Mixtures of organic contaminants found at Superfund sites generally enter groundwater through the vadose zone, a variably saturated zone lying between the land surface and the groundwater table. Chlorinated contaminants include alkyl halides such as trichloroethylene, volatile, aromatic compounds such as the chlorinated benzenes, and non-volatile compounds such as the chlorinated phenols. Understanding the transport and fate of these compounds in the vadose zone requires combining knowledge of sorption behavior, cosolute effects, and other chemical and microphysical factors with principles of water and solute movement in water-unsaturated media. The overall objective of the research is to test and validate our ability to predict the migration of chlorinated-organic contaminants dissolved in water in the vadose zone. Our approach will involve soil- column and intermediate-scale transport experiments using model compounds, and mathematical modeling of the experimental results. Laboratory batch and small-scale column experiments will be used to develop some chemical parameters, and to develop a better understanding of the dominant chemical and microphysical processes controlling sorption and transport. The mobile fluid in our experiments will be water, and concentrations of contaminants will be below solubility limits. Both volatile and non-volatile compounds will be used; for volatile compounds, gas-phase transport by diffusion will be addressed. The result of this research will provide sorely needed validation for tools used to describe contaminant transport in unsaturated media. We will use existing models, but will need to estimate the model parameters from our data. Our results will also provide better means for parameter estimation in both research and applications of vadose-zone transport modeling.
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