Water-chemistry data from the Edwards aquifer for 1976-85, consisting of nearly 1,500 chemical analyses from 280 wells and 3 springs, were used to statistically evaluate relations among ground-water chemistry, hydrogeology, and land use. Five land uses associated with sampled wells were classified on the basis of published information and field surveys. Four major subareas of the aquifer were defined to reflect the relative susceptibility of ground water to contamination originating from human activities using hydrogeologic and tritium data.

Water from an agricultural area over the unconfined zone of the aquifer had the largest median concentration of nitrite plus nitrate. Large nitrite plus nitrate concentrations were spatially associated with large tritium concentrations and nitrogen isotopic ratios characteristic of streamflow recharge. Detections of fecal-coliform bacteria were associated mainly with water from wells completed in the unconfined zone.

Most of the occurrences of tetrachloroethylene, l,2-(trans)-dichloroethylene, trichlorofluoromethane, 1,1,1-trichloroethane, and 2,4-D in ground water were associated with wells completed in the unconfined zone of the aquifer. Fatty acids detected in water from some wells may commonly be present naturally in ground water.

The percentage of samples in which arsenic, barium, lead, and zinc were detected was similar among subareas; the samples were from the freshwater parts of the aquifer. Large lead and zinc concentrations were associated with volumes of pumpage less than 1,000 gallons.

In general, the quality of ground water in the freshwater parts of the aquifer (north of the "bad-water" line) is suitable for all uses including human consumption. Two areas that are exceptions are: (1) Northeast of Garner Field in Uvalde, Texas, where PCE (tetrachloroethylene) has been detected in groundwater samples, and (2) north-central Bexar County near the former West Avenue landfill where PCE and benzene have been detected in ground-water samples. Concentrations of these organic compounds in water from many wells in the two areas exceed the maximum contaminant level for human consumption set by the U.S. Environmental Protection Agency.