Occurrence of pharmaceutical compounds in the San Antonio segment of the Edwards (Balcones fault zone) aquifer, south-central Texas, June 2018–August 2020

The occurrence of pharmaceutical compounds (hereinafter referred to as “pharmaceuticals”) in surface water that recharges the San Antonio segment of the Edwards aquifer in south-central Texas is of concern with respect to potential effects on groundwater quality. This study, conducted during June 2018–August 2020 by the U.S. Geological Survey in cooperation with the San Antonio Water System, used a source-to-sink approach (that is, from aquifer recharge to aquifer discharge) to compare the occurrences and concentrations of pharmaceuticals in recharging (losing) streams to those in relatively shallow and relatively deep groundwater. Individual pharmaceutical concentrations in surface water were all less than 200 nanograms per liter, and most were less than 50 nanograms per liter. The two most common pharmaceuticals in surface water were metformin (50-percent detection frequency) and caffeine (25-percent detection frequency). In contrast to surface-water sites, few detections of pharmaceuticals above the detection limit were quantified at groundwater sites. On the basis of studied sample sites that are considered to be representative of the Edwards aquifer recharge zone, groundwater results collectively indicate that pharmaceuticals currently (2020) do not substantially impair water quality in the shallow unconfined part of the Edwards aquifer. Although groundwater pharmaceutical detections were not common, two detections of acetaminophen occurred in samples collected from a relatively deep confined part of the aquifer, indicating that this part of the aquifer is potentially vulnerable to pharmaceutical contamination. The near absence of wastewater treatment plant discharges and Texas Land Application Permit facilities within the drainage areas of streams in this study could explain reduced inputs of pharmaceuticals relative to pesticides on the Edwards aquifer recharge zone and their reduced occurrence in unconfined groundwater. In the western part of the study area, numerous pharmaceutical detections in the Frio River indicate that pharmaceutical sources in rural areas also exist with potential water-quality effects. Although reported pharmaceutical concentrations for the San Antonio segment of the Edwards aquifer are currently (2020) low, ongoing development in the region has the potential to increase contaminant loads, including for pharmaceuticals.