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In cooperation with the U.S. Air Force, Aeronautical Systems Center, Environmental Management Directorate, Wright-Patterson Air Force Base, Ohio

Hydrogeology and Simulation of Ground-Water Flow in the Paluxy Aquifer in the Vicinity of Landfills 1 and 3, U.S. Air Force Plant 4, Fort Worth, Texas

By Eve L. Kuniansky and Stanley T. Hamrick

U.S. Geological Survey
Water-Resources Investigations Report 98–4023


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CONTENTS

Abstract

Introduction

Purpose and Scope

Description of Study Area

Previous Investigations

Acknowledgments

Hydrogeology

Goodland-Walnut Confining Unit

Paluxy Aquifer

Hydraulic Properties

Ground-Water Withdrawals

Direction of Ground-Water Flow, May 1993

Hydraulic Connection With Lake Worth

Recharge

Glen Rose Confining Unit

Simulation of Ground-Water Flow in the Paluxy Aquifer

Grid Design and Boundary Conditions

Internal Boundaries

Model Layers

Simulated Recharge and Discharge

Model Calibration

Simulated Water Levels

Sensitivity Analysis

Particle-Tracking Analysis

Summary

References

FIGURES

1-3.   Maps showing:
  1.   Location of study area and approximate extent of contaminants in the Paluxy Formation
  2.   Location of landfills 1 and 3 and selected wells at Air Force Plant 4 and vicinity, Fort Worth, Texas
  3.   Surface geology in western Tarrant County, Texas
4.   Generalized hydrogeologic section at Air Force Plant 4 and Naval Air Station, Fort Worth area, Texas
5-16.   Maps showing:
  5.   Regional potentiometric surface, flow directions, and location of water-supply wells and domestic wells completed in the Paluxy aquifer at Air Force Plant 4 and vicinity, Fort Worth, Texas, May 1993
  6.   Model grid designed for simulation of the Paluxy aquifer at Air Force Plant 4 and vicinity, Fort Worth, Texas
  7.   Detail of model grid in the vicinity of landfills 1 and 3 at Air Force Plant 4, Fort Worth, Texas
  8.   Location of constant head cells, active cells, and inactive cells in model layer 1, upper zone of the Paluxy aquifer at Air Force Plant 4 and vicinity, Fort Worth, Texas
  9.   Location of constant head cells and active cells in model layer 2, middle zone of the Paluxy aquifer at Air Force Plant 4 and vicinity, Fort Worth, Texas
  10a-b.  

Difference between observed and simulated ground-water levels in the Paluxy aquifer:

a. At Air Force Plant 4 and vicinity, Fort Worth, Texas
b. At Air Force Plant 4, Fort Worth, Texas

  11.   Model-estimated transmissivity for layer 1, upper zone of the Paluxy aquifer at Air Force Plant 4 and vicinity, Fort Worth, Texas
  12.   Model-estimated transmissivity for layer 2, middle zone of the Paluxy aquifer at Air Force Plant 4 and vicinity, Fort Worth, Texas
  13.   Model-estimated transmissivity for layer 3, lower zone of the Paluxy aquifer at Air Force Plant 4 and vicinity, Fort Worth, Texas
  14.   Simulated ground-water levels in layer 1, upper zone of the Paluxy aquifer, with and without recovery wells, at Air Force Plant 4 and vicinity, Fort Worth, Texas
  15.   Simulated ground-water levels in layer 2, middle zone of the Paluxy aquifer, with and without recovery wells, at Air Force Plant 4 and vicinity, Fort Worth, Texas
  16.   Simulated ground-water levels in layer 3, lower zone of the Paluxy aquifer, with and without recovery wells, at Air Force Plant 4 and vicinity, Fort Worth, Texas
17.   Graphs showing sensitivity of the model to changes in parameters and stresses
18.   Map showing location of forward and backward particle-tracking pathlines at Air Force Plant 4, Fort Worth, Texas

TABLES

1.   Stratigraphic units, lithology, and water-yielding characteristics of the units in the vicinity of Air Force Plant 4
2.   Hydraulic properties in the Paluxy aquifer estimated from aquifer pumping tests
3.   Hydraulic conductivities in the Paluxy aquifer estimated from slug tests
4.   Vertical hydraulic conductivity determined from laboratory triaxial cell tests on core samples obtained from the Paluxy aquifer
5.   Average daily production for White Settlement municipal supply wells completed in the Paluxy aquifer
6.   Water budget for steady-state simulation, May 1993
7.   Water budget for steady-state simulation with recovery wells

Abstract

Ground-water contamination of the surficial terrace alluvial aquifer has occurred at U.S. Air Force Plant 4, a government-owned, contractor-operated facility, northwest of Fort Worth, Texas. A poorly constructed monitoring well, P–22M, open to the underlying middle zone of the Paluxy aquifer was installed at landfill 3, October 1987, allowing leakage of contaminated ground water to reach the Paluxy aquifer. This well was plugged and abandoned in November 1995. Additionally, volatile organic compounds have been detected in fractures in the Goodland-Walnut confining unit, the hydrogeologic unit separating the terrace alluvial aquifer from the underlying Paluxy aquifer, beneath the western part of landfill 1. Volatile organic compounds in concentrations near the analytical detection limit were detected in the upper Paluxy prior to the drilling of well P–22M.

The ground-water-flow simulation model described in this report was developed to examine the best logistically feasible location to install recovery wells to capture the low concentration (less than 100 micrograms per liter) trichloroethylene plume beneath landfills 1 and 3 (west Paluxy plume). Once the recovery wells were installed (1996), the simulation model was recalibrated with new data. This report documents the capture area of the installed recovery wells. Four geologic units are pertinent to this site-specific model. From oldest to youngest, these are the Glen Rose Formation, Paluxy Formation, Walnut Formation, and Goodland Limestone. The Glen Rose Formation is relatively impermeable in the study area and forms the confining unit underlying the Paluxy Formation. The Paluxy Formation forms the Paluxy aquifer, which is a public drinking water supply for the City of White Settlement. The Walnut Formation and Goodland Limestone form the Goodland-Walnut confining unit overlying the Paluxy aquifer. Near landfill 3, gamma-ray logs indicate three distinct zones of the Paluxy Formation; upper, middle, and lower. The formation is about 170-feet thick near landfill 3, and each zone is about 57-feet thick.

Two steady-state simulations using the computer program MODFLOW were analyzed using the particle-tracking computer program, MODPATH. One simulation is the calibration simulation using Paluxy aquifer water-level data for May 1993. The second simulation includes the installed recovery wells. A variably spaced grid was designed for the model. The smallest grid cells, 25 by 25 feet, are in the vicinity of landfills 1 and 3. The largest cells, 4,864.5 by 1,441.5 feet, are at the northwestern corner of the model grid near the Parker-Tarrant County line. The modeling was accomplished with three layers representing the upper, middle, and lower zones of the Paluxy aquifer. Particles, which represent contaminant molecules moving in solution with the ground water, were tracked from well P–22M and an area below landfill 1, at the top of the upper zone of the Paluxy aquifer, for 9 years (forward tracking). The forward tracking estimates where contaminants might move by advection from 1987 to 1996. Analysis of backward tracking from the new recovery wells indicates that the simulated contributing area to the recovery wells intercepts the contaminant plume, minimizing off-site migration of the west Paluxy plume. To determine the effectiveness of the recovery wells, monitoring wells southeast of Building 14 have been installed (1996–97) for sampling.


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