Chemical heterogeneity of glaciofluvial deposits: Outcrop study and implications for reactive transport
Spatial variations in the reactive properties of geologic systems and their influence on contaminant transport are poorly understood. Consequently, an outcrop study was conducted in a glaciofluvial deposit in Deerfield, New Hampshire in order to: (1) identify the sediment properties controlling heavy metal adsorption, (2) evaluate the extent to which geologic information can be used to characterize their spatial variation, and (3) assess the impact of spatial variations on heavy metal transport. Four hundred seventy-six, spatially-located sediment samples were collected from an eight square meter vertical exposure of outwash sands and gravels. Lithologic facies were mapped on outcrop photographs. Sample color, permeability, porosity, grain size, surface area, lead (Pb2+) sorption, carbon content, magnetic mineral content, and dithionite citrate-extractable iron, manganese, and aluminum content were measured in the laboratory. Fifty-seven percent of the variation in Pb2+ sorption can be explained by a linear combination of sediment permeability and extractable iron, manganese, and aluminum, indicating that Pb2+ sorption is controlled by (hydr)oxide grain coatings. Reactive surface area, estimated from sample grain size and (hydr)oxide mass together with observations of grain coating morphology and numerical abundance, accounts for 65 percent of the sorption variation. Three sorption-related properties: permeability, extractable iron, and extractable manganese are strongly related to sediment facies and/or color and thus can be mapped over a wide range of spatial scales. Differences in the geometries of iron and manganese enrichment, petrographic observations, and SEM-EDS analyses indicate the grain coatings originated from the post-depositional weathering of biotite and garnet, coupled with local, redox-driven redistribution of the liberated iron and manganese. Numerical simulations show that spatial variations in (hydr)oxide grain coatings increase plume mobility and dispersion when the spatial scale of the heterogeneity is similar to the scale of the problem. Overall, the outcrop study findings suggest that Pb2+ partition coefficients can be estimated from relatively simple and inexpensive measurements of permeability and dithionite-citrate extractable metals. The results further suggest that information regarding sediment facies and color can help produce more efficient and geologically realistic descriptions of chemical heterogeneity.
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