Theme: 04. Addressing Challenging Site Conditions

Topic Area: 4e. DNAPL Source Zone Remediation: Lessons Learned

Formatted Title:

Recalcitrant DNAPL Source Zone Characterization and Remediation

Background/Objectives:

Remedial investigation and actions at this industrial facility date back to the late 1980s. Historical releases of chlorinated ethenes (CEs) to the subsurface resulted in impacts to soils and groundwater at the site located within the Atlantic Coastal Plain physiographic province of New Jersey. Contamination has impacted the Wenonah formation (sand, some silt and clay) to ~10 feet below grade surface (ft bgs) and the Marshalltown formation (dense clay with trace silt and sand) from ~10 to 60 ft bgs, which was historically assumed to serve as an aquitard (K=~9.9E-7 cm/s) for vertical migration. Previous attempts to remediate the soil and groundwater were incomplete and insufficient where localized groundwater concentrations continued to be indicative of residual source material nearby. The primary reasons for poor performance of prior remediation efforts were a limited understanding of the impacted volumes, the contaminant mass bound in the soils, and access constraints.

Approach/Activities:

A high-resolution (direct sensing and high-density sampling) remedial design investigation (RDI) was completed in the source area to close data gaps, greatly improve the Conceptual Site Model (CSM), and optimize the treatment volume. A targeted remediation program was designed and implemented for in-situ chemical reduction and in-situ bioremediation of the CE source area residing within the low permeability unit. High pressure injection techniques were employed to emplace a combination of zero valent iron, sodium lactate, nutrients, pH buffer, and bioaugmentation culture.

Results/Lessons Learned:

The RDI identified significant concentrations in soil (Total CEs >22,700 mg/kg) and groundwater (Total CEs >180,000 µg/L) in a highly limited area extending to depths of approximately 40 ft bgs, which were previously unknown. The presentation will discuss source zone RDI strategies and lessons learned, empirical soil organic carbon content water partitioning coefficients, and DNAPL behavior in a low permeability formation as they relate to remedial design ramifications. Contaminant reductions of 90 to 99.9% were observed with little to no rebound at one year following treatment. The talk will present the challenges of balancing amendment loadings with formation acceptance, optimization of lateral and vertical spacing, injection/distribution monitoring, and performance evaluation extending through approximately 18 to 24 months after treatment.

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Reva Pepalla