plume. This approach reduced the amount of calibration time
needed to simulate a test plume and was considered valid since information on concentrations
were available in close proximity to the SA boundary. The initial calculation of dispersion using
estimated inputs for velocity, diffusion, and dispersivity, yielded a value of 6,709. This term was
eventually adjusted with an increase by a factor of about 3.5x to obtain a similar shape and area
as the west plume. Transverse dispersion was set at 0 through trial and error calibration of this
term. Based on the nature of the channel-derived alluvial deposits within the aquifer in this
area, it is expected that longitudinal dispersion would dominate the transport and plume
characteristics, a condition that is further supported by the capture zones for the dewatering
wells in this area of the SA.
The simulated Parkview plume boundary was defined at a concentration of 0.5 µg/l, the same
concentration at the boundary of the mapped plumes. Figure 6.4-1 shows the pattern of
influence the dewatering wells near the west plume could potentially induce in this area. Three
contours are displayed in this figure- the Parkview plume area, the simulated test plume
simulated without insertion of new dewatering wells, and potential plume shape following 1.7
years of dewatering well extraction. In this test scenario, the hypothetical plume (at a
concentration of 0.05 µg/l) widens by about 600 ft on the north and 875 ft on the south side of
the plume. The test plume shortens however, over this time period, which would be expected
by further dispersion and diffusion induced from the pumping wells. Note that this scenario
assumes a constant concentration at the simulated plume source and does not account for
decreasing concentrations from remediation efforts or natural attenuation. It is assumed that
over time, remediation actions in this area will negate the concern over lateral migration of
DL = aL * vx +D* Eqn. 1 (Fetter 1994)
Grand Island Study Session - 5/15/2012 Page 90 / 124
Grand Island Dewatering Study DRAFT - Groundwater Model Report
Olsson Project No. 011-2231 Page 36 of 43
contaminants at levels above NDEQ regulatory limits, and that the SA dewatering wells in this
area will not create long-term disturbance of the Parkview plume to a point of adversely
affecting public water supplies in Grand Island. If lateral dispersion of contaminants persist,
interceptor wells could be used on the north and south sides of the Parkview plume to capture
any laterally migrating contaminant induced by the dewatering wells.
Figure 6.4-2 displays the results of the hypothetical test near the VCP plume. The hypothetical
simulation for this area of the model was similar to that at the Parkview plume, but with an initial
concentration of 1.0 ug/l defined at the plume source. Due to grid cell resolution and lack of
detailed information on locale permeabilities, it was not possible to create a plume shape that
resembled the entire length of the VCP plume (the mapped plume width was considerably
narrower than the smallest cell in the model grid). Because of this condition, an attempt was
made to simulate a hypothetical plume only in the VCP plume source area. The model
indicates that over the dewatering period for the SA, the simulated plume (at a concentration
contour of 0.5 ug/l) shifts southward by approximately 250 feet, but does not elongate or come
within close proximity of public supply wells or other dewatering wells. It should be expected
that the simulated plume in this area would be less influenced by pumping in relation to the
Parkview area considering the greater spacing and few number of dewatering wells in the
northeast extent of the SA. It is possible however that the extremely narrow portion of the
plume not simulated in this test could widen from influence of the wells on the north and south
sides of the VCP plume. Since the currently mapped plume is extremely narrow, even
Figure 6.4-1 - Results of a hypothetical contaminant transport analysis at the Parkview plume in the SA.
Displayed are the mapped Parkview plume extent from the EPA (magenta), the simulated test plume pre-
dewatering well activity (orange), and plume extent following extraction well pumping (blue). Dewatering well
locations shown by yellow triangles.
Grand Island Study Session - 5/15/2012 Page 91 / 124
Grand Island Dewatering Study DRAFT - Groundwater Model Report
Olsson Project No. 011-2231 Page 37 of 43
under the influence of some municipal and industrial wells currently operating in the area, it is
unlikely that the addition of the wells that surround this plume exert significant influence on this
plume. The capture zone shapes indicate that the wells on the south side of the northeast
plume would more likely interfere with the VCP plume in this area as the capture zones for the
north side wells are close to perpendicular and away from the VCP plume.
6.5 Water Table Recovery
A final analysis was conducted to assess the recovery trends of the water table across both the
NWA and SA if all wells were shut off following water table dewatering below the critical level in
each area. This test revealed that between 1.5 to 2 months, the water table begins to approach
and exceed the critical level in isolated areas of both the NWA and SA, primarily along the south
border and northeast portion of the SA and in the southwest portion of the NWA. Testing
revealed that within one month of resuming pumping at all wells, the water table again dropped
below the critical level over both areas. These conditions are assuming water table conditions
as observed in the 2009-2011 timeframe, which based on area hydrographs was similar to
water levels observed in the late 1990s, under normal or above normal climatic conditions. In
extended wet periods, the wells will likely need to be run continuously to maintain water table
levels below the critical surface. However, during periods of below average precipitation, the
wellfields will be able to remain idle over longer periods or run an alternating schedules where
not all wells operate simultaneously.
