Sorption-desorption processes contributing to natural attenuation of trichloroethene in porous media.

Monitored natural attenuation (MNA), which relies on natural processes to reduce contaminant concentrations over time, is widely used to manage dilute chlorinated solvent groundwater plumes. In heterogeneous aquifers, the accumulation and subsequent release of chlorinated solvents can strongly influence the long-term effectiveness of MNA. This research investigated rate-limited sorption-desorption processes influencing trichloroethene (TCE) release from lower-permeability media. Batch reactor studies were conducted with four natural soils to establish equilibrium linear distribution coefficients (K), which ranged from 0.4 to 1.0 mL/g. Column transport studies were then completed using the same soils at two flow rates, with periods of flow interruption, to assess rate-limited desorption. A numerical simulator that incorporates a "two-site" sorption model was used to fit the effluent concentration data and obtain parameters for fraction of sorption sites at instantaneous equilibrium (f) and the rate of sorption for time-dependent sites (k), which ranged from 0.2 to 0.6 and 0.3 day to 3 day, respectively. Subsequent model simulations were used to illustrate the importance of these sorption parameters on the extended release of TCE from low permeability porous media. These finding demonstrate that in addition to back diffusion, rate-limited sorption-desorption processes should be considered when evaluating MNA as long-term remedies for chlorinated solvent plumes.