GEOLOGIC CONTROLS ON DNAPL MOBILITY AT MGP SITES

Extensive DNAPL contamination is often found at former MGP sites, and accounts for a large portion of the overall exposure risk to both human and ecological receptors. These liquids were introduced into the subsurface continuously over long time periods (often several decades at each site) and in many cases were driven into the ground by gas pressure at the point of introduction. Consequently, large volumes of DNAPL are often found in the subsurface, and both vertical and lateral spreading of the DNAPL plumes can be quite extensive. Over 70 MGP sites are currently undergoing remedial investigations in New York State, with significant off-site DNAPL migration noted at nearly all of them. DNAPL plumes several hundred feet in length are common, and plumes well over 1000 feet have been documented. This migration is often controlled by small-scale geologic features, some of which may not be intuitively obvious at the time of initial evaluation. We have found significant bedrock plumes where RQD values consistently exceed 90 percent, and have seen overburden cores where downward migration has been stopped at medium-grained sand lenses in otherwise gravelly aquifers. Careful borehole logging and thoughtful analysis of the geologic environment surrounding each site are essential in defining and interpreting the nature and extent of DNAPL contamination at MGP sites. The integrity and continuity of confining units is particularly important in unconsolidated materials. In bedrock, identification of persistent joints and bedding planes is equally important.