Figure 6.4-2 - Results of a hypothetical contaminant transport analysis at the VCP plume in the SA. Displayed are
the mapped VCP plume extent from NDEQ (magenta), the simulated test plume pre-dewatering well activity
(orange), and plume extent following extraction well pumping (blue). Dewatering well locations shown by yellow
triangles.
Grand Island Study Session - 5/15/2012 Page 92 / 124
Grand Island Dewatering Study DRAFT - Groundwater Model Report
Olsson Project No. 011-2231 Page 38 of 43
7.0 CONCLUSIONS AND RECOMMENDATIONS
This report covers the development, calibration and utilization of a groundwater model to
investigate dewatering options of the Quaternary aquifer underlying Grand Island, NE. The
process of this project for model development involved, in order, 1) creation and calibration of a
steady state model to water levels observed in the late 1990s, 2) calibration of water level
changes and recharge in a transient model that represents the period of 1999 to 2011, 3) design
of wellfields for two areas of Grand Island with a criteria of using 15 ft below ground level as a
dewatered threshold, including assessment of recovery time 4) evaluation of capture zone areas
for each well installed, and 5) test hypothetical contaminant plumes to further assess the
influence of the dewatering wellfield designs.
7.1 Key Findings
Dewatering of the northwest area of concern (NWA) required 11 dewatering wells, each
pumping at 500 gpm for a duration of 6.5 to 7 months.
Dewatering of the south area (SA) of concern requires 22 wells with pumping rates
ranging from 400 to 500 gpm. Most of the south area dewaters within at about the 7
month timeframe, but near complete dewatering takes between 9 and 10 months.
12-year capture zones for the NWA are typically 0.75 to 1.5 miles in length and are
oriented slight southwest to the northeast and do not approach the west model
boundary.
12-year capture zones in the SA have more variability in shape, length, and orientation.
The final design of the wellfield layout was determined after trial and error model runs to
ensure no dewatering well capture zones cross the GCA of VCP plume in the SA.
A hypothetical contaminant transport analysis revealed that the dewatering wells in the
SA have the potential influence the contaminant plumes, with greater potential of
influence on the GCA plume on the west than the VCP plume in the northeast part of the
SA. These conclusions are based however with a simulated constant source
contaminant concentration and no natural attenuation. The model demonstrates that to
dewater the entire SA, wells will need to be installed in close proximity to the
contaminant plumes and gradients induced from pumping these wells could cause
lateral migration of contaminants.
Water table recovery, under normal climatic conditions, approaches the critical surface
after 1.5 to 2 months following initial dewatering of both the SA and NWA wellfields.
Resumption of pumping following this recovery in both wellfields reduces the water table
below the critical level within one month.
7.2 Recommendations
Because potential exists for disturbance of the GCA and VCP plumes by the dewatering
wells in the SA, it is recommended that the city communicates with the EPA and NDEQ
regarding the current status of each contaminant plume in the SA and the projected
timeframes for cleanup of each plume and discuss possibilities such as interceptor wells
Grand Island Study Session - 5/15/2012 Page 93 / 124
Grand Island Dewatering Study DRAFT - Groundwater Model Report
Olsson Project No. 011-2231 Page 39 of 43
between the dewatering wells and the plumes if contaminant is observed to migrate
laterally from each plume following initiation of dewatering operations.
If further analysis is required to assess potential impacts to the GCA and VCP plumes,
the contaminant transport models for these areas that were developed intentionally for
fate and transport assessment should be used.
Under normal climatic conditions and achievement of complete dewatering across the
NWA and SA, pumping durations across the wellfields will be able to maintain dewatered
conditions with a cyclical pattern of pumping, such as 1 month with wells in operation
followed by one to two months of no pumping. Changing climatic conditions will dictate
modification of this type of schedule with wet periods requiring constant pumping and dry
periods requiring less pumping and/or longer recovery periods with wells that remain
idle.
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Grand Island Dewatering Study DRAFT - Groundwater Model Report
Olsson Project No. 011-2231 Page 40 of 43
REFERENCES
Cannia, J.C., Woodward, D.W., and Cast, L., 2006. Cooperative Hydrology Study
Hydrostratigraphic Units and Aquifer Characterization Report. Cooperative Hydrology Study, 91
p.
