Distribution of volatile organic compounds in porewater of the seasonally unsaturated Chalk aquifer

Abstract

Abstract The Chalk aquifer is contaminated at many sites at outcrop by volatile organic compounds (VOCs). Most studies of VOCs concentrate on the saturated zone, ignoring potentially significant accumulations of dissolved contaminants within porewaters of the unsaturated zone. Of particular interest in the Chalk is the large zone of the aquifer over which seasonal fluctuation of the water table occurs. VOCs accumulated in this zone (resulting from vertical migration and diffusion from contaminated ground water at seasonal high water levels) can form a reservoir of contaminants which will be released into the aquifer over a long period. Better understanding of the VOC behaviour in the seasonally unsaturated zone is needed to improve risk assessment and optimize remediation response. Determination of VOC occurrence and concentration in the seasonally unsaturated zone at a contaminated site on the Chalk outcrop is described. Some 500 samples of chalk have been taken from cores drilled in the unsaturated zone over a depth interval of 20 m. Headspace analysis has been used to determine porewater chemistry for a range of VOCs. Vertical profiles of contaminant con- centration are presented and interpreted in terms of VOC transport mechanisms. Hydrocarbon contamination at the site poses an additional element of complexity. The presence of chloride associated with VOC contamination provides a conservative comparison for the distribution behaviour of the VOCs.