USGS LOGO

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


Document Accessibility: Adobe Systems Incorporated has information about PDFs and the visually impaired. This information provides tools to help make PDF files accessible. These tools convert Adobe PDF documents into HTML or ASCII text, which then can be read by a number of common screen-reading programs that synthesize text as audible speech. In addition, an accessible version of Acrobat Reader 5.0 for Windows (English only), which contains support for screen readers, is available. These tools and the accessible reader may be obtained free from Adobe at http://access.adobe.com/.

pdf version (2.57 MB)

plate 1 (578 KB)

plate 2 (617 KB)


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

1.  
Hydrographs showing water levels measured in selected observation wells in the shallow alluvial aquifer underlying the study area, 1993–98
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.




U.S. Department of the Interior, U.S. Geological Survey
Persistent URL: http://pubs.water.usgs.gov/wri004197
Page Contact Information: GS Pubs Web Contact
Last modified: Friday, September 16 2005, 04:23:59 PM
FirstGov button  Take Pride in America button