DISSOLUTION OF DENSE CHLORINATED SOLVENTS INTO GROUNDWATER .3. MODELING CONTAMINANT PLUMES FROM FINGERS AND POOLS OF SOLVENT

Chlorinated hydrocarbon (CHC) solvents are frequently observed groundwater contaminants. Groundwater concentrations of these compounds are generally below 1 mg/L, even at sites where it is known or at least strongly suspected that actual liquid CHC solvent is present in the saturated zone, and even though (1) these compounds are soluble at hundreds to thousands of milligrams per liter and (2) saturation is achieved relatively quickly when water flows through a porous medium containing droplets of CHC solvent. In this work, analytical transport models are used to examine why concentrations observed in the field are typically so low. The results suggest that the main reason is that CHC solvent below the water table tends to accumulate as stagnant pools located on the tops of low-permeability layers, or on top of an underlying aquitard. The rate at which CHC solvent dissolves from a pool into the flowing groundwater is controlled by vertical dispersion. Large fingers of solvent are not a likely source morphology in the saturated zone, and water flowing through many small fingers would cause such fingers to dissolve too quickly to be able to provide the type of long-term contamination that is observed at many CHC solvent spill sites.