Sorption-desorption processes contributing to the natural attenuation of cis-1,2-dichloroethene in porous media.

Monitored natural attenuation (MNA) is widely used to manage groundwater plumes with persistent chlorinated solvents exceeding regulatory standards. In heterogeneous aquifers, accumulation and release of these contaminants can impact MNA's effectiveness. Research often focuses on tetrachloroethene (PCE) and trichloroethene (TCE), but incomplete reductive dichlorination can lead to cis-1,2-dichloroethene (DCE) accumulation. This study investigates rate-limited sorption-desorption processes governing DCE release from lower-permeability media. Batch reactor studies with two soils established equilibrium linear distribution coefficients (K) of 0.15 mL/g and 0.25 mL/g. Column transport studies were then completed using the same soils at two flow rates with flow interruptions to assess rate-limited desorption. A numerical simulator with a "two-site" sorption model was used to fit the effluent concentration data, yielding parameters for the fraction of sorption sites at instantaneous equilibrium (f) and the rate of sorption for time-dependent sites (k) ranging from 0.2 to 0.6 and 0.4-2 1/day, respectively. Soils with small f and k exhibit prolonged DCE release, which can benefit MNA at sites with an active DCE-to-ethene dechlorinating microbial community. These persistent, low concentrations of DCE can support microbial reductive dichlorination by providing sufficient residence time for the complete biodegradation to non-toxic ethene. This work emphasizes the importance of developing conceptual site models that capture sorption-desorption processes contributing to natural attenuation of chlorinated solvents in heterogeneous aquifers.