Theme: 07. Per- and Polyfluorinated Alkyl Substances (PFAS)

Topic Area: 7h. PFAS Fate and Transport

Formatted Title:

Bust a Move: Can the Dynamic Dance of Pumping and Reinjection Accelerate PFAS Removal?

Background/Objectives:

Aquifers provide drinking water to 38% of the residents of the United States, but this crucial resource is threatened by recalcitrant contaminants that resist cleanup via conventional groundwater remediation technology. In recent years, no class of recalcitrant contaminants has generated more concern than per- and polyfluoroalkyl substances (PFAS), whose undisputed usefulness in industrial, commercial, and retail products drives enormous production rates that inevitably contaminate the environment including aquifers. Because no in situ cleanup technology currently exists for PFAS, remediation has generally relied on pump and treat with ex situ treatment for PFAS-impacted aquifers. This prompts us to revisit a classical problem in groundwater remediation: How can we optimize mass removal via pump and treat?

Approach/Activities:

This work tests the hypothesis that the combination of dynamic pumping with targeted reinjection of treated/clean water can accelerate cleanup of PFAS via enhanced flushing of contaminant mass from each portion of the impacted aquifer—the pure advection/transport, slow advection/storage, and pure storage zones (i.e., the three-compartment conceptual model). Comprising both academic research on engineered injection and extraction (EIE) and field experience with dynamic groundwater recirculation (DGR™), we present an inventive design approach to optimize mass removal at PFAS-impacted sites. As a case study, we model the subsurface hydraulics at a U.S. Department of Defense Superfund site in the Southwestern United States to compare mass removal via conventional, steady pump-and-treat schemes to innovative EIE/DGR™ strategies. Using high-resolution aquifer characterization data within the context of the three-compartment model, we model the various groundwater flow conditions and corresponding mass removal using MODFLOW in conjunction with the volumetric-tracking/flow-allocation code MODALL using a new combined metric we term the plume capture function (PCF).

Results/Lessons Learned:

Modeling results suggest two key conclusions. First, mass removal is greatly improved with application of an EIE/DGR™-based remedy; this improvement is expected to enhance current PFAS remediation strategies, which will be the subject of future modeling work and field studies. Second, modeling results suggest that, in many respects, PFAS behaves like other recalcitrant groundwater contaminants. The principal challenge, however, is working with the exceptionally low PFAS concentration standards applied at many remediation sites: reducing concentrations of traditional contaminants from 1 ppm to 1 ppb is relatively easy but achieving the same three-log reduction 10 ppb to 10 ppt is harder. As concentrations become fleetingly small, transport mechanisms begin to unravel.

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Roseanna Neupauer