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

By

Kasmarek, Mark C.

U.S. Geological Survey, Houston, TX,

STROM, Eric W.

U.S. Geological Survey, Austin, TX

In November 1997, the U.S. Geological Survey, in cooperation with the City of Houston, began an investigation based on numerical simulation (MODFLOW model) of the Chicot and Evangeline aquifers in the greater Houston area to better understand the hydrology, flow, and associated land-surface subsidence. The focus of the study was Harris and Galveston Counties, but the other counties were included to achieve the appropriate boundary conditions. Transient simulations were made 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 and 1996 match closely. Hydrographs were also used as water-level calibration targets. 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 the same period until acceptable matches were achieved.

Water-level measurements indicate that by 1977, large ground-water withdrawals in southeastern Harris County had caused the potentiometric surfaces to decline several hundred feet below sea level in the Chicot and Evangeline aquifers. Although these areas now (1996) show significant recovery, new centers of decline have developed to the northwest. In 1996, water simulated from clay storage in the Chicot and Evangeline aquifers was about 19 percent and 10 percent, respectively, of the total water withdrawn from the aquifers.