Multi‐Year Passive In Situ Treatment of Per‐ and Polyfluoroalkyl Substances (PFAS) With an HRX Well®

ABSTRACT The Horizontal Reactive Treatment Well (HRX Well®) is a relatively new remediation technology that can passively treat groundwater and reduce contaminant mass discharge within a contaminant plume. The HRX Well is a large‐diameter horizontal well that captures impacted groundwater through an upgradient screen, treats the groundwater via treatment media placed in the center cased section of the well, and then discharges treated water back to the aquifer through a downgradient screen. Herein, the results of the first field demonstration of an HRX Well designed to treat per‐ and polyfluoroalkyl substances (PFAS) are presented. Based on the results of treatability tests and numerical design modeling, the 645‐ft long HRX Well was constructed with removable cartridges containing granular activated carbon (GAC) for passive PFAS treatment. It has operated continuously with minimal operation and maintenance activity for more than 3 years. Total PFAS treatment efficiencies ranged from 53% to 74%, resulting in a sustained and average PFAS mass discharge reduction of approximately 5 mg/day. The HRX Well compares favorably (effectiveness, implementability, and lifecycle costs) to conventional alternatives, such as pump‐and‐treat. The estimated treatment zone width is approximately 20 ft, which could be increased by pumping. Multiple HRX Wells could be installed to address a wide treatment zone, and HRX Wells could be paired with other technologies in an overall plume treatment strategy. This study highlights the importance of understanding groundwater dynamics and PFAS concentration trends when designing the HRX Well and interpreting results. Furthermore, this demonstration indicates it may be prudent to design most HRX Wells with an active (pumping) configuration option. This feature adds only modest cost, but allows more operational control (i.e., the ability to specify flow rates) and increases capture. There are only a few demonstrated technologies for the treatment of PFAS‐impacted groundwater, and these results affirm that HRX Wells may be a viable In Situ remediation technology for PFAS at some sites.