In cooperation with the Department of the Navy, Southern Division Naval Facilities Engineering Command
Computer-Model Analysis of Ground-Water Flow and Simulated Effects of Contaminant Remediation at Naval Weapons Industrial Reserve Plant, Dallas, Texas
By René A. Barker and Christopher L. Braun
U.S. Geological Survey
Water-Resources Investigations Report 00–4197
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CONTENTS
Abstract
Introduction
Purpose and Scope
Study Area
Methods of Investigation
Digital Computer Model
Data Collection, Storage, and Retrieval
Acknowledgments
Hydrogeologic Setting
Shallow Alluvial Aquifer
Eagle Ford Shale
Computer-Model Analysis of Ground-Water Flow
Model Description
Calibration Strategy
Boundary Conditions
Recharge
Discharge
Outflow to Drains
Evapotranspiration
Aquifer Properties
Hydraulic Conductivity
Transmissivity
Specific Yield
Steady-State Conditions
Water Levels
Water Budget
Transient Conditions
Water Levels
Water Budget
Sensitivity Testing
Simulated Effects of Contaminant Remediation
Withdrawals at Areas of Concern 1, 2, and 3
Withdrawals at Area of Concern 3
Summary
References Cited
PLATES
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1. |
Hydrographs showing water levels measured in selected observation wells in the shallow alluvial aquifer underlying the study area, 1993–98 |
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2. |
Hydrographs showing simulated rates of recharge to (in) and discharge from (out) the shallow alluvial aquifer underlying the study area, without and with remediation wells and recovery trench, 1992–98 |
FIGURES
1–3. |
Maps showing: | |
1. |
Location of study area | |
2. |
Locations of Naval Weapons Industrial Reserve Plant, Naval Air Station, and off-site water bodies | |
3. |
Locations of model area, roads, railroads, runways, on-site water bodies, and buildings associated with Naval Weapons Industrial Reserve Plant and Naval Air Station | |
4. |
Diagrammatic section showing conceptualized hydrogeologic setting and estimated long-term average rates of ground-water recharge (inflow) and discharge (outflow) | |
5. |
Hydrographs showing relation among rate of precipitation, altitude of Mountain Creek Lake (including Cottonwood Bay), and altitude of ground-water levels in wells near Cottonwood Bay, 1992–98 | |
6–15. |
Maps showing: | |
6. |
Distribution of long-term average water levels in the alluvial aquifer | |
7. |
Distribution of long-term average saturated thickness in the alluvial aquifer | |
8. |
Locations of stabilization systems and recent remediation activity at three Areas of Concern (AOC) | |
9. |
Configuration of the top of Eagle Ford Shale, which comprises the base of the alluvial aquifer | |
10. |
Configuration of finite-difference grid and boundary conditions used for the digital computer model of ground-water flow in the alluvial aquifer | |
11. |
Calibrated distribution of long-term average infiltration to and long-term average evapotranspiration from the alluvial aquifer | |
12. |
Calibrated distribution of hydraulic conductivity in the alluvial aquifer | |
13. |
Calibrated distribution of transmissivity in the alluvial aquifer under natural (unstressed) conditions | |
14. |
Calibrated distribution of specific yield in the alluvial aquifer | |
15. |
Comparison of observed and simulated long-term average (steady-state) water levels | |
16. |
Distribution of differences between observed and simulated long-term average water levels | |
17. |
Hydrographs showing comparison of observed (blue) and simulated (red) water levels, 1992–98, for selected wells in the study area | |
18–20. |
Maps showing: | |
18. |
Comparison of observed and simulated March 1998 water levels | |
19. |
Comparison of observed August 1998 water levels and average of water levels observed between August and October 1993–97 | |
20. |
Comparison of observed and simulated August 1998 water levels | |
21. |
Graph showing long-term average monthly and average annual evaporation rates applicable to evaporation losses from the west and east drainage lagoons | |
22. |
Graphs showing responses of simulated long-term average water levels to 25-percent increases and 25-percent decreases in the calibrated values of hydraulic conductivity and the calibrated rates of infiltration in the steady-state model | |
23. |
Hydrographs showing responses of simulated water levels in selected observation wells to 50-percent increases (blue) and to 50-percent decreases (green) in the calibrated values (red) of specific yield in the transient model | |
24. |
Graph showing schedule of remediation activities during January 1996–December 1998 at Areas of Concern (AOC) 1, 2, and 3 | |
25–27. |
Maps showing: | |
25. |
Simulated distribution of water levels, as of December 31, 1998, following remediation during July 1996–December 1998 at Areas of Concern 1, 2, and 3 | |
26. |
Simulated distribution of water-level drawdown, as of December 31, 1998, following remediation during July 1996–December 1998 at Areas of Concern 1, 2, and 3 | |
27. |
Simulated distribution of backward particle-tracking flowlines depicting capture zones in the alluvial aquifer, resulting from 2.5- and 5-year remediation periods at Areas of Concern (AOC) 1, 2, and 3 | |
28–30. |
Diagrams showing: | |
28. |
Simulated rates and directions of ground-water flow, as of December 31, 1998, between northern shoreline of Cottonwood Bay and trench at Area of Concern 3, assuming inactive recovery trench | |
