CONTAMINANTS AS TRACERS: UTILIZING ENVIRONMENTAL SIGNALS TO CHARACTERIZE REGIONAL CONTRIBUTIONS TO A KARST SPRING SYSTEM

The Edwards (Balcones Fault Zone) Aquifer (Edwards BFZ) is a large karst aquifer in south-central Texas that serves over two million people in addition to various endangered and protected species. The Edwards Aquifer Authority (EAA) is a regional groundwater conservation district assigned to manage, enhance, and protect the aquifer system. Comal Springs, in New Braunfels, Texas, is a major discharge feature of the Edwards BFZ Aquifer. The springs comprise multiple major discharge orifices and represent a composite of discharge from recharge areas totaling more than 3150 km² spread across six counties. Identifying relative contributions of flow from different parts of the Edwards BFZ Aquifer to Comal Springs is an important component of understanding the vulnerability of the springs to contamination or loss of flow from development and pumping. Per- and polyfluoroalkyl substances (PFAS) are a group of man-made chemicals that are environmentally persistent, readily transportable in groundwater, and widely detected in environmental settings across the U.S. As part of an established water quality sampling program, the EAA has sampled for PFAS in groundwater, surface water, and spring water over the past several years.

PFAS are detected in waters of the Edwards BFZ Aquifer and Comal Springs at concentration levels below human health concern (PFOA+PFOS below 6 ng/L) but with consistency and spatial variability that suggest PFAS may be used as tracers of opportunity to evaluate the aquifer flow system. Concentrations of detected PFAS in Comal Springs discharge have remained relatively stable PFAS data from samples collected within the saturated zone of Edwards BFZ Aquifer indicate the highest concentrations occur in wells near the recharge zone in developed urban and suburban areas, which is consistent with detections of other contaminants in the aquifer system. Simple mixing calculations indicate the contribution of PFAS from this area of the recharge zone is larger than would be expected based on estimates of volumetric recharge. This suggests, contrary to the current conceptualization that incorporates significant dilution, local flow paths from areas vulnerable to contamination may have a disproportionate effect on spring water quality.