Natural Attenuation of Tetrachlorethene by Biological and Tidal Processes

Stephen R. Walbridge and Rod R. Rustad, published in the Proceedings of the Third International Conference on Remediation of Chlorinated and Recalcitrant Compounds at Monterey, California in May of 2002.

ABSTRACT
Tetrachloroethene (PCE) in groundwater at a manufacturing facility in New England is attenuating through a combination of biodegradation and tidal processes before it discharges to a tidal creek. Modeling studies assessed the relative contributions of these two major processes in reducing PCE concentrations within the plume, from near solubility concentrations at the source area to concentrations below risk-based criteria for discharge of groundwater to surface water. Simulations using the BIOCHLOR model showed that biodegradation could reasonably account for only part of the observed PCE concentration reduction. A one-dimensional transient numerical model of groundwater flow and transport simulated the long-term effects of tidal water level fluctuations on PCE concentrations to assess whether these fluctuations could account for the remainder of the concentration reduction. Groundwater levels in wells near the tidal creek fluctuate by up to several feet in response to tidal changes. The dynamically changing flow velocities within the tidally-affected zone can reduce PCE concentrations in groundwater migrating toward the creek by accentuating hydrodynamic dispersion and mixing of groundwater with water from the creek. Model simulations indicate that these processes could reduce concentrations within the aquifer by more than 90 percent within a zone of only 60 feet from the creek. This reduction, combined with that produced by biodegradation processes, appears capable of producing a stable plume front of relatively low PCE concentrations, as has been observed near the creek since monitoring began in 1997. Based on these results, a monitored natural attenuation remedy has been accepted for the PCE plume.

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