Formatted Title
Multiple Lines of Evidence to Assess In Situ Remediation Success of Chlorinated Ethenes with Sorption-Supported Degradation
Background/Objectives
Advances in chlorinated ethene assessment techniques, training, and tactics over the past 20 years create the opportunity to build robust conceptual site models (CSMs). Robust CSMs and technological advancements have fostered poignant combined remedies such as colloidal activated carbon coupled with in situ biological and abiotic reduction. Performance monitoring for sorption-supported in situ degradation can be challenging to assess; relying solely on aqueous concentration data as the only performance metric can be misleading due to the mechanisms at play.
At a former dry cleaner presenting unacceptable vapor intrusion (VI) risk to nearby residents in Temperance, Michigan, anthropogenic and hydrogeologic factors led to a shallow, bifurcated groundwater plume of tetrachloroethene (PCE) and related daughter products that intruded the basements of residences by way of sump pump dewatering systems. Decades of sump water operation largely controlled local hydraulic gradients, complicating both near-term vapor mitigation and long-term vapor source reduction. PCE mass storage in saturated soil at concentrations above the soil saturation limit presented a long-term source for VI transport. Guided by a robust CSM, in situ chemical reduction (ISCR) via sulfidated micro zero-valent iron and enhanced reductive dechlorination (ERD) via addition of electron donor and halorespiring bacteria were implemented as the destructive mechanisms for PCE and related daughter products. At many locations these technologies were coupled with colloidal activated carbon to expedite the reduction in chlorinated ethenes in the groundwater. These technologies were implemented in source areas and in barrier-type configurations at mid-plume and distal plume locations.
Groundwater performance monitoring of VOCs and dissolved gasses suggests that greater than 95% of the chlorinated ethene mass has been destroyed. Groundwater monitoring along source – transport route – and receptor flow paths (i.e., CSM) demonstrate compliance with stringent VI screening levels. Beyond groundwater, project stakeholders were concerned that underlying clay previously demonstrated to be contaminated could allow for PCE diffusion back into the groundwater, ultimately causing rebound and long-term risk. A saturated soil and underlying clay post remediation performance assessment utilizing membrane interface probe (MIP) with hydraulic profile tool (HPT) and multiple detectors (e.g., PID, FID, XSD) was performed. In addition, confirmatory saturated soil and underlying clay samples were collected and analyzed for chlorinated ethenes.
Approach/Activities
Multiple high-resolution site characterization (HRSC) techniques were used, including passive soil gas surveys, geophysical techniques, and discrete saturated soil analysis to refine the CSM and guide a surgical in situ remediation design/implementation. Post remediation performance monitoring focused on not only aqueous concentrations of chlorinated ethenes and dissolved gases but also soil vapor, saturated soil, and MIP-generated data. The MIP-generated data included chlorinated ethenes via the halogen-specific detector (XSD) and the presence of ethene/methane via the FID. These data were corroborated with laboratory analysis.
Results/Lessons Learned
Post-remediation groundwater concentrations at the site have shown a greater than 95% reduction in chlorinated ethenes with groundwater detections at the receptor below regulatory VI screening levels. Long-term VI risk was a concern due to the potential for diffusion out of the underlying clay and into the aqueous phase. A comparison of both pre-remediation and post-remediation saturated soil and underlying clay sample analytical in conjunction with MIP logs will be presented which show that remediation at the site is protective of long-term VI risk along the source, transport, receptor pathway.