Temporal partitioning of a chlorinated solvent release between DNAPL, aqueous, and sorbed phases in transmissive and low permeability zones

Chlorinated solvent source zones and plumes are commonly the product of an initial release to the subsurface of a dense nonaqueous phase liquid (DNAPL). Given release of a sufficient volume, DNAPL often accumulates above low permeability units that behave as capillary barriers. With time, DNAPL constituents partition into water and sorb to solids. Furthermore, with time, advection carries aqueous phase constituent downgradient of the release (forming plumes) and diffusion drives aqueous phase constituents into low permeability zones. This research demonstrates that chlorinated solvent releases evolve through time. At early time the problem is largely one of DNAPL in transmissive zones. Through time the problem evolves into one that is dominated by aqueous and sorbed phase constituents in low permeability zones. Evolution of a release is demonstrated using two-dimensional analytical solutions for an idealized two-layer system with a DNAPL-like source in the transmissive zone, immediately above the low permeability layer. Results indicate that key factors governing the relative fractions of contaminants in transmissive and low permeability zones are time, low permeability zone retardation coefficients, and low permeability zone effective diffusion coefficients. The primary implication of this work is that remedies for older chlorinated solvent releases may need to address contaminants in low permeability zones, as well as contaminants in transmissive zones.