AROMATIC HYDROCARBON CONTAMINATION OF CLAY STRATA BELOW A PETROCHEMICAL SITE IN WALES, UK

Clay strata have been implicitly assumed in the UK to provide good protection of underlying aquifers. Unfortunately this assumption is not underpinned by significant research. The Environment Agency (for England and Wales) has recently initiated a program of research to address this issue that focuses upon organic contaminants. As part of that program, we report on the initial findings of an on-going field study to investigate organic contaminant penetration of clay strata below a former petrochemical site in Wales, UK. Detailed investigation of the site by consultants indicated two areas of the site worthy of more detailed research: (i) a thin, relatively continuous clay stratum in a shallow sandy aquifer predominantly contaminated with aromatic hydrocarbons; and, (ii) a deeper, thick underlying clay aquitard that has overlying high concentrations of 1,2-dichloroethane due to migration to depth of this dense nonaqueous-phase liquid (DNAPL). To date, the focus has been upon (i), in particular the role of a clay stratum 6-7.5m below ground surface in part of a site contaminated nearer surface by NAPL benzene, toluene, styrene and ethylbenzene. Detailed depth profiles were determined from four locations using methanol extraction of sub-cores. Contamination profiles (and other site data) demonstrated accumulation of aromatic contamination in the clays, complete penetration of the clay in places and varying trends in vertical profiles. Preliminary advection-diffusion-sorption modelling suggested that increased knowledge of spatial variability in the clay continuity, permeability and thickness is required to more fully elucidate the role of the clay stratum. Occurrence of preferential flow paths in the clay has been tentatively shown through the use of thin sections and the role of biodegradation is being assessed through a companion Agency-funded project.