At present (1996), VOC contamination in the SCTX study area appears to be associated with urban development. Analysis of VOCs in surface water, primarily from NPDES urban-stormwater sampling sites in San Antonio, indicates that the three most commonly detected VOCs are 1,2,4-trimethylbenzene, toluene, and xylene. Detections of VOCs in water from the Edwards aquifer are limited to two localized areas in the SCTX study area: one in the city of San Antonio and one in Uvalde County. The five most commonly detected VOCs in water samples from the Edwards aquifer, in descending order, are tetrachloroethene, trichloroethene, bromoform, chloroform, and dibromochloromethane.
The fractured nature of the limestone in the study area increases the potential for contamination of the Edwards aquifer. Any VOCs entering the recharge zone from streams originating in the catchment area or direct infiltration can enter the Edwards aquifer immediately and contaminate the water supply. The SCTX NAWQA plans to sample and analyze for VOCs in surface water and ground water throughout the study area with emphasis on areas of development. VOCs would be monitored at NPDES sites, at SCTX NAWQA basic fixed sites, and in the SCTX NAWQA ground-water study-unit surveys to determine baseline water-quality conditions.
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