INVESTIGATION OF A POTENTIAL HEXAVALENT CHROMIUM PLUME AT A NATIONAL PRIORITIES LIST (SUPERFUND) SITE OVER THE EDWARDS AQUIFER

The San Antonio segment of the Edwards Aquifer in south-central Texas, USA, is a large and prolific karst aquifer that serves as a sole source aquifer to many communities in the region and provides springflow that supports several endangered and threatened species. A small metal plating facility that operated from 1994–2002 created significant heavy metal contamination, including hexavalent chromium (Cr⁶⁺), at the surface and was subsequently referred to the National Priorities List (NPL) in 2018 after Cr⁶⁺ and cyanide were detected in nearby Edwards Aquifer wells.

The Edwards Aquifer Authority is a regional groundwater management agency and is responsible for monitoring water quality in the aquifer. As a result of the site’s Superfund designation, we embarked on an effort to characterize the aquifer system in the vicinity of the site, collect water quality samples, and evaluate the potential sources of chromium in the aquifer. Using available well-logs, previous models of the aquifer structure, and experience from similarly faulted zones, we constructed a 3-D structural model of the area. Data from previous U.S. EPA Unregulated Contaminant Monitoring Rule sampling was combined with new water quality analyses and aquifer rock geochemistry to assess the presence of chromium in the aquifer system. Data shared by cooperating agencies such as the U.S. Geological Survey, Texas Commission on Environmental Quality, Texas Department of State Health Services, and the U.S. EPA and its consultants provided information on potential migration of Cr⁶⁺ at the site. This information included results of samples from soil borings and shallow and intermediate groundwater at the site.

After nearly four years of data collection and analyses, we concluded that the Edwards Aquifer was not adversely impacted. Several factors combined to produce the original “unexpected” detections, but the potential for the site to have been a contaminant source was real. A preferred path forward and resolution of issues at the site were recommended by the U.S. EPA and accepted by the cooperating agencies, and the site was removed from the NPL in 2023. This site investigation, which was facilitated by strong interagency cooperation, is a great example of a robust and risk-informed assessment of potential contamination in an important aquifer system.