Theme: 01. Remediation Technology Innovations

Topic Area: 01n. Monitored Natural Attenuation: Innovative Monitoring Approaches/Lines of Evidence and Lessons Learned

Formatted Title:

Sorption-Desorption Processes Contributing to the Natural Attenuation of Chlorinated Solvents

Background/Objectives:

The realization that some sites contaminated with chlorinated solvents are unlikely to achieve remediation goals in acceptable timeframes has led to an approach that first incorporates a source removal to the extent practical, and then focuses on long-term management of the remaining residual contamination. A management plan that can accurately predict processes occurring naturally, either biotically or abiotically, in the subsurface provides guidance to achieve long-term remedial goals. The overall goal of this study was to determine kinetic and equilibrium parameters for abiotic processes that contribute to natural attenuation of chlorinated solvents in heterogenous aquifers. These parameters will then support the development of models that are capable of predicting long-term chlorinated solvent behavior in complex systems.

Approach/Activities:

A combination of column studies and batch reactor studies was conducted with four different soils to investigate the effects of rate-limited sorption-desorption, non-linear sorption, and non-ideal sorption and desorption of trichloroethylene (TCE). Column studies utilized flow interruptions to investigate desorption kinetics. Sorption batch studies conducted in ampules and desorption batch studies conducted in crimp top vials were provide equilibrium isotherms and kinetic data, as well as investigate non-ideal behavior. Instantaneous equilibrium and non-equilibrium models were used to evaluate effluent concentration data collected from the column experiments.

Results/Lessons Learned:

For all soils tested, an increase in TCE concentration was observed following flow interruption periods ranging from 24 to 168 hrs. Column experiments that were conducted at a faster flow rate (50 cm/day versus 15 cm/day) exhibited earlier TCE breakthrough and longer flow interruption periods to observe increases in TCE concentrations. The instantaneous equilibrium model was not able to accurately fit the column data, while nonequilibrium models, including the a one-site model and a two-site model, more accurately captured the column effluent data. In addition, TCE sorption batch studies completed for the same four soils revealed rate-limited behavior and linear sorption coefficients (K_D) ranged from 0.39 mL/g to 0.99 mL/g. These experimental results demonstrate the effects of rate-limited sorption-desorption processes and show the importance of utilizing nonequilibrium sorption-desorption models when evaluating monitored natural attenuation of chlorinated solvents. Additional studies are underway to assess soil microporosity and evaluate long-term abiotic reactivity of TCE in the presence of iron-bearing minerals.

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Allison Stein