Estimated depth to the water table and estimated rate of recharge in outcrops of the Chicot and Evangeline aquifers near Houston, Texas

Abstract

In 1989, the U.S. geological survey, in cooperation with the Harris Galveston coastal subsidence district, began a field study to determine the depth to the water table and to estimate the rate of recharge in outcrops of the Chicot and Evangeline aquifers near Houston, Texas. The study area comprises about 2,000 square miles of outcrops of the Chicot and Evangeline aquifers in northwest Harris county, Montgomery county, and southern Walker county. Because of the scarcity of measurable water-table wells, depth to the water table below land surface was estimated using a surface geophysical technique, seismic refraction. The water table in the study area generally ranges from about 10 to 30 feet below land surface and typically is deeper in areas of relatively high land surface altitude. The water table has demonstrated no long term trends since ground water development began, with the probable exception of the water table in the Katy area: There the water table is more than 75 feet deep, probably due to ground water pumpage from deeper zones. An estimated rate of recharge in the aquifer outcrops was computed using the interface method in which environmental tritium is a ground water tracer. The estimated average total recharge rate in the study area is 6 inches per year. This rate is an upper bound on the average recharge rate during the 37 years 1953-90 because it is based on the deepest penetration (about 80 feet) of postnuclear- testing tritium concentrations. The rate, which represents one of several components of a complex regional hydrologic budget, is considered reasonable but is not definitive because of uncertainty regarding the assumptions and parameters used in its computation.