Chen, Xunhong 2005. Statistical and geostatistical features of streambed hydraulic
conductivities in the Platte River, NE. Environmental Geology,(2005) 48: 693-701.
Conservation and Survey Division, 1998. The groundwater atlas of Nebraska. Institute of
Agriculture and Natural Resources, University of Nebraska-Lincoln. Resource Atlas No.
4a/1998, 44 p.
Doherty, J. 1994. PEST: Model Independent Parameter Estimation.
Dreezen, V.H. 1999. Hall County, NE Test-Hole Logs: UNL CSD Test Hole Report No. 40, 125
p.
Fetter, C.W. 1994. Applied Hydrogeology, 3rd Ed. MacMillan Publishing, p. 86.
Gutentag, E.D., Heimes, F.J., Krothe, N.C., Luckey, R.R., and Weeks, J.B., 1984, Geohydrology
of the High Plains aquifer in parts of Colorado, Kansas, Nebraska, New Mexico, Oklahoma,
South Dakota, Texas, and Wyoming: U.S. Geological Survey Professional Paper 1400-B, 63 p.
Johnson, L.A. 1970. Permeability of Wisconsin Sand and Gravel. Pick-Sloan Missouri Basin
Program, Nebraska Mid-State Division.
Penman, H.L. 1948. Natural evaporation from open water, bare soil and grass. Proc. Roy. Soc.
A, 193, 120-45.
Peterson, S.M. and C.P. Carney, 2002. Estimated discharge to streams from the High Plains
Aquifer in the Eastern Model Unit of the COHYST Study Area for the period prior to major
groundwater irrigation. http://cohyst.dnr.ne.gov/adobe/dc012EMU_baseflw_02.pdf.
Peterson, S.M., 2007. Groundwater flow model of the Eastern Model Unit of the Nebraska
Cooperative Hydrology Study (COHYST) area. Platte River Cooperative Hydrology Study, 87 p.
Available on the world wide web at:
http://cohyst.dnr.ne.gov/adobe/dc012EMU_GFMR_090507.pdf.
Pollock, D. W. 1989. Documentation of computer programs to compute and display pathlines
using results from the U.S. Geological Survey modular three-dimensional finite difference
ground-water flow model, U.S. Geological Survey Open-File Report 89-381, 188p.
Stanton, J. 1999. Areas of gain and loss along the Platte River, central Nebraska, spring 1999.
US Geological Survey Water Resources Investigation Report. 2000-4065.
Szilagyi, J., F.E. Harvey, and J.F. Ayers. 2005. Regional Estimation of Total Recharge to
Ground Water in Nebraska. Ground Water 43, no. 1: 63–69.
Grand Island Study Session - 5/15/2012 Page 95 / 124
Grand Island Dewatering Study DRAFT - Groundwater Model Report
Olsson Project No. 011-2231 Page 41 of 43
United States Census Bureau, 2010. State and county quick facts, accessed on the world wide
web on February 29, 2012 at http://quickfacts.census.gov/qfd/states/31/3119595.html.
University of Nebraska Lincoln - Conservation and Survey Division, 1973. Topographic Regions
of Nebraska. Available on the world wide web at: http://digitalcommons.unl.edu/caripubs/62.
University of Nebraska Lincoln - Conservation and Survey Division, 1995. Configuration of the
Water Table, 1995. Accessed online at http://snr.unl.edu/data/geographygis/NebrGISwater.asp.
Zheng, C. and P.P. Wang. 1999. MT3DMS: A Modular Three-Dimensional Multispecies
Transport Model for Simulation of Advection, Dispersion, and Chemical Reactions of
Contaminants in Groundwater Systems; Documentation and User’s Guide. U.S. Army Engineer
Research and Development Center.
Grand Island Study Session - 5/15/2012 Page 96 / 124
APPENDIX A
Additional calibration hydrographs for the transient model.
