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.
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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