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In cooperation with the City of Houston

Hydrogeology and Simulation of Ground-Water Flow and Land-Surface Subsidence in the Chicot and Evangeline Aquifers, Houston Area, Texas

By Mark C. Kasmarek and Eric W. Strom

U.S. Geological Survey
Water-Resources Investigations Report 02–4022


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Contents

Abstract

Introduction

Purpose and Scope

Previous Studies

Physiographic Setting

Acknowledgments

Hydrogeology of the Chicot and Evangeline Aquifers

Hydrogeologic Units and Geologic Setting

Aquifer Properties

Recharge and Discharge

Ground-Water Development

Potentiometric Surfaces and Land-Surface Subsidence

Simulation of Ground-Water Flow and Land-Surface Subsidence in the Chicot and Evangeline Aquifers

Numerical Model

Grid Design

Boundaries and Stresses

Model Calibration

Transmissivity

Vertical Hydraulic Conductance

Potentiometric Surfaces

1977 Ground-Water-Flow Conditions

1996 Ground-Water-Flow Conditions

Predevelopment Ground-Water-Flow Conditions

Storage in Sands

Land-Surface Subsidence and Storage in Clays

Sensitivity Analysis

Model Limitations

Summary

Selected References

Figures

1.   Map showing location of the Houston area, Texas
2.   Generalized hydrogeologic section showing the northwest-to-southeast dip of the hydrogeologic units of interest in the Houston area, Texas
3.   Chart showing correlation of geologic and hydrogeologic units of the Gulf Coast aquifer system in the Houston area, Texas
4–7.   Maps showing:
  4.   Approximate areal extent of the Chicot aquifer in the Houston area, Texas
  5.   Altitude of the base of the Chicot aquifer in the Houston area, Texas
  6.   Approximate areal extent of the Evangeline aquifer in the Houston area, Texas
  7.   Altitude of the base of the Evangeline aquifer in the Houston area, Texas
8.   Hydrographs showing water levels in wells screened in the outcrops of the (a) Chicot aquifer in Montgomery County and (b) Evangeline aquifer in Liberty County, Houston area, Texas
9.   Map showing outcrop of areas that are predominantly clay in the Beaumont Clay, Houston area, Texas
10.   Graph showing total ground-water withdrawal in the Houston area, Texas, 1891–1996
11–21.   Maps showing:
  11.   Measured potentiometric surface of the Chicot aquifer, Houston area, Texas, January 1996
  12.   Measured potentiometric surface of the Evangeline aquifer, Houston area, Texas, January 1996
  13.   Finite-difference grid used in the numerical model of the Chicot and Evangeline aquifers, Houston area, Texas
  14.   Modeled transmissivity of the Chicot aquifer, Houston area, Texas
  15.   Modeled transmissivity of the Evangeline aquifer, Houston area, Texas
  16.   Total cumulative sand thickness of the Chicot aquifer, Houston area, Texas
  17.   Total cumulative sand thickness of the Evangeline aquifer, Houston area, Texas
  18.   Cumulative clay thickness from land surface to the centerline of the Chicot aquifer, Houston area, Texas
  19.   Cumulative clay thickness from the centerline of the Chicot aquifer to the centerline of the Evangeline aquifer, Houston area, Texas
  20.   Simulated leakance of the Chicot aquifer, Houston area, Texas
  21.   Simulated leakance of the Evangeline aquifer, Houston area, Texas
22.   Hydrographs showing simulated and measured water levels in selected observation wells screened in the Chicot aquifer in Fort Bend and Harris Counties, Houston area, Texas
23.   Hydrographs showing simulated and measured water levels in selected observation wells screened in the Evangeline aquifer in Harris County, Houston area, Texas
24–28.   Maps showing:
  24.   Data points (wells) used to construct the 1977 and 1996 water-level-altitude maps of the Chicot and Evangeline aquifers, Houston area, Texas, and to determine respective root-mean-square errors
  25.   Simulated and measured potentiometric surfaces in the Chicot aquifer, Houston area, Texas, 1977
  26.   Simulated and measured potentiometric surfaces in the Evangeline aquifer, Houston area, Texas, 1977
  27.   Simulated and measured potentiometric surfaces in the Chicot aquifer, Houston area, Texas, 1996
  28.   Simulated and measured potentiometric surfaces in the Evangeline aquifer, Houston area, Texas, 1996
29.   Diagram showing simulated 1996 flow rates for the Chicot and Evangeline aquifers, Houston area, Texas
30.   Map showing simulated predevelopment potentiometric surface in the Chicot aquifer, Houston area, Texas
31.   Map showing simulated predevelopment potentiometric surface in the Evangeline aquifer, Houston area, Texas
32.   Diagram showing simulated predevelopment flow rates for the Chicot and Evangeline aquifers, Houston area, Texas
33–40.   Maps showing:
  33.   Total cumulative clay thickness of the Chicot aquifer, Houston area, Texas
  34.   Total cumulative clay thickness of the Evangeline aquifer, Houston area, Texas
  35.   Inelastic storativity of the Chicot aquifer, Houston area, Texas
  36.   Inelastic storativity of the Evangeline aquifer, Houston area, Texas
  37.   Measured land-surface subsidence, Houston area, Texas, 1906–95
  38.   Simulated land-surface subsidence, Houston area, Texas, 1891–1995
  39.   Measured land-surface subsidence, Houston area, Texas, 1978–95
  40.   Simulated land-surface subsidence, Houston area, Texas, 1978–95
41.   Graph showing sensitivity of the model of the Chicot and Evangeline aquifers, Houston area, Texas, to changes in aquifer properties and ground-water withdrawal
42.   Graph showing sensitivity of the model of the Chicot and Evangeline aquifers, Houston area, Texas, to changes in clay and sand storage properties

