The sorption equilibrium coefficient of tetrachloroethene related to lithocomponent distribution and lithofacies of the Borden aquifer, Canadian Armed Forces Base Borden, Ontario, Canada
George, Shannon Star
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Sorption is an important variable for modeling contaminant fate and transport, and is examined in this study as the tetrachloroethene (PCE) sorption equilibrium coefficient (Kd). Sorption of hydrophobic and moderately hydrophobic organic contaminants (HOC) occurs in the presence of sediments containing fractions of organic carbon (FOC), such as humic and fulvic acids, kerogen, soot, and charcoal. Sedimentary aquifers are comprised of lithologic components (lithocomponents) originating from source rocks of different geologic histories and chemical compositions. Sedimentary lithocomponents containing carbon cause HOC sorption and subsequent contaminant plume retardation within aquifers. This study tests a method for modeling PCE Kd using a grain scale analysis of lithocomponents and relating Kd to specific lithofacies of the Borden aquifer. This study is important because it shows that the distribution of specific lithocomponents affects aquifer chemical heterogeneity, and that lithocomponent characterization can be used to model PCE sorption spatial heterogeneity. This study resolves that very dark calcareous lithocomponents are the most significant lithocomponents contributing to increased PCE Kd within the Borden aquifer and cause aquifer sorption heterogeneity. Sorption within vertical cores was predicted by determining the Kd of individual lithocomponents and cataloging the mass fraction distribution of those lithocomponents in continuously sampled vertical cores. Kd predictions were compared to the measured values of those samples. Sorption heterogeneity of the Borden aquifer is related to the distribution of dark and very dark calcareous, and shale grains that contain fractions of organic carbon greater than 0.1%. Using a lithofacies based approach in conjunction with lithocomponents characterization resolves sorption heterogeneity at the field scale. HOC aquifer sorption heterogeneity was resolved using a lithofacies based approach using a hierarchical classification system.