Dissolution of Dense Chlorinated Solvents Into Groundwater. 2. Source Functions for Pools of Solvent

When a spill of dense chlorinated hydrocarbon (CHC) solvent occurs into the subsurface, penetration into the saturated zone is likely to lead to the accumulation of pools of solvent on the tops of low-permeability layers. Steady-state dissolution from such pools was modeled. The dissolution rate depends on the molecular diffusion coefficient, the vertical dispersivity α_v, the groundwater velocity v, the solubility, and the pool length. A value for α_v for dissolution of trichloroethylene in a sand-filled physical model was found to be ~ 0.00023 m. This is consistent with vertical dispersivities reported from field experiments in heterogeneous media. With α_v = 0.00023 m, at typical groundwater velocities, dissolution from CHC pools was found to be very slow; pools containing a few hundred to a few thousand killograms of CHC solvent will persist for decades to centuries. Pool lifetimes can be shortened by increasing the groundwater velocity through pumping. However, lifetimes will not usually be reduced by more than a factor of 5. Even after a pool has totally dissolved away, contaminants may remain within the aquitard on which the pool originally formed. The release into the aquifer of CHC solvents dissolved in the pores of the aquitard can persist at a significant rate for many years. The implication of these results is that, for many sites contaminated with CHC solvents, the focus of remediation may need to be on long-term control as opposed to attempts at “quick fixes”.