DISSOLUTION OF DENSE CHLORINATED SOLVENTS INTO GROUND-WATER .1. DISSOLUTION FROM A WELL-DEFINED RESIDUAL SOURCE

When found dissolved in ground water, chlorinated hydrocarbon (CHC) solvents such as tetrachloroethylene (PCE) are nearly always encountered at concentrations far below their solubilities. This is true even in cases when it is known or at least strongly suspected that actual liquid solvent is present in the aquifer. In contrast, laboratory studies using columns packed with porous media containing various organic fluids have indicated that when water is forced to flow through a zone containing an immobile organic fluid, saturation concentrations can be achieved rapidly. This study investigated the concentrations and mass removal rates obtainable when simulated ground water is free to flow at least partially around a zone of porous medium that contains solvent in a state of stable, residual saturation. Reductions in the mass removal rate due to reduced permeability in the solvent zone were found to be minimal under the conditions of the experiment. Thus, near the source the plume was essentially the same width as the source zone, and PCE concentrations in the center of the plume were at or near saturation, even at water velocities of 100 cm/day. The fact that CHC aqueous concentrations in ground-water samples are typically far below saturation is, therefore, likely not due to limited dissolution from the fingers of residual solvent. Probable explanations include the preferential formation of thin pools rather than fingers of CHC, dispersion during transport, and dilution by uncontaminated water in monitoring or pumping wells.