Tables

1.   Stress periods used in the model of the Chicot and Evangeline aquifers, Houston area, Texas
2.   Root-mean-square errors of simulated water levels in the Chicot and Evangeline aquifers, Houston area, Texas, 1977 and 1996

Abstract

In November 1997, the U.S. Geological Survey, in cooperation with the City of Houston Utilities Planning Section and the City of Houston Department of Public Works & Engineering, began an investigation of the Chicot and Evangeline aquifers in the greater Houston area in Texas to better understand the hydrology, flow, and associated land-surface subsidence. The principal part of the investigation was a numerical finite-difference model (MODFLOW) developed to simulate ground-water flow and land-surface subsidence in an 18,100-square-mile area encompassing greater Houston.

The focus of the study was Harris and Galveston Counties, but other counties were included to achieve the appropriate boundary conditions. The model was vertically discretized into three 103-row by 109-column layers resulting in a total of 33,681 grid cells. Layer 1 represents the water table using a specified head, layer 2 represents the Chicot aquifer, and layer 3 represents the Evangeline aquifer.

Simulations were made under transient conditions for 31 ground-water-withdrawal (stress) periods spanning 1891–1996. The years 1977 and 1996 were chosen as potentiometric-surface calibration periods for the model. Simulated and measured potentiometric surfaces of the Chicot and Evangeline aquifers for 1977 match closely. Water-level measurements indicate that by 1977, large ground-water withdrawals in east-central and southeastern areas of Harris County had caused the potentiometric surfaces to decline as much as 250 feet below sea level in the Chicot aquifer and as much as 350 feet below sea level in the Evangeline aquifer. Simulated and measured potentiometric surfaces of the Chicot and Evangeline aquifers for 1996 also match closely. The large potentiometric-surface decline in 1977 in the southeastern Houston area showed significant recovery by 1996. The 1996 centers of potentiometric-surface decline are located much farther northwest. Potentiometric-surface declines of more than 200 feet below sea level in the Chicot aquifer and more than 350 feet below sea level in the Evangeline aquifer were measured in observation wells and simulated in the flow model.

Simulation of land-surface subsidence and water released from storage in the clay layers was accomplished using the Interbed-Storage Package of the MODFLOW model. Land-surface subsidence was calibrated by comparing simulated long-term (1891–1995) and short-term (1978–95) land-surface subsidence with published maps of land-surface subsidence for about the same period until acceptable matches were achieved.

Simulated 1996 Chicot aquifer flow rates indicate that a net flow of 562.5 cubic feet per second enters the Chicot aquifer in the outcrop area, and a net flow of 459.5 cubic feet per second passes through the Chicot aquifer into the Evangeline aquifer. The remaining 103.0 cubic feet per second of flow is withdrawn as pumpage, with a shortfall of about 84.9 cubic feet per second supplied to the wells from storage in sands and clays. Water simulated from storage in clays in the Chicot aquifer is about 19 percent of the total water withdrawn from the aquifer.

Simulated 1996 Evangeline aquifer flow rates indicate that a net flow of 14.8 cubic feet per second enters the Evangeline aquifer in the outcrop area, and a net flow of 459.5 cubic feet per second passes through the Chicot aquifer into the Evangeline aquifer for a total inflow of 474.3 cubic feet per second. A greater amount, 528.6 cubic feet per second, is withdrawn by wells; the shortfall of about 54.8 cubic feet per second is supplied from storage in sands and clays. Water simulated from storage in clays in the Evangeline aquifer is about 10 percent of the total water withdrawn from the aquifer.




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