Diffusive solute transport and reaction processes in natural and contaminated porous media

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The research program is focused on diffusion and reaction processes in porous media, primarily shallow groundwater systems, with or without anthropogenic contamination. The long-term objective is to improve understanding of coupled diffusion-reaction processes and their controls on solute mobility in groundwater and surface water systems. The proposed research program will address two objectives: 1. In contaminated fractured rock aquifers where natural attenuation of CrVI or chlorinated solvent contamination can be demonstrated, we will build conceptual models by identifying the reactants, products, mechanisms and relative contributions of specific reactions. The conceptual models will be further developed into a numerical framework with the reactive transport code, MIN3P (Mayer et al. 2001). 2. We will improve the understanding of growth mechanisms and time-series variability of chemical and isotopic compositions in growth bands of Fe-Mn nodules that occur at the water-sediment interface in lakes. Knowledge that anthropogenic contaminants in aquifers may undergo natural attenuation by chemical and biological processes integrates well within a risk-based assessment of options for management of contaminated groundwater systems. In cases where it can be shown that natural attenuation mechanisms are active, and risk to human and ecosystem health are low, regulators may decide that aggressive, disruptive and costly remediation approaches are not necessary. We are working with partners in Boeing Corp. and the US EPA to study diffusion and reaction processes that contribute to natural attenuation of Cr(VI) and chlorinated solvents in the matrix pore space of fractured bedrock aquifers. Our contributions include measurement of diffusion transport rates, and the development of geochemical and mineralogical methods to identify mineral reactants and reaction products that allow us to define attenuation reaction processes. Freshwater Fe-Mn nodules have been observed in lakes at the water-sediment interface. The Fe and Mn is thought to be derived from the underlying sediment porewater. In glaciated terrains the nodules are likely as old as the lakes. We will study their origin by conducting detailed studies of the porewater chemistry and stable isotopic composition. Porewater data from the cores will allow for a quantitative study of the upward transport rates for Fe, Mn and trace elements. The nodules, which grow radially similar to tree rings, will be analyzed to determine their age profiles (210Pb, 226Ra, 14C) along the growth direction. This will allow us to define growth rates and total age. These studies will contribute to the understanding of major and trace-element budgets and fluxes to lakes through time. We aim to construct time series for trace-element fluxes at the regional scale that will be valuable in understanding post-industrialization contaminant dispersion.