The variability and intrinsic remediation of a BTEX plume in anaerobic sulphate-rich groundwater

Data from long-term groundwater sampling, limited coring, and associated st udies are synthesised to assess the variability and intrinsic remediation/n atural attenuation of a dissolved hydrocarbon plume in sulphate-rich anaero bic groundwater. Fine vertical scale (0.25- and 0.5-m depth intervals) and horizontal plume-scale (> 400 m) characteristics of the plume were mapped o ver a 5-year period from 1991 to 1996. The plume of dissolved BTEX (benzene , toluene, ethylbenzene, xylene) and other organic compounds originated fro m leakage of gasoline from a subsurface fuel storage tank. The plume was up to 420 m long, less than 50 m wide and 3 m thick. In the first few years o f monitoring, BTEX concentrations near the point of leakage were in approxi mate equilibrium with non-aqueous phase liquid (NAPL) gasoline. NAPL compos ition of core material and long-term trends in ratios of BTEX concentration s in groundwater indicated significant depletion (water washing: volatilisa tion and possibly biodegradation) of benzene from residual NAPL after 1992. Large fluctuations in BTEX concentrations in individual boreholes were sho wn to be largely attributable to seasonal groundwater flow variations. A co mbination of temporal and spatial groundwater quality data was required to adequately assess the stationarity of plumes, so as to allow inference of i ntrinsic remediation. Contoured concentration data for the period 1991 to 1 996 indicated that plumes of toluene and o-xylene were, at best, only parti ally steady state (pseudo-steady state) due to seasonal groundwater flow ch anges. From this analysis, it was inferred that significant remediation by natural biodegradation was occurring for BTEX component plumes such as tolu ene and o-xylene, but provided no conclusive evidence of benzene biodegrada tion. Issues associated with field quantification of intrinsic remediation from groundwater sampling are highlighted, preferential intrinsic biodegrad ation of selected organic compounds within the BTEX plume was shown to be o ccurring, in parallel with sulphate reduction and bicarbonate production. R atios of average hydrocarbon concentrations to benzene for the period 1991 to 1992 were used to estimate degradation rates (half-lives) at various dis tances along the plume. The estimates varied with distance, the narrowest r ange being, for toluene, 110 to 260 days. These estimates were comparable t o rates determined previously from an in situ tracer test and from plume-sc ale modelling. (C) 1999 Elsevier Science B.V. All rights reserved.