Analyses of samples collected from Barton Springs at approximately weekly Intervals and from Barton Creek and five wells in the Austin area during selected storm-runoff periods generally show that recharge during storm runoff resulted in significant temporal and area! variations in the quality of ground water in the recharge zone of the Edwards aquifer. Recharge during storm runoff resulted in significant increases of bacterial densities in the ground water. Densities of fecal coliform bacteria in samples collected from Barton Springs, the major point of ground-water discharge, ranged from less than 1 colony per 100 milliliters during dry weather in November 1981 and January and August 1982 to 6,100 colonies per 100 milliliters during a storm in May 1982. Densities of fecal streptococcal bacteria ranged from 1 colony per 100 miniliters during dry weather in December 1981 to 11,000 colonies per 100 miniliters during a storm in May 1982.

Recharge during storm runoff resulted in significant decreases in the specific conductance and the concentration of total nitrate nitrogen in the ground water. Specific-conductance values of samples from Barton Springs ranged from 438 micromhos per centimeter at 25° Celsius after a storm in October 1981 to 682 micromhos after a relatively long period of deficient rainfall in September 1982. The specific-conductance values and, thus, the mineralization of the ground water in the recharge zone generally were inversely related to the quantity of recharge. Nitrate nitrogen was the most prevalent form of nitrogen in the ground water. Concentrations of total nitrate nitrogen in samples from Barton Springs ranged from 0.51 milligram per liter after a storm in October 1981 to 1.6 milligrams per liter during dry weather in February and September 1982.

Although the values of these and other properties or constituents in ground water varied temporally and areally, available data indicate that the values of most of the major and minor elements in ground water in the recharge zone of the Edwards aquifer were significantly less than the primary maximum or secondary maximum contaminant levels set by the U.S. Environmental Protection Agency for public water systems.

Bacteriological data for Barton Springs and selected wells indicate that the ground water in the aquifer is susceptible to bacterial pollution, especially during storm runoff. The water may require disinfection if used for drinking or culinary purposes.