Samples collected at the six NPDES sites were analyzed for most of the VOCs listed in table 2. Detections were recorded for 9 of the 63 VOCs: 1,2,4-trimethylbenzene, benzene, ethylbenzene, methylene chloride, methyl-tert-butyl ether (MTBE), naphthalene, tetrachloroethene, toluene, and xylene. Number of sites, total number of samples, and number of samples above detection limit per volatile organic compound for NPDES surface-water sites are listed in table 3. The three VOCs with the most detections at NPDES sites are 1,2,4-trimethylbenzene, toluene, and xylene. Concentrations of these compounds are less than 2 mg/L. Figure 6 and table 4 show percent detections of the 9 VOCs detected at each of the NPDES sites. Detections per sampling site ranged from 1 VOC at station 08178430 to 7 VOCs at station 08177720. Three of the VOCs (1,2,4-trimethylbenzene, MTBE, and toluene) detected at station 08177720, San Pedro Avenue at Olmos Creek, were detected in at least 50 percent of the samples collected.
Concentrations of VOCs detected in samples from NPDES surface-water sites grouped by site-associated land use (commercial, residential, or light industrial) and sampling date are listed in tables 5–7. Eight VOCs were detected in 40 samples collected in commercial land-use areas; 4 VOCs were detected in 14 samples collected in residential land-use areas; and 3 VOCs were detected in 8 samples collected in light industrial land-use areas. The boxplots in figure 7 show the range and distribution of VOC concentrations grouped by land use.
The number of VOCs analyzed for in samples from the other USGS surface-water sites varied from site to site, and the total number of samples per VOC varied from 60 to 82. The 12 VOCs detected in these samples comprise 1,1,1-trichloroethane, 1,2,4-trimethylbenzene, bromoform, bromodichloromethane, chloroethane, cis-1,2-dichloroethene, dibromochloromethane, methylene chloride, tetrachloroethene, toluene, trichloroethene, and trichlorofluoromethane (table 8).
Twenty-eight VOCs were detected in water samples from 89 Edwards aquifer wells; the total number of detections per VOC ranged from 1 detection for 7 VOCs to 95 detections for tetrachloroethene, the most commonly detected (table 9). The range in concentration for the detected VOCs are listed in table 9. Locations of Edwards aquifer wells with VOC detections are shown in figure 8. VOCs detected at a given well ranged from 1 to 10. Two wells in San Antonio had 10 VOC detections. The largest VOC concentration was 130 mg/L trichloroethene, and the second largest concentration was 120 mg/L tetrachloroethene.
Percent detections for all VOCs, 5 VOC subgroups, and the 5 most commonly detected VOCs in Edwards aquifer wells are shown in figure 9. Less than 50 percent of the samples had VOC detections. The majority of the detections are halogenated alkanes and alkenes. The five most commonly detected VOCs in Edwards aquifer wells are tetrachloroethene, trichloroethene, bromoform, chloroform, and dibromochloromethane. The range and distribution of concentrations of the five most commonly detected VOCs in Edwards aquifer wells are shown by boxplots in figure 10.
Back to Table of Contents
Forward to Implications for Future Monitoring
| AccessibilityFOIAPrivacyPolicies and Notices | |
| |
|