Weathering, Structural Style and Frequency in Fractured Bedrock Contaminant Source Areas at Two Sites in the UK

Case studies are presented of two superficially similar fractured bedrock sites in the UK, both subject to impacts from chlorinated solvent spills. The sites were investigated using high resolution site characterisation techniques which have allowed a detailed understanding of the behaviour of contaminant mass in the source areas. Despite the similarities, differences in the amount of weathering, the 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, whilst clay infilled fractures in the underlying fresh shale prevented the contaminants from entering the structures at depth and produced a shallow and relatively short plume. Peaty soils, co-spilled hydrocarbons and the relatively high organic carbon content of the shale had caused extensive dechlorination, such that the parent solvent was usually present at concentrations one order of magnitude lower than the degradation compounds. 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 at high concentration. A lack of pore space, rapidly consumed carbon sorption sites and little to no biodegradation has produced a deep, long, fast-moving and highly concentrated plume that is impacting a nearby surface water receptor. Both sites are described by site-specific conceptual site models, as well as by standard 14-compartment models of phase and location. The decision to obtain high-resolution structural logging and chemical sampling data were the most important choices in the improvement of the conceptual site model, and ultimately in the selection of a sustainable remedial option.