At present (1996), the largest VOC concentration in surface water of the SCTX study area is within the city of San Antonio where population density is high. Urban development in San Antonio includes a large amount of impervious cover that enhances runoff and thus potential contaminants entering the streams. The streams are hydraulically connected to the Edwards aquifer, which is the sole source of drinking water for the city of San Antonio. Therefore, any contaminants entering the streams potentially could pollute the drinking-water supply.
Detections of VOCs in surface water in the SCTX study area seem to be associated with urban development. As water draining from the catchment area flows through urban development on the recharge area, the possibility of contaminating the Edwards aquifer exists. The number of detections of VOCs in surface-water samples collected in and around San Antonio compared to the relative lack of VOC detections outside the San Antonio area indicate the greater potential for VOC contamination because of increasing development in the recharge zone. Economic growth in the Hill Country is promoting development. Therefore, analyzing surface water near developing cities in the catchment area could facilitate awareness of the presence of VOC contaminants as development increases.
Even though VOC concentrations at NPDES surface-water sites are small (less than 2 mg/L), permit compliance monitoring of VOCs is expected to continue at these sites and at new sites within the San Antonio area. The SCTX NAWQA plans to incorporate these future data into the data analysis. In addition, nine surface-water sites were selected throughout the SCTX study unit as basic fixed sites (Gilliom and others, 1995), including eight sites in the San Antonio region. The locations of the eight surface-water sites in the San Antonio region of the SCTX study unit are shown in figure 11. VOCs would be measured at some of these stations during intensive sampling periods.
On the basis of available data, VOC contamination of water in the Edwards aquifer is greatest at two locations: in the city of San Antonio and at a site in Uvalde County (fig. 8). VOC contamination in ground water could be associated with three sources. In San Antonio the West Avenue landfill and a gasoline service station near Thousand Oaks Drive are coincident with sites of VOC detections. An abandoned industrial laundry facility near the municipal airport at Uvalde, Tex., is at the site of VOC detections in Uvalde County.
The West Avenue landfill site was a municipal solid-waste facility operated by the city of San Antonio from 1967 to 1972. The landfill occupied an old quarry where limestone was mined from rock units overlying the Edwards aquifer. Vertical permeability of the limestone enhanced by faults could increase the potential for vertical flow to the Edwards aquifer from overlying formations. The USGS, in cooperation with the EUWD, began a study in 1981 to determine possible VOC contamination of Edwards aquifer wells. Water from wells near the landfill have detectable concentrations of tetrachloroethene. Since the closing of the landfill in 1972, methane has been detected in a number of methane-collecter wells (Edwards Underground Water District, 1984).
The Thousand Oaks Drive service station was constructed in 1983 in northwest San Antonio. The underground storage tank was filled with unleaded gasoline in September 1983 after the tank had passed an air test for structural integrity. During December 1983, approximately 11,200 gal of gasoline leaked from the underground storage tank. The leak resulted from a hole that developed when the filled tank settled on a large rock in the bedding material. Investigation of the site indicated that an elongated contaminant plume formed in the rock units overlying the Edwards aquifer. Although the tank is not located in the recharge zone, hydrocarbons were detected in some nearby domestic wells. MTBE concentrations in Edwards aquifer wells in the vicinity of the service station range from 2 to 898 mg/L. The migration pathway from the rock units overlying the Edwards aquifer appears to have been crossflow through well bores and flow through faults or fractures in the area (Geraghty and Miller Hydrocarbon Services, 1989).
An industrial laundry facility was located near the present-day municipal airport in Uvalde, from 1966 to 1979. Before the industrial laundry facility was established, the site had been a pipe reclaimer/dealership, a farm machinery dealership, and a retail hardware store. After the industrial laundry facility was destroyed by a fire, the municipal airport was established nearby in 1979. While the airport was being built, a concrete sump tank from the industrial laundry facility was discovered. Sludge samples taken from the sump showed 1,2-dichloroethene concentrations of as much as 208 mg/kg. From 1984 to 1988 the EUWD studied surrounding wells to determine possible contamination of Edwards aquifer wells. In August 1985, three tetrachloroethene compounds were detected in Edwards aquifer wells in the vicinity of the airport. In October 1985, tetrachloroethene was detected in four more Edwards aquifer wells. By November 1985, tetrachloroethene had been detected in 11 Edwards aquifer wells (Edwards Underground Water District, 1988).
The karstic features of the Edwards aquifer, which make the limestone aquifer so productive, also can make it susceptible to contamination. The secondary porosity of the Edwards aquifer creates preferential ground-water flowpaths that enhance the potential for migration of contaminants in the aquifer (R.A. Barker, U.S. Geological Survey, written commun., 1995). VOC contamination of ground water in the SCTX study area appears to be limited to the two localized areas in the city of San Antonio and Uvalde County described above. San Antonio is located primarily on strata that confine the Edwards aquifer, but residential and commercial development has expanded to the outcrop of the Edwards aquifer where some VOCs have been detected. In Uvalde County some development also has occurred on the Edwards aquifer outcrop.
The SCTX NAWQA proposes to sample ground water throughout the SCTX study unit with particular emphasis on areas where the Edwards aquifer crops out. These samples will be analyzed for VOCs. The study would be part of the ground-water study-unit survey to determine baseline water-quality conditions (Gilliom and others, 1995). Synoptic sampling studies also might be done in San Antonio and where VOC concentrations are detected.
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