29. |
Simulated rates of ground-water discharge, as of December 31, 1998, to recovery trench at Area of Concern 3 | |
30. |
Simulated rates and directions of ground-water flow, as of December 31, 1998, between northern shoreline of Cottonwood Bay and recovery trench at Area of Concern 3 | |
TABLES
1. |
Summary of stabilization systems and recent remediation activity at three Areas of Concern (AOC) at the Naval Weapons Industrial Reserve Plant (NWIRP), Dallas, Texas |
2. |
Simulated long-term average (steady-state) rates of recharge to and discharge from the shallow alluvial aquifer underlying the study area |
3. |
Simulated rates of recharge to and discharge from the shallow alluvial aquifer underlying the study area, as of December 31, 1998 |
4a. |
Simulated rates of ground-water discharge from the shallow alluvial aquifer underlying the study area, as of December 31, 1998, following remediation during July 1996–December 1998 at Areas of Concern 1, 2, and 3 |
4b. |
Simulated effects on rates of recharge to and discharge from the shallow alluvial aquifer underlying the study area, as of December 31, 1998, following remediation during July 1996–December 1998 at Areas of Concern 1, 2, and 3 |
VERTICAL DATUM AND ABBREVIATIONS
Sea Level: In this report, “sea level” refers to the National Geodetic Vertical Datum of 1929—a geodetic datum derived from a general adjustment of the first-order level nets of both the United States and Canada, formerly called Sea Level Datum of 1929.
Abbreviations:
acre-ft, acre-foot
ft, foot
ft/d, foot per day
ft/ft, foot per foot
ft/mi, foot per mile
ft2/d, foot squared per day
ft3/d, cubic foot per day
gal/min, gallon per minute
in/yr, inch per year
mi, mile
mi2, square mile
Abstract
In June 1993, the Department of the Navy, Southern Division Naval Facilities Engineering Command (SOUTHDIV), began a Resource Conservation and Recovery Act (RCRA) Facility Investigation (RFI) of the Naval Weapons Industrial Reserve Plant (NWIRP) in north-central Texas. The RFI has found trichloroethene, dichloroethene, vinyl chloride, as well as chromium, lead, and other metallic residuum in the shallow alluvial aquifer underlying NWIRP.
These findings and the possibility of on-site or off-site migration of contaminants prompted the need for a ground-water-flow model of the NWIRP area. The resulting U.S. Geological Survey (USGS) model: (1) defines aquifer properties, (2) computes water budgets, (3) delineates major flowpaths, and (4) simulates hydrologic effects of remediation activity. In addition to assisting with particle-tracking analyses, the calibrated model could support solute-transport modeling as well as help evaluate the effects of potential corrective action. The USGS model simulates steady-state and transient conditions of ground-water flow within a single model layer.
The alluvial aquifer is within fluvial terrace deposits of Pleistocene age, which unconformably overlie the relatively impermeable Eagle Ford Shale of Late Cretaceous age. Over small distances and short periods, finer grained parts of the aquifer are separated hydraulically; however, most of the aquifer is connected circuitously through randomly distributed coarser grained sediments. The top of the underlying Eagle Ford Shale, a regional confining unit, is assumed to be the effective lower limit of ground-water circulation and chemical contamination.
The calibrated steady-state model reproduces long-term average water levels within +5.1 or –3.5 feet of those observed; the standard error of the estimate is 1.07 feet with a mean residual of 0.02 foot. Hydraulic conductivity values range from 0.75 to 7.5 feet per day, and average about 4 feet per day. Specific yield values range from 0.005 to 0.15 and average about 0.08. Simulated infiltration rates range from 0 to 2.5 inches per year, depending mostly on local patterns of ground cover.
Computer simulation indicates that, as of December 31, 1998, remediation systems at NWIRP were removing 7,375 cubic feet of water per day from the alluvial aquifer, with 3,050 cubic feet per day coming from aquifer storage. The resulting drawdown prevented 1,800 cubic feet per day of ground water from discharging into Cottonwood Bay, as well as inducing another 1,325 cubic feet per day into the aquifer from the bay. An additional 1,200 cubic feet of water per day (compared to pre-remediation conditions) was prevented from discharging into the west lagoon, east lagoon, Mountain Creek Lake, and Mountain Creek swale.
Particle-tracking simulations, assuming an aquifer porosity of 0.15, were made to delineate flowpath patterns, or contaminant “capture zones,” resulting from 2.5- and 5-year periods of remediation activity at NWIRP. The resulting flowlines indicate three such zones, or areas from which ground water is simulated to have been removed during July 1996–December 1998, as well as extended areas from which ground water would be removed during the next 2.5 years (January 1999–June 2001).
Simulation indicates that, as of December 31, 1998, the recovery trench was intercepting about 827 cubic feet per day of ground water that—without the trench—would have discharged into Cottonwood Bay. During this time, the trench is simulated to have removed about 3,221 cubic feet per day of water from the aquifer, with about 934 cubic feet per day (29 percent) coming from the south (Cottonwood Bay) side of the trench.
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