Abstract

Case studies are presented of two superficially similar fractured bedrock sites in the UK affected by chlorinated solvents. Site investigation used high-resolution site characterization techniques that have allowed a detailed understanding of the distribution of contaminant mass in the source areas, and the development of advanced conceptual site models at both sites. Differences in the amount of weathering, the geological structural style and the amount of organic carbon present meant that the contaminant fate and transport at each site is completely different. At one site deep, pervasive weathering had increased matrix porosity and permeability, whereas clay-infilled fractures in the underlying fresh shale prevented the contaminants from entering fractures at depth and produced a shallow and relatively short plume. At the second site there were very thin soils over a low-grade slate with almost no weathering, and solvent had penetrated to greater depths and diffused into the matrix. A lack of pore space, carbon and biodegradation has produced a deep, long, fast-moving and highly concentrated plume that is affecting a nearby surface water receptor. The decision to obtain high-resolution structural logging and chemical sampling data was the most important choice in the development of the conceptual site model.

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