Aquifer characteristics from well-field production records-- Edwards Limestone, San Antonio area, Texas.

Abstract

The Edwards limestone aquifer, the main source of water to the San Antonio area of south central Texas, has been the subject of numerous hydrologic investigations. The most comprehensive studies have related changes in aquifer head and storage to the differences between recharge and discharge. Projections of future demands from the aquifer are used to predict future aquifer heads, but these are limited to yearend estimates and do not include maximum summertime drawdowns. The Theis nonequilibrium formula, with a modification introduced by Stallman for continuously varying discharge, was used with well-field production records to determine apparent values of aquifer characteristics. Although the aquifer model does not strictly apply to the mathematical model imposed by the formula, useful approximations of the coefficients of transmissivity (12 million gallons per day per foot) and storage (0. 6 x l0) were derived. Testing the coefficients with independent historical data gave some credence to the magnitude of the above results. These apparent coefficients were used with projected aquifer demands and year-end aquifer heads to predict maximum possible summertime drawdowns in a key well for 1970-2000. The 2000 summertime head was projected to be 100 feet lower than the record summertime low of 1956.