Grand Island Study Session - 5/15/2012 Page 97 / 124
G-100378A
G-100378D
G-100377
(see figure 5.1-1 for locations in model domain)
-0.44
0.55
1.53
2.52
3.50
4.49
5.48
6.46
7.45
8.43
9.42
0.17 438.45876.731315.021753.302191.582629.863068.143506.433944.714382.99Drawdown
Time
Observed
Computed
-0.45
0.44
1.33
2.21
3.10
3.99
4.88
5.77
6.65
7.54
8.43
0.17 438.45 876.731315.021753.302191.582629.863068.143506.433944.714382.99Drawdown
Time
Observed
Computed
-4.11
-3.34
-2.57
-1.80
-1.04
-0.27
0.50
1.27
2.03
2.80
3.57
0.17 438.45876.731315.021753.302191.582629.863068.143506.433944.714382.99Drawdown
Time
Observed
Computed
Grand Island Study Session - 5/15/2012 Page 98 / 124
APPENDIX B
Detailed Layout of the Updated Dewatering System
Grand Island Study Session - 5/15/2012 Page 99 / 124
011-2231PROJECT NO:
DRAWN BY:
05/07/12DATE:
EJS
EXHIBIT
FAX 308.384.8752
TEL 308.384.8750
Grand Island, NE 68802-1072
P.O. Box 1072
201 East 2nd Street
GRAND ISLAND DEWATERING
COLLECTION SYSTEM LAYOUT RINDEPENDENCE AVENUENORTH ROADW. 13TH STREET
CAPITAL AVENUE
HIGHWAY 2
STATE STREET U.S. HWY 281112345610711Grand IslandStudy Session - 5/15/2012Page 100 / 124
OLD POTASH HIGHWAY
US HIGHWAY 30UPRRHILLTOP ROADUS HIGHWAY 281HUSKER HIGHWAY RAILROADBLAINE STREETSTOLLEY PARK ROAD
BIKE TRAILU.S. HIGHWAY 281US HIGHWAY 30011-2231PROJECT NO:
DRAWN BY:
05/07/12DATE:
EJS
EXHIBIT
FAX 308.384.8752
TEL 308.384.8750
Grand Island, NE 68802-1072
P.O. Box 1072
201 East 2nd Street
GRAND ISLAND DEWATERING
COLLECTION SYSTEM LAYOUT R 2
WOOD RIVER
BYPASS WOOD RIVERUS HIGHWAY 34
W. 13TH STREET789
912341011Grand IslandStudy Session - 5/15/2012Page 101 / 124
011-2231PROJECT NO:
DRAWN BY:
05/07/12DATE:
EJS
EXHIBIT
FAX 308.384.8752
TEL 308.384.8750
Grand Island, NE 68802-1072
P.O. Box 1072
201 East 2nd Street
GRAND ISLAND DEWATERING
COLLECTION SYSTEM LAYOUT R 3US HIGHWAY 281HUSKER HIGHWAY RAILROADBLAINE STREETSTOLLEY PARK ROAD BIKE TRAIL
WOOD RIVER
BYPASS WOOD RIVERUS HIGHWAY 34912
3410115Grand IslandStudy Session - 5/15/2012Page 102 / 124
STOLLEY PARK ROAD
ROUSH LANE
STATE FAIR BLVD
FONNER PARK ROAD
LOCUST STREETSTUHR ROADBISMARK ROADGRAND ISLANDPOLICE DEPARTMENTUS HIGHWAY 30011-2231PROJECT NO:
DRAWN BY:
05/07/12DATE:
EJS
EXHIBIT
FAX 308.384.8752
TEL 308.384.8750
Grand Island, NE 68802-1072
P.O. Box 1072
201 East 2nd Street
GRAND ISLAND DEWATERING
COLLECTION SYSTEM LAYOUT R 4WOOD RIVER1312146781516Grand IslandStudy Session - 5/15/2012Page 103 / 124
SWIFT ROAD US HIGHWAY 30SHADY BEND ROADSEEDLING MILE ROAD
011-2231PROJECT NO:
DRAWN BY:
05/07/12DATE:
EJS
EXHIBIT
FAX 308.384.8752
TEL 308.384.8750
Grand Island, NE 68802-1072
P.O. Box 1072
201 East 2nd Street
GRAND ISLAND DEWATERING
COLLECTION SYSTEM LAYOUT R 5WOOD RIVER222021191817Grand IslandStudy Session - 5/15/2012Page 104 / 124
APPENDIX C
Detailed Breakdown of Probable Opinion of Cost
Grand Island Study Session - 5/15/2012 Page 105 / 124
Item Estimated
Quantity Unit Unit $ Total $
8" PVC Water Main 10560 LF 25.00$ $264,000
10" PVC Water Main 725 LF 29.00$ $21,025
12" PVC Water Main 900 LF 37.00$ $33,300
14" PVC Water Main 2985 LF 42.00$ $125,370
16" PVC Water Main 3090 LF 46.00$ $142,140
18" PVC Water Main 1170 LF 61.00$ $71,370
20" PVC Water Main 6755 LF 75.00$ $506,625
24" PVC Water Main 17780 LF 110.00$ $1,955,800
30" DI Water Main 2540 LF 140.00$ $355,600
36" DI Water Main 2735 LF 188.00$ $514,180
Utility Crossing 18 EA 1,720.00$ $30,960
