Formatted Title

The HRX Well® for Effective Long-Term In Situ cVOC and PFAS Mass Discharge Control at Three Sites

Background/Objectives

Contaminant mass flux and mass discharge represent the most appropriate measure of plume dynamics and risk to receptors and, therefore, remedial technologies focusing on long-term mass flux/discharge reduction are increasingly favored, especially at low-permeability sites where implementation of injection-based in situ treatment is more challenging. The Horizontal Reactive Treatment Well (HRX Well^®) is a new in situ remediation concept that is particularly well-suited for sites where passive long-term mass discharge reduction control is a primary performance objective. The HRX Well utilizes horizontal wells oriented parallel to groundwater flow. The design leverages natural “flow-focusing” induced by the engineered hydraulic conductivity contrast between the aquifer and the well to passively capture groundwater.  Groundwater is treated in situ as it flows through the treatment media in the well. It then exits the horizontal well along its down-gradient screen and the downgradient positions of the aquifer cleaned over time. Many different types of granular treatment media are already available (e.g., zero valent iron [ZVI], granular activated carbon [GAC]) additionally, miniaturized reactors that utilize sonolytic, electrochemical, and other processes could be modified to be deployed within an HRX Well. Therefore, this approach could address a wide range of contaminants, including difficult-to-treat contaminants like chlorinated volatile organic compounds (cVOCs), metals, and per- and polyfluoroalkyl substances (PFAS).

Approach/Activities

Full-scale HRX Wells have been installed and operated continuously at three sites: at Site 1 for cVOCs using ZVI since 2018, at Site 2 PFAS using both GAC and sonolysis since 2021, and at Site 3 cVOCs using ZVI and GAC since 2022. These wells have been operated in both fully passive and active (flow enhancement via pumping) configurations.

Results/Lessons Learned

In general, the field results demonstrate that under passive operation capture and treatment widths of tens of feet can be achieved for many low-permeability aquifer settings, and that reductions in contaminant mass flux are nearly immediate. In higher permeability settings capture can be increased through pumping.  At Site 1, the capture and treatment zone widths exceed 50 feet for a single HRX Well, contaminant mass flux within this zone is reduced by >99.9% and concentrations at downgradient performance wells within the treatment zone have reduced by an average of more than 60%.  At Site 2, the capture and treatment zone widths are 20 to 40 ft with sustained reductions in PFAS mass flux with concentration decreases observed in three of four nearby monitoring wells.  At Site 3, capture and treatment zone exceed 70 ft, with cVOC concentration decreases observed in three nearby monitoring wells. The HRX Well offers several technical and practical advantages over conventional permeable reactive barrier (PRB) treatment schemes, including highly efficient treatment media usage, the ability to use multiple media types in series (i.e., treatment trains), and easy/inexpensive media replacement. The HRX Well requires minimal aboveground footprint, can access deep zones and contamination under surface infrastructure, and consumes no/minimal energy and water.