During 2011–12, the U.S. Geological Survey, in cooperation with the San Antonio River Authority, evaluated detections, concentrations, and distributional patterns of selected compounds of emerging concern (hereinafter referred to as “CECs”) from water-quality samples (hereinafter referred to as “samples”) collected at a total of 20 sampling sites distributed throughout the San Antonio River Basin, Texas. Of the 54 wastewater compounds analyzed, 32 were detected in at least one sample collected from the San Antonio River Basin, and 22 of those compounds were not detected in any samples. The flame retardants tris (2-chloroethyl) phosphate and tris (dichloroisopropyl) phosphate, both possible endocrine disruptors, were the most frequently detected wastewater compounds with 28 of the 33 samples analyzed for wastewater compounds having measureable concentrations of those compounds. Of the 13 analyzed pharmaceuticals, 4 compounds were detected in a least one sample. Carbamazepine, an anticonvulsant, was the most frequently detected prescription pharmaceutical with 24 detections in 34 samples analyzed for pharmaceuticals. Of the 17 steroidal hormones, 4 were detected in at least one sample from the San Antonio River Basin. Estrone was detected in 9 of 34 samples analyzed for steroidal hormones, making it the most frequently detected steroidal hormone. Of the 4 sterols, all 4 were detected in at least one sample from the San Antonio River Basin. Cholesterol, detected in 19 of 34 samples analyzed for sterols, was the most frequently detected sterol.

Three synoptic sampling events were completed as part of this study. The first and second synoptic sampling events included samples collected at the same 12 sampling sites. During the first and second synoptic sampling events, the lowest number of detections (2 and 0, respectively) and the lowest total concentrations of all measured compounds (0.62 and not measureable, respectively) occurred in samples collected at the Macdona site (Medina River near Macdona, Tex.). The highest number of detections (21 and 23, respectively) and highest total concentrations of all measured compounds (7.75 and 3.97 micrograms per liter [µg/L], respectively) occurred in samples collected at the SAR Elmendorf site (San Antonio River near Elmendorf, Tex.). The third synoptic sampling event included samples collected at seven sites that were added to the study after the first two synoptic sampling events were completed. During the third synoptic sampling event, the lowest number of detections (two) and the lowest total concentration (0.14 µg/L) of compounds were measured in samples collected at the North Prong site (North Prong Medina River above confluence Wallace Creek near Medina, Tex.). The highest number of detections (21) occurred at the SAR Mitchell site (San Antonio River at Mitchell Street, San Antonio, Tex.). The Dos Rios site (the Dos Rios wastewater treatment plant outfall at San Antonio, Tex.) had the highest total concentration of all measured compounds (4.37 µg/L) in the third synoptic sampling event. Because Ecleto Creek flows only intermittently at the Ecleto site (Ecleto Creek near Runge, Tex.), samples from the Ecleto site were collected at different times than were samples from the other sites and were not included in a synoptic sampling event. The presence of wastewater compounds at the Ecleto site indicates that at least some wastewater compounds can be introduced into surface waters in rural parts of the San Antonio River Basin during runoff or because of onsite wastewater system seepage. The steroidal hormone and sterols detected at the Ecleto site, including estrone, cholesterol, beta-sitosterol, and beta-stigmastanol, likely were derived from cattle waste rather than from wastewater effluent.

The distributional patterns of detections and concentrations of individual compounds and compound classes show the influence of wastewater-treatment plant (WWTP) outfalls on the quality of water in the San Antonio River Basin. In the Medina River Subbasin, the minimal influence of wastewater is evident as far downstream as the Macdona site. Downstream from the Macdona site, the Medina River receives treated municipal wastewater from both the Medio Creek Water Recycling Center site from an unnamed tributary at the plant and the Leon Creek Water Recycling Center site from Comanche Creek at the plant, and corresponding increases in both the number of detections and the total concentrations of all measured compounds at all downstream sampling sites were evident. Similarly, the San Antonio River receives treated municipal wastewater as far upstream as the SAR Witte site (San Antonio River at Witte Museum, San Antonio, Tex.) and additional WWTP outfalls along the Medina River upstream from the confluence of the Medina and San Antonio Rivers. Consequently, all samples collected along the main stem of the San Antonio River had higher concentrations of CECs in comparison to sites without upstream WWTPs. Sites in urbanized areas without upstream WWTPs include the Leon 35 site (Leon Creek at Interstate Highway 35, San Antonio, Tex.), the Alazan site (Alazan Creek at Tampico Street, San Antonio, Tex.), and the San Pedro site (San Pedro Creek at Probandt Street, at San Antonio, Tex.). The large number of detections at sites with no upstream wastewater source demonstrated that CECs can be detected in streams flowing through urbanized areas without a large upstream source of treated municipal wastewater. A general lack of detection of pharmaceuticals in streams without upstream outfalls of treated wastewater appears to be typical for streams throughout the San Antonio River Basin and may be a useful indicator of point-source versus nonpoint-source contributions of these compounds in urban streams. Observations of lower concentrations of compounds at the furthest downstream sampling sites in the basin indicate some natural attenuation of these compounds during transport; however, a more focused assessment is needed to make this determination.