Theme: 10. Characterization, Fate and Transport

Topic Area: 10f. High-Resolution Site Characterization (HRSC)

Formatted Title:

High Resolution Site Characterization for PCE Source Area to Support a Time Critical Removal Action

Background/Objectives:

The site is an active dry cleaner located in a commercial building that is also occupied by a restaurant in northern Delaware. Past operations at the dry cleaner resulted in a tetrachloroethene (PCE) groundwater plume in a complex geological unit overlaying a karst-like bedrock setting. The site lies atop crystalline bedrock units of the Wissahickon, Cockeysville (dolomite marble), and Baltimore Gneiss Formations. These units were highly metamorphosed, folded and overturned to form anticlines, and stacked with a series of thrust faults. The Wissahickon and a regolith overlies the Cockeysville Formation, whereas the regolith is comprised of layers and lenses of sand, silt, and clay formed by weathering of the dolomite. Three of the six public water supply wells are impacted by PCE and withdraw their water from the Cockeysville Formation: a primary source of groundwater. As part of a remedial investigation, high-resolution site characterization (HRSC) technologies were used to identify the source area and the distribution of PCE to the top of the Cockeysville Formation. Data visualization tools were used to refine the conceptual site model and support selection of a treatment technology to be implemented as part of a time critical removal action. An advanced surface geophysical mapping tool, multi-channel analysis of surface waves (MASW), was used to map the surface of the Cockeysville Formation to help understand the subsurface conditions.

Approach/Activities:

HRSC consisted of membrane interface probe/hydraulic profiling tool (MiHPT) technologies at 50 boring locations, with 12 of the 50 borings located inside the building. Boring depths ranged from 25 feet (ft) below ground surface (bgs) to 74 ft bgs. Confirmation soil and groundwater samples were conducted to compare to the MIP responses. Soil and groundwater sampling were targeted to characterize a range of MIP and HPT responses. The data were used to generate three-dimensional plume models and cross-sectional views to facilitate data interpretation and presentation and to form the basis for selection of applicable treatment technologies.

MASW was used to model the shear-wave velocities of the subsurface based on measured surface waves. The shear-wave velocity data were calibrated using site-specific boring logs to estimate the depth to the top of the Cockeysville Formation across the site. The MASW data were processed using a surface wave recognition and modeling program and exported using a grid-based visualization platform.

The data visualization tools were used to support a time critical removal action that will consist of an electrical resistance heating (ERH) thermal remediation at the two source area locations. It is proposed that 46 electrodes will be used to treat the source area with ERH with eight performance monitoring wells to be used to ensure the ERH system operational goals are met.

Results/Lessons Learned:

MiHPT results indicated two distinct zones of elevated MIP response: the front portion of building occupied by the dry cleaner and the back portion of the dry cleaner/underneath the building occupied by the restaurant. Subsurface soil samples were collected, and the results confirmed high PCE concentrations in soil at 11,000 milligrams per kilograms (mg/kg) at a depth of 6 to 8 ft and at 50,000 mg/kg at a depth of 22 to 28 ft in these two source area locations. Groundwater samples indicate that the PCE concentrations extend vertically to the karstic bedrock interface.

MASW results confirmed a variable bedrock surface ranging from 42 to over 200 ft deep. A pronounced bedrock trough was observed behind and possibly beneath the building occupied by the dry cleaner and restaurant.

Results from the ERH thermal remediation are currently pending and will be presented with conclusions regarding remediation of the PCE source areas within the Wissahickon and regolith.

Christian Matta

Affiliation

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High Resolution Site Characterization for PCE Source Area to Support a Time Critical Removal Action