Theme: 01. Remediation Technology Innovations

Topic Area: 01m. In Situ Technologies: Lessons Learned

Formatted Title:

Phased Pre-Design Investigation Approach in Support of CSM Refinement and Effective Injection Design

Background/Objectives:

Historic operations at a former manufacturing site located in the midwestern USA created an ongoing environmental liability within subsurface soils and groundwater due to the presence of residual chlorinated volatile organic compounds (cVOCs). Dissolved phase cVOCs were initially detected in a sand and gravel hydrologic unit at concentrations up to 10’s of parts per million (ppm). Previous in situ chemical oxidation and soil excavation activities conducted at the site were not effective in reducing site constituents of concern in groundwater, and the current project team was tasked with designing a pilot remedial program that could serve as the final site remedy. Based on the available site historical analytical data, three areas were identified to implement a site-specific in situ chemical reduction (ISCR) remedy.

The objectives for the pilot ISCR treatment program are:

to mitigate the ongoing sources of dissolved phase cVOCs from subsurface saturated soils beneath the primary source area;
to mitigate the flux of cVOCs in groundwater migrating downgradient from the primary source area; and
to support ISCR treatment as the final site remedy for groundwater.

Approach/Activities:

A phased pre-design investigation (PDI) approach resulted in further refinement of the ISCR treatment areas. The PDI included the phased installation and sampling of 27 temporary monitoring wells to refine the lateral and vertical boundaries for each treatment area. Three phased investigations were completed to refine the dimensions of the source area. Results from the PDI event indicated that the highest cVOC concentrations in groundwater associated with the main source area extended well outside the previously existing monitoring well network and had increased by an order of magnitude to the 100’s of ppm. As a result of the phased PDI, the source area ISCR pilot treatment area nearly doubled in size and the amendment dosage increased. The subsequently refined injection design was implemented in a phased approach.

Results/Lessons Learned:

Because the phased approach required several phases to determine the full extent of the main source area, the PDI timeframe lasted longer than initially expected. Main source area characterization activities overlapped with the start of the implementation of injection activities, which were scheduled based on client fiscal requirements. However, the phased approach was instrumental in understanding the main source area and designing an injection approach which would meet the first goal of the program as stated above. Injection activities were initiated at the downgradient area and the source injection was completed last during the first injection phase. The client and regulator agreed that a second injection phase was required, and the pilot program was expanded the following year to cover the remaining extent of the source area. Performance monitoring activities are currently ongoing and indicate significantly reduced cVOC concentrations in all treatment areas, the presence of reducing biogeochemical conditions, and generation of degradation products at performance monitoring locations.

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Jill Tribley

Biography