Trace metals found at Superfund sites enter ground water through the vadose zone, a heterogeneous unsaturated zone lying between the land surface and the ground water table. Trace metals of interest for this study include cadmium (Cd), lead (Pb) and copper (Cu). Understanding the transport and fate of these metals in the vadose zone requires combining knowledge about chemical behavior (e.g. complexation, precipitation, dissolution and adsorption) with solute movement in unsaturated media. Our central hypothesis is that chemical factors are important in determining the rate of trace metal movement and that the most important complexation reaction in natural water systems will be with natural organic matter (NOM). Specific hypotheses that we will attempt to test are: (1) different metals will bind to NOM with different affinities, with the affinity being controlled by distribution of binding sites; (2) certain metals will complete for the same sites on the NOM; surface speciation will be extremely important in determining aqueous-phase concentrations; and (3) in unsaturated systems non-equilibrium transport will dominate - mass transfer will be controlled by rates of adsorption, desorption, precipitation and dissolution reactions. Our approach will be to use batch experiments and soil-column and intermediate-scale transport studies using trace metals and NOM. Laboratory batch and column experiments will be used to measure chemical parameters. We will use existing models, but use our data to estimate the parameters needed. Our results will provide insight into the rate-controlling processes of trace metal transport and provide parameters needed for vadose-zone transport modeling.
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