Pump System 11 EA 81,650.00$ $898,150
Pump Controls 11 EA 3,350.00$ $36,850
Decommission Well 3 EA 1,000.00$ $3,000
Observation Well 5 EA 7,500.00$ $37,500
Observation Well Control System 7 EA 6,550.00$ $45,850
Remove Driveway 1867 SY 5.00$ $9,333
6" P.C. Concrete Driveway 1867 SY 36.00$ $67,200
Gravel Surfacing 294 TON 28.00$ $8,227
Remove and Replace 4" P.C. Concrete Walk 2950 SF 4.50$ $13,275
Remove and Replace 6" Concrete Bikeway 100 SY 45.00$ $4,500
Remove Ashp./Conc. Roadway 1333 SY 11.00$ $14,667
Replace Concrete Roadway 1333 SY 65.00$ $86,667
6" Concrete Pavement 750 SY 36.00$ $27,000
Seeding 38 AC 1,250.00$ $46,875
Slip Form Channel Lining 25015 SY 35.00$ $875,521
Hand Pour Channel Lining 1944 SY 38.00$ $73,889
Steel for Outlet Structure 670 LB 1.50$ $1,005
Concrete for Outlet Structure 10 CY 600.00$ $6,000
Dewatering - 8" Water Main 9240 LF 15.00$ $138,600
Dewatering - 10" Water Main 725 LF 15.00$ $10,875
Dewatering - 12" Water Main 1000 LF 15.00$ $15,000
Dewatering - 14" Water Main 2985 LF 20.00$ $59,700
Dewatering - 16" Water Main 3490 LF 20.00$ $69,800
Dewatering - 18" Water Main 3270 LF 20.00$ $65,400
Dewatering - 20" Water Main 2285 LF 20.00$ $45,700
Dewatering - 24" Water Main 8360 LF 20.00$ $167,200
Dewatering - 30" Water Main 2620 LF 20.00$ $52,400
Dewatering - 36" Water Main 3005 LF 20.00$ $60,100
Directional Drilling - 8" Water Main 595 LF 90.00$ $53,550
Directional Drilling - 10" Water Main 0 LF 110.00$ $0
Directional Drilling - 12" Water Main 100 LF 120.00$ $12,000
Directional Drilling - 14" Water Main 0 LF 155.00$ $0
Directional Drilling - 16" Water Main 400 LF 175.00$ $70,000
Directional Drilling - 18" Water Main 2100 LF 200.00$ $420,000
Directional Drilling - 20" Water Main 795 LF 242.00$ $192,390
Directional Drilling - 24" Water Main 870 LF 300.00$ $261,000
Directional Drilling - 30" Water Main 80 LF 400.00$ $32,000
Directional Drilling - 36" Water Main 270 LF 500.00$ $135,000
TOTAL 8,136,593.83$
PHASE 1
Grand Island Study Session - 5/15/2012 Page 106 / 124
Item Estimated
Quantity Unit Unit $ Total $
8" PVC Water Main 16045 LF 25.00$ $401,125
10" PVC Water Main 2775 LF 29.00$ $80,475
12" PVC Water Main 2260 LF 37.00$ $83,620
14" PVC Water Main 3995 LF 42.00$ $167,790
16" PVC Water Main 0 LF 46.00$ $0
18" PVC Water Main 2765 LF 61.00$ $168,665
20" PVC Water Main 0 LF 75.00$ $0
24" PVC Water Main 0 LF 110.00$ $0
30" DI Water Main 0 LF 140.00$ $0
36" DI Water Main 0 LF 188.00$ $0
Utility Crossing 16 EA 1,720.00$ $27,520
Pump System 16 EA 81,650.00$ $1,306,400
Pump Controls 16 EA 3,350.00$ $53,600
Decommission Well 3 EA 1,000.00$ $3,000
Observation Well 5 EA 7,500.00$ $37,500
Observation Well Control System 7 EA 6,550.00$ $45,850
Remove Driveway 1150 SY 5.00$ $5,750
6" P.C. Concrete Driveway 1150 SY 36.00$ $41,400
Gravel Surfacing 581 TON 28.00$ $16,281
Remove and Replace 4" P.C. Concrete Walk 2950 SF 4.50$ $13,275
Remove and Replace 6" Concrete Bikeway 67 SY 45.00$ $3,000
Remove Ashp./Conc. Roadway 1200 SY 11.00$ $13,200
Replace Concrete Roadway 1200 SY 65.00$ $78,000
6" Concrete Pavement 1133 SY 36.00$ $40,800
Seeding 21 AC 1,250.00$ $25,986
Slip Form Channel Liner 0 SY 35.00$ $0
Hand Pour Channel Liner 0 SY 38.00$ $0
Steel for Outlet Structure 0 LB 1.50$ $0
Concrete for Outlet Structure 0 CY 600.00$ $0
Dewatering - 8" Water Main 8785 LF 15.00$ $131,775
Dewatering - 10" Water Main 2960 LF 15.00$ $44,400
Dewatering - 12" Water Main 383 LF 15.00$ $5,738
