Theme: 10. Characterization, Fate and Transport

Topic Area: 10h. HRSC Suites of Tools to Improve CSMs

Formatted Title:

Using HRSC Techniques to Identify Contaminant Transport Pathways and Revise 3D CSM for Remedy Re-Evaluation

Background/Objectives:

Operable Unit (OU) 2 at Hill Air Force Base Superfund site consists of an approximately 20-acre chlorinated solvent groundwater plume originating from a dense nonaqueous phase liquid (DNAPL) source area; the primary contaminant of concern is trichloroethene (TCE). The existing site conceptual site model (CSM), developed through extensive site investigation activities in the 1990s and 2000s, understood the source area contamination as being restricted to sands and gravels within a buried paleochannel incised into an underlying low-permeability clay unit that restricted downward contaminant migration. As such, a vertical containment wall encircling the source area and associated source recovery well extraction field were constructed in 1996 to hydraulically control groundwater flow into and out of the source area and recover DNAPL. The source area extraction system has since recovered much of the pooled DNAPL; however, the DNAPL continues to source the downgradient TCE groundwater plume that flows downslope within a landslide zone. Long-term plume monitoring has indicated that the higher concentration portion of the TCE plume (i.e., 1,000 to 10,000 µg/L) is expanding from the source containment area and downslope. Contaminant migration pathways through the containment area and the low-permeability clay unit were not supported by the CSM. As such, a Supplemental Site Investigation (SSI) was performed using high-resolution site characterization (HRSC) techniques to identify contamination transport mechanisms and update the CSM.

Approach/Activities:

HRSC activities at OU 2 included membrane interface probe-hydraulic profiling tool (MIP-HPT) and dye-laser induced fluorescence (dye-LIF) techniques to collect detailed vertical contamination profiles and formation hydraulics to identify localized migration pathways from the source area. A total of 26 MIP-HPT borings were completed to depths up to 55 feet below ground surface (bgs) across a transect where the higher concentration source area plume is expanding and near contaminated seeps and springs along hillslope slump block slide planes to assess potential migration pathways. The MIP-HPT collected photoionization detector (PID), flame ionization detector (FID), halogen specific detector (XSD), electrical conductivity (EC), and hydraulic profiling data. Also, 19 dye-LIF borings were completed to depths up to 58 feet bgs in the source area to vertically map the extent of residual DNAPL. Soil and groundwater sampling and soil logging were completed at 8 selected MIP-HPT and dye-LIF borings to confirm HRSC results. All HRSC investigation data was assimilated to the existing 3D CSM.

Results/Lessons Learned:

The OU 2 SSI identified vertical and horizontal contaminant migration through the low-permeability clay unit within and downgradient to source containment area, serving as a secondary contaminant source as groundwater overtops the clay unit and allows downgradient contaminant migration. Results from the investigation were integrated into the existing site data set to update the 3D CSM. The presentation will present the technical HRSC findings from the SSI and the resulting contaminant transport pathways via the revised 3D hydrogeologic model. The improved understanding of site contaminant fate and transport from the SSI and 3D CSM update are being used to assess and aid in future remedy evaluations.

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Peifen Tamashiro