Wednesday, April 14, 2010: 2:10 p.m.
Continental C (Westin Tabor Center, Denver)
A key element to support the design of a remedial action is the development of a comprehensive conceptual site model (CSM). Development of a CSM for a site in Colorado was unique since the source of contamination was disposal of trichloroethene (TCE) from 1960-1965 around a building constructed to a depth of 30 ft below ground surface. The CSM was developed to understand historic operations, current extent of source areas, contaminant distribution underneath and around the building, extent of the plume and characteristics impacting contaminant fate and transport. As part of CSM development, data gaps were identified and additional site characterization activities were performed. These activities included soil gas and electric resistance surveys, subsurface soil sampling and groundwater monitoring. Following completion of these activities, an updated CSM summarized that TCE groundwater contamination originates from the subsurface building and extends in all directions because the building sits at the center of a groundwater mound. The contamination resides in a perched sandstone aquifer with relatively low hydraulic conductivity but significant secondary porosity. Residual DNAPL is not suspected, but residual sorbed phase contamination potentially sustains the plume. Limited to no contamination was found within the subsurface soil, supporting the origins of contamination from the subsurface building drains and infrastructure.
The CSM was used to develop a pilot test to use hydraulic fracturing to emplace a zero valent iron/complex carbon amendment (“EHC-G®”) into the subsurface, especially beneath the building. The purpose of this pilot test is to evaluate the remedial effectiveness of in situ chemical reduction for reducing TCE concentrations. The radius of fracture-emplacement in the bedrock was upwards to 79 ft (vertical) and 65 ft (horizontal). Initial results within 90 days of amendment emplacement indicates a 50-90% decline in TCE, evidence of reducing redox potential and an increase in the Dehalococcides spp. population.