Dewatering - 14" Water Main 0 LF 20.00$ $0
Dewatering - 16" Water Main 0 LF 20.00$ $0
Dewatering - 18" Water Main 1085 LF 20.00$ $21,700
Dewatering - 20" Water Main 0 LF 20.00$ $0
Dewatering - 24" Water Main 0 LF 20.00$ $0
Dewatering - 30" Water Main 0 LF 20.00$ $0
Dewatering - 36" Water Main 0 LF 20.00$ $0
Directional Drilling - 8" Water Main 915 LF 90.00$ $82,350
Directional Drilling - 10" Water Main 185 LF 110.00$ $20,350
Directional Drilling - 12" Water Main 765 LF 120.00$ $91,800
Directional Drilling - 14" Water Main 380 LF 155.00$ $58,900
Directional Drilling - 16" Water Main 0 LF 175.00$ $0
Directional Drilling - 18" Water Main 100 LF 200.00$ $20,000
Directional Drilling - 20" Water Main 0 LF 242.00$ $0
Directional Drilling - 24" Water Main 0 LF 300.00$ $0
Directional Drilling - 30" Water Main 0 LF 400.00$ $0
Directional Drilling - 36" Water Main 0 LF 500.00$
TOTAL 3,090,249.69$
PHASE 2
Grand Island Study Session - 5/15/2012 Page 107 / 124
Item Estimated
Quantity Unit Unit $ Total $
8" PVC Water Main 2705 LF 25.00$ $67,625
10" PVC Water Main 735 LF 29.00$ $21,315
12" PVC Water Main 0 LF 37.00$ $0
14" PVC Water Main 3935 LF 42.00$ $165,270
16" PVC Water Main 0 LF 46.00$ $0
18" PVC Water Main 0 LF 61.00$ $0
20" PVC Water Main 0 LF 75.00$ $0
24" PVC Water Main 0 LF 110.00$ $0
30" DI Water Main 0 LF 140.00$ $0
36" DI Water Main 0 LF 188.00$ $0
Utility Crossing 2 EA 1,720.00$ $3,440
Pump System 6 EA 81,650.00$ $489,900
Pump Controls 6 EA 3,350.00$ $20,100
Decommission Well 0 EA 1,000.00$ $0
Observation Well 1 EA 7,500.00$ $7,500
Observation Well Control System 1 EA 6,550.00$ $6,550
Remove Driveway 478 SY 5.00$ $2,389
6" P.C. Concrete Driveway 478 SY 36.00$ $17,200
Gravel Surfacing 37 TON 28.00$ $1,037
Remove and Replace 4" P.C. Concrete Walk 0 SF 4.50$ $0
Remove and Replace 6" Concrete Bikeway 0 SY 45.00$ $0
Remove Ashp./Conc. Roadway 320 SY 11.00$ $3,520
Replace Concrete Roadway 320 SY 65.00$ $20,800
6" Concrete Pavement 0 SY 36.00$ $0
Seeding 5 AC 1,250.00$ $6,650
Slip Form Channel Liner 0 SY 35.00$ $0
Hand Pour Channel Liner 0 SY 38.00$ $0
Steel for Outlet Structure 0 LB 1.50$ $0
Concrete for Outlet Structure 0 CY 600.00$ $0
Dewatering - 8" Water Main 3005 LF 15.00$ $45,075
Dewatering - 10" Water Main 735 LF 15.00$ $11,025
Dewatering - 12" Water Main 0 LF 15.00$ $0
Dewatering - 14" Water Main 3985 LF 20.00$ $79,700
Dewatering - 16" Water Main 0 LF 20.00$ $0
Dewatering - 18" Water Main 0 LF 20.00$ $0
Dewatering - 20" Water Main 0 LF 20.00$ $0
Dewatering - 24" Water Main 0 LF 20.00$ $0
Dewatering - 30" Water Main 0 LF 20.00$ $0
Dewatering - 36" Water Main 0 LF 20.00$ $0
Directional Drilling - 8" Water Main 300 LF 90.00$ $27,000
Directional Drilling - 10" Water Main 0 LF 110.00$ $0
Directional Drilling - 12" Water Main 0 LF 120.00$ $0
Directional Drilling - 14" Water Main 50 LF 155.00$ $7,750
Directional Drilling - 16" Water Main 0 LF 175.00$ $0
Directional Drilling - 18" Water Main 0 LF 200.00$ $0
Directional Drilling - 20" Water Main 0 LF 242.00$ $0
Directional Drilling - 24" Water Main 0 LF 300.00$ $0
Directional Drilling - 30" Water Main 0 LF 400.00$ $0
Directional Drilling - 36" Water Main 0 LF 500.00$ $0
TOTAL 1,003,846.24$
PHASE 3
Grand Island Study Session - 5/15/2012 Page 108 / 124
Item Estimated
Quantity Unit Unit $ Total $
8" PVC Water Main 29310 LF 25.00$ $732,750
10" PVC Water Main 4235 LF 29.00$ $122,815
