ABSTRACT
Natural attenuation of petroleum hydrocarbons in groundwater has been extensively investigated (Davis et al., 1999 and Wiedemeier et al., 1999). Here, for the first time, the sustainability of natural attenuation processes acting on contaminant plumes associated with large and complex Light Non-Aqueous Phase Liquid (LNAPL) source zones in a coastal carbonate aquifer where methanogenesis is the dominant degradation process. The research was complicated by multiple LNAPL source zones, variable groundwater recharge rates and a layered sand aquifer which sustains vertically-downward migration to an underlying limestone unit through a separating clay aquitard.
Multilevel Samplers (MLS) were sampled over three years to characterise the distribution of the multicomponent hydrocarbon groundwater plume and geochemical indicators. A transient three-dimensional site-wide groundwater model was constructed, and alternative hydrogeological models were studied to identify the most plausible conceptual model generating the observed plume behavior and biogeochemical response. Hydrogeological and hydrochemical information were integrated into a two-dimensional cross-sectional reactive transport model assuming a multi-component dissolving NAPL source; using PHT3D (Frommer et al., 2003) for a selected 2D depth transect along the main flow direction. Physical transport and reactive model parameters were constrained using the chemical concentrations observed in the field. Over 100 calibration trials were undertaken.
Scenario-modelling showed that discontinuities in the clay aquitard could sustain the necessary vertical gradients without expressing as "sinks" in site-scale water table contour maps. Despite uncertainties, the reactive transport simulations were able to reproduce the general features of the observed MLS data. Toluene and the xylene isomers were virtually degraded within the LNAPL source area. Consistent with observed data, simulations showed sulphate was consumed at the plume fringe, and bicarbonate and methane were produced within the plume core. Steep gradients illustrated the effect of low dispersion coefficients. The reactive transport simulations show that calcium was consumed and calcite precipitated under methanogenic conditions. Methanogenesis accounted for 84% of the mass removed by biodegradation, while the remainder was due to sulphate reduction. Calcite appears to precipitate during hydrocarbon biodegradation due to the high concentration of bicarbonate and sulphate In the groundwater. This behavior differs from previously reported studies. The hydrocarbon plume in this study was evidently stable in its extent and the degradation reactions appeared sustainable.
