Abstract

Elevated concentrations of sediment-associated contaminants are typically associated with urban areas such as San Antonio, Texas, in Bexar County, the seventh most populous city in the United States. This report describes an assessment of selected sediment-associated contaminants in samples collected in Bexar County from sites on the following streams: Medio Creek, Medina River, Elm Creek, Martinez Creek, Chupaderas Creek, Leon Creek, Salado Creek, and San Antonio River. During 2007–09, the U.S. Geological Survey periodically collected surficial streambed-sediment samples during base flow and suspended-sediment (large-volume suspended-sediment) samples from selected streams during stormwater runoff. All sediment samples were analyzed for major and trace elements and for organic compounds including halogenated organic compounds and polycyclic aromatic hydrocarbons (PAHs). Selected contaminants in streambed and suspended sediments in watersheds of the eight major streams in Bexar County were assessed by using a variety of methods—observations of occurrence and distribution, comparison to sediment-quality guidelines and data from previous studies, statistical analyses, and source indicators.

Trace elements concentrations were low compared to the consensus-based sediment-quality guidelines threshold effect concentration (TEC) and probable effect concentration (PEC). Trace element concentrations were greater than the TEC in 28 percent of the samples and greater than the PEC in 1.5 percent of the samples. Chromium concentrations exceeded sediment-quality guidelines more frequently than concentrations of any other constituents analyzed in this study (greater than the TEC in 69 percent of samples and greater than the PEC in 8 percent of samples). Mean trace element concentrations generally are lower in Bexar County samples compared to concentrations in samples collected during previous studies in the Austin and Fort Worth, Texas, areas, but considering the relatively large ranges and standard deviations associated with the concentrations measured in all three areas, the trace element concentrations are similar. On the basis of Mann-Whitney U test results, the presence of a military installation in a watershed was associated with statistically significant higher chromium, mercury, and zinc concentrations in streambed sediments compared to concentrations of the same elements in a watershed without a military installation.

Halogenated organic compounds analyzed in sediment samples included pesticides (chlordane, dieldrin, DDT, DDD, and DDE), polychlorinated biphenyls (PCBs), and brominated flame retardants. Three or more halogenated organic compounds were detected in each sediment sample, and 66 percent of all concentrations were less than the respective interim reporting levels. Halogenated organic compound concentrations were mostly low compared to consensus-based sediment quality guidelines—TECs were exceeded in 11 percent of the analyses and PECs were exceeded in 1 percent of the analyses. Chlordane compounds were the most frequently detected halogenated organic compounds with one or more detections of chlordane compounds in every watershed; concentrations were greater than the TEC in 6 percent of the samples. Dieldrin was detected in 50 percent of all samples, however all concentrations were much less than the TEC. The DDT compounds (p,p'-DDT, p,p'-DDD, and p,p'-DDE) were detected less frequently than some other halogenated organic compounds, however most detections exceeded the TECs. p,p'-DDT was detected in 13 percent of the samples (TEC exceeded in 67 percent); p,p'-DDD was detected in 19 percent of the samples (TEC exceeded in 78 percent); and p,p'-DDE was detected in 35 percent of the samples (TEC exceeded in 53 percent). p,p'-DDE concentrations in streambed-sediment samples correlate positively with population density and residential, commercial, and transportation land use. One or more PCB congeners were detected in 90 percent of the samples. Additionally, total PCB concentrations in streambed-sediment samples correlated positively with residential, industrial, and mining land uses. The PCB mixtures in the samples from Leon Creek, Salado Creek, and the San Antonio River were dominated by PCB congeners 138, 180, and 187. The similar PCB congener patterns suggest similar PCB sources in the three watersheds. The results of the Mann-Whitney U test indicate that the presence of a military installation in a watershed was associated with statistically significant higher total chlordane, p,p'-DDE, and total PCB concentrations. Brominated flame retardant compounds were detected most frequently in samples from the Medio Creek, Leon Creek, and San Antonio River watersheds. No sediment-quality guidelines exist for brominated flame retardant compounds. Mean halogenated organic compound concentrations generally are lower in Bexar County samples compared to concentrations in samples collected during previous studies in the Austin and Fort Worth areas.

The PAHs were detected most frequently in samples from the Leon Creek and San Antonio River watersheds. Total PAH concentrations exceeded the TEC in 14 percent of the samples and none of the concentrations exceeded the PEC. Total PAH concentrations in streambed-sediment samples correlate positively with population density; residential, commercial, transportation, and mining land-use categories; and industrial wastewater outfall, wastewater liftstation, and petroleum storage tank potential source of contamination categories. Mean total PAH concentrations generally are lower in Bexar County samples compared to concentrations in samples collected during previous studies in the Austin and Fort Worth areas, but considering the large ranges and standard deviations associated with the concentrations measured in all three areas, total PAH concentrations are similar. PAH sources were investigated by comparing the ratios of individual PAH compounds to total PAH of several common PAH sources, such as coal combustion (power plants), automobile-related sources, and coal-tar parking lot sealcoat, and the Bexar County streambed- and suspended-sediment samples. The contribution from each PAH source was estimated by using a chemical mass balance model, which indicated that parking lot coal-tar sealcoat dust is the largest PAH source to the average Bexar County sediment, accounting for 70.2 to 78.9 percent of the PAHs in the mixture.

The overall degree of sediment contamination of the Bexar County sediment samples was assessed by using the mean PEC quotient. The incidence of sediment toxicity is highly correlated to the mean PEC quotient. Samples from Leon Creek at Lackland Air Force Base, Leon Creek at Quintana Road, Leon Creek at Interstate Highway 35, Leon Creek at Southwest Military Drive (Loop 13), and San Antonio River at Loop 410 (in order of decreasing values) had the highest mean PEC quotients.

First posted July 26, 2011