12" PVC Water Main 3160 LF 37.00$ $116,920
14" PVC Water Main 10915 LF 42.00$ $458,430
16" PVC Water Main 3090 LF 46.00$ $142,140
18" PVC Water Main 3935 LF 61.00$ $240,035
20" PVC Water Main 6755 LF 75.00$ $506,625
24" PVC Water Main 17780 LF 110.00$ $1,955,800
30" DI Water Main 2540 LF 140.00$ $355,600
36" DI Water Main 2735 LF 188.00$ $514,180
Utility Crossing 36 EA 1,720.00$ $61,920
Pump System 33 EA 81,650.00$ $2,694,450
Pump Controls 33 EA 3,350.00$ $110,550
Decommission Well 6 EA 1,000.00$ $6,000
Observation Well 11 EA 7,500.00$ $82,500
Observation Well Control System 15 EA 6,550.00$ $98,250
Remove Driveway 3494 SY 5.00$ $17,472
6" P.C. Concrete Driveway 3494 SY 36.00$ $125,800
Gravel Surfacing 912 TON 28.00$ $25,546
Remove and Replace 4" P.C. Concrete Walk 5900 SF 4.50$ $26,550
Remove and Replace 6" Concrete Bikeway 167 SY 45.00$ $7,500
Remove Ashp./Conc. Roadway 2853 SY 11.00$ $31,387
Replace Concrete Roadway 2853 SY 65.00$ $185,467
6" Concrete Pavement 1883 SY 36.00$ $67,800
Seeding 64 AC 1,250.00$ $79,511
Slip Form Channel Lining 25015 SY 35.00$ $875,521
Hand Pour Channel Lining 1944 SY 38.00$ $73,889
Steel for Outlet Structure 670 LB 1.50$ $1,005
Concrete for Outlet Structure 10 CY 600.00$ $6,000
Dewatering - 8" Water Main 21030 LF 15.00$ $315,450
Dewatering - 10" Water Main 4420 LF 15.00$ $66,300
Dewatering - 12" Water Main 1383 LF 15.00$ $20,738
Dewatering - 14" Water Main 6970 LF 20.00$ $139,400
Dewatering - 16" Water Main 3490 LF 20.00$ $69,800
Dewatering - 18" Water Main 4355 LF 20.00$ $87,100
Dewatering - 20" Water Main 2285 LF 20.00$ $45,700
Dewatering - 24" Water Main 8360 LF 20.00$ $167,200
Dewatering - 30" Water Main 2620 LF 20.00$ $52,400
Dewatering - 36" Water Main 3005 LF 20.00$ $60,100
Directional Drilling - 8" Water Main 1810 LF 90.00$ $162,900
Directional Drilling - 10" Water Main 185 LF 110.00$ $20,350
Directional Drilling - 12" Water Main 865 LF 120.00$ $103,800
Directional Drilling - 14" Water Main 430 LF 155.00$ $66,650
Directional Drilling - 16" Water Main 400 LF 175.00$ $70,000
Directional Drilling - 18" Water Main 2200 LF 200.00$ $440,000
Directional Drilling - 20" Water Main 795 LF 242.00$ $192,390
Directional Drilling - 24" Water Main 870 LF 300.00$ $261,000
Directional Drilling - 30" Water Main 80 LF 400.00$ $32,000
Directional Drilling - 36" Water Main 270 LF 500.00$ $135,000
TOTAL $12,230,689.75
Combined Total
Grand Island Study Session - 5/15/2012 Page 109 / 124
City of Grand Island
Tuesday, May 15, 2012
Study Session
Item -2
Analysis of Utility Rates for City / Non-City Customers
Staff Contact: Tim Luchsinger
Grand Island Study Session - 5/15/2012 Page 110 / 124
Council Agenda Memo
From:Timothy Luchsinger, Utilities Director
Meeting:May 15, 2012
Subject:Analysis of Utility Rates for City / Non-City Customers
Item #’s:2
Presenter(s):Timothy Luchsinger, Utilities Director
Background
The Utilities Department provides electricity in its State-established service area, which
extends beyond the City limits. Water has traditionally been offered only to customers
within the City limits, but has been provided on a limited basis to areas affected by
groundwater quality issues. There are currently no differential utility rates for City/non-
City customers. Council requested a review of cost of service and survey of other
municipal utilities to determine whether a surcharge or other means of rate adjustment is
warranted.
