A SPATIAL ANALYSIS: USING CHARACTERIZATION OF BORDEN AQUIFER LITHOLOGIES TO RESOLVE SORPTION HETEROGENEITY

Sorption of organic compounds in sedimentary aquifers is a key variable in reactive contaminant transport. Incorporation of dense carbon within the geologic record determines sorption potential of sedimentary lithologies. Dense carbon such as soot and charcoal are formed by partial combustion of organic matter. Shale and dark calcareous lithologies have high equilibrium sorption coefficients (Kd), which are attributed to the presence of dense carbon lithic fragments within sedimentary grains. The spatial distribution of these lithic fragments governs aquifer sorption potential. This study identifies units with distinct Kd values (chemofacies) for modeling and prediction of perchloroethylene (PCE) contaminant plumes. Vertical sediment cores of unconsolidated Quaternary deposits from the Borden aquifer at the Canadian Armed Forces Base, Ontario were sampled. Rigorous characterization of sample lithology and grain size distribution explains spatial Kd variability within and among sedimentary units. Generalized Kd values obtained from traditional octanol-water partitioning coefficients are often too low and can greatly underestimate plume retardation and contaminant desorption potential. Kd values are obtained for lithologies within the aquifer and are used to predict overall unit sorption potential. Detailed grain scale assessment of aquifer sediments promotes a better understanding of spatial aquifer heterogeneity. Defining chemofacies using Kd distribution allows for accurate and practical sampling methods for assessing reactive contaminant modeling and remediation.