Three sites with different upstream land use in the San Antonio region of the SCTX NAWQA study unit (study area) were selected as intensive fixed sites. The sampling strategy for intensive fixed sites is based on obtaining information about the occurrence and seasonal patterns of selected water-quality constituents, which include nutrients, pesticides, and VOCs. The three intensive fixed sites selected to determine the effects of agriculture and urbanization on surface-water quality in the study area are Medina River at LaCoste (agriculture indicator site), Salado Creek (lower station) at San Antonio (urban indicator site), and San Antonio River near Elmendorf (integrator site). Surface-water quality is a major issue in the SCTX study unit because of the potential for contamination of the Edwards aquifer.
Samples were collected at the three intensive fixed sites during April 1996-April 1998. Sample collection usually was done when pesticides and VOCs were likely to be detected. Most samples were collected monthly, although pesticide sampling was more frequent during the spring, and VOC sampling was more frequent during the winter. Some samples were collected with autosamplers, which can collect composite samples during storms. Quality-control samples comprised field blanks, equipment blanks, replicates, trip blanks, and field matrix spikes.
Seasonal patterns in nitrite plus nitrate nitrogen concentrations were not evident at all three intensive fixed sites. Nitrite plus nitrate nitrogen concentrations often exceeded the MCL (10 mg/L) for drinking water at the integrator site, which is downstream of municipal wastewater treatment plants. Nitrite plus nitrate nitrogen concentrations in samples collected during stormflow were smaller than concentrations in samples collected during base flow at the agriculture and integrator sites.
Total phosphorus concentrations at the agriculture indicator site were mostly less than 0.10 mg/L, the maximum concentration recommended by EPA for streams not discharging directly into reservoirs, and the majority of concentrations at the urban indicator site also were less than 0.10 mg/L. All total phosphorus concentrations at the integrator site were greater than 0.10 mg/L.
Samples collected at the three intensive fixed sites were analyzed for 84 pesticides. Concentrations of some pesticides detected tended to increase in the spring during peak pesticide application. More pesticides were detected at the urban indicator site and at the integrator site than at the agriculture indicator site. Four pesticides (atrazine, deethylatrazine, diazinon, and prometon) were detected in at least 38 percent of samples at all sites. The largest concentrations of atrazine (0.75 mg/L), diazinon (0.33 mg/L), and prometon (0.24 mg/L) were at the urban indicator site. The concentrations of detected pesticides that have an MCL were less than that MCL at all three intensive fixed sites.
Samples collected for VOCs were analyzed for 86 compounds. More VOCs were detected at the urban indicator site than at the agriculture indicator site, most likely because more sources of VOCs are located in urbanized areas. The most VOCs detected (33 in 13 samples) and the largest concentrations of two VOCs (chloroform, 4.6 mg/L and tetrahydrofuran, 3.9 mg/L) were in samples collected at the integrator site. Some of the VOCs detected (THMs), especially at the integrator site, could be associated with the chlorination process at wastewater treatment plants in the watershed. More VOCs were detected in samples collected at the integrator site during winter storm events than during base flow. The concentrations of all detected VOCs that have an MCL were less than the MCL at all three intensive fixed sites.
Sampling at the three intensive fixed sites during April 1996-April 1998 indicates that more detections and larger concentrations of nutrients, pesticides, and VOCs tend to be associated with an urban or integrated urban setting. Continued sampling of these constituents at the sites could provide additional information on changes in surface-water quality with urban development.
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