Discussion
The impact of the costs of services provided to utility customers outside of the Grand
Island city limits is minimal or undetermined, as well as the contribution received by the
resulting revenue. Other municipal utilities may have a higher utility rate for out-of-town
or suburban, customers, but appear to have been set for other benefits or costs determined
by policy makers in those areas. The results of the Department’s analysis will be
presented with the attached slides for further discussion by Council.
Conclusion
This item is presented to the City Council in a Study Session to allow for any questions to
be answered and to create a greater understanding of the issue at hand. Council may
request further action for their consideration at a scheduled meeting.
Grand Island Study Session - 5/15/2012 Page 111 / 124
UTILITIESAnalysis of Utility Rates for City / Non-City CustomersCity Council Study SessionGrand IslandStudy Session - 5/15/2012Page 112 / 124
UTILITIESRate AnalysisUrban (Inside City limits) / Suburban (Outside City limits)Cost of ServicesComparison with other municipal utilitiesGrand IslandStudy Session - 5/15/2012Page 113 / 124
UTILITIESNebraska Electric Service AreasGrand IslandStudy Session - 5/15/2012Page 114 / 124
UTILITIESElectric Service AreaEstablished in 1964Determined by Nebraska Power Review BoardNewly annexed areas must be requested to be added by electric utility within one year of annexationLocal governing bodies set ratesGrand IslandStudy Session - 5/15/2012Page 115 / 124
UTILITIESOutside of City Limits – Electric System 2000 of 24,600 (8%) of electric customers are outside of City limits$260,000 of $55,000,000 (0.5%) of electric revenue is from sales outside of City limits112 miles of 459 miles (24%) of electric circuits are outside of City limits10% of annual line maintenance budget is for lines outside of City limitsGrand IslandStudy Session - 5/15/2012Page 116 / 124
UTILITIESSuburban Charges – Electric System(NE municipalities over 10,000 pop.)Fremont10% higher for residential single phase under ten horsepower Hastings91% higher customer charge21% higher energy rateGrand IslandStudy Session - 5/15/2012Page 117 / 124
UTILITIESWater System OverviewGrand IslandStudy Session - 5/15/2012Page 118 / 124
UTILITIESOutside of City Limits – Water System 85 of 15,800 (0.5%) of water customers are outside of City limits$38,000 of $4,200,000 (0.9%) of water revenue is from sales outside of City limits13 miles of 278 miles (5%) of water distribution lines are outside of City limitsGrand IslandStudy Session - 5/15/2012Page 119 / 124
UTILITIESSuburban Charges – Water System(NE municipalities over 10,000 pop.)Half of surveyed municipalities have no difference in ratesThose establishing different rates varied from 25% to 100% of the urban rateGrand IslandStudy Session - 5/15/2012Page 120 / 124
UTILITIESOther ConsiderationsAll utilities customers contribute to City general fund by Utilities Department in-lieu-of-tax paymentSuburban customers do not pay 1.5% City sales tax on utility bills (~ $4,500 annually)Primary motive for annexation has been fire protectionGrand IslandStudy Session - 5/15/2012Page 121 / 124
UTILITIESOther Considerations (cont.)Electric distribution expense was $3,000,000 of $56,000,000 (5.3%) total electric expense in 2011Water distribution expense was $920,000 of $5,000,000 (18.6%) total water expense in 2011 Grand IslandStudy Session - 5/15/2012Page 122 / 124
UTILITIESSummaryOut-of City utilities revenues are less than 1% of total revenuesCost of electricity and water production and administrative costs are independent of customer locationDistribution expense is a minor component of total utilities expenseGrand IslandStudy Session - 5/15/2012Page 123 / 124
UTILITIESDiscussionMinimal impact on overall revenues, costs to Utilities DepartmentOther City resources – Benefits? Costs?Annexation implications – Desired? Feasible?Urban/suburban rates established by policy issues?Grand IslandStudy Session - 5/15/2012Page 124 / 124