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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