CHARACTERIZING HETEROGENEITY IN SEDIMENTARY AQUIFERS FOR REACTIVE TRANSPORT PREDICTION USING SEDIMENTARY ARCHITECTURE

Models for reactive solute transport require representation of how both hydraulic and reactive attributes (such as permeability (k) and the sorption distribution coefficient (Kd)), vary in space. Currently, transport modeling methods that characterize the spatial occurrence of Kd (or other reactive attributes) in sedimentary aquifers lag behind efforts to evaluate k. This project seeks to fill this knowledge gap by capturing the spatial variability of aquifer reactivity, using sedimentary architecture to provide the spatial framework. Sediment erosion, transport and deposition all include processes which naturally sort sediment grains and create unit types that differ in composition (mineralogy) as well as grain size.

Prior research has demonstrated that hydrophobic organic compound (HOC) Kd is lowest for sand-sized grains (compared to other grain sizes) sieved from bulk aquifer samples (both Borden and other aquifers). The present study draws on perchloroethene (PCE) batch sorption measurements for >700 samples (characterized by lithofacies) obtained from aquifer cores. Consistent with prior studies, the ln Kd cumulative distribution functions observed in this study for the more coarse-textured and poorly sorted lithofacies were greater than those observed for the more fine-grained and well-sorted lithofacies.

PCE sorption to lithocomponents extracted from the Borden aquifer were extremely large for dark to very dark calcareous grains, Kd = 40-600 ml/g, compared to ~0.3 ml/g observed for a bulk (depth-integrated) aquifer sample at a comparable aqueous concentration. Sorption to light calcareous grains was moderate (1-2 ml/g). Lithologic analysis of grains >0.84 mm for 350 samples demonstrated that a high relative sample Kd (e.g. >0.8 ml/g for the Borden aquifer) is correlated to the occurrence (wt %) of dark and very dark carbonaceous lithocomponents despite their very low abundance. This result demonstrates that a very small portion of the aquifer system controls potential mass storage.