Toxic Substances Hydrology Program
Natural Attenuation Accelerates Pump-and-Treat Cleanup of TCE in Fractured Rock
Natural biodegradation of trichloroethylene (TCE) in a contaminated fractured rock aquifer in New Jersey contributes substantially to site cleanup operations. An active pump-and-treat system is presently removing about 630 kilograms of TCE per year (kg/yr) at the site. In addition, naturally occurring biodegradation processes remove an additional 500 kg/yr. Because natural TCE biodegradation does not incur the economic costs of a pump-and-treat approach, it is a welcome addition to overall cleanup operations at this site.
These findings were recently reported in Remediation Journal by a team of U.S. Geological Survey (USGS) scientists working at the Naval Air Warfare Center (NAWC) Research Site in West Trenton, New Jersey. The scientists estimated TCE biodegradation rates by combining data on the concentrations of two degradation products of TCE-cis 1-2 dichloroethylene (cis-DCE) and chloride with estimates of subsurface porosities and groundwater residence times, as measured over a 20-year period in a network of 88 observation wells.
The scientists showed that TCE biodegradation rates varied with depth; rates were much higher in the shallow, highly weathered bedrock (about 500 kg/yr) than in the deeper, fractured bedrock (about 20 kg/yr). These differences reflect the higher porosity, greater contaminant mass, and greater amounts of natural organic matter in the weathered bedrock. Because the efficiency of pump-and-treat TCE removal has decreased from 2,000 kg/yr in 1998 to 630 kg/yr in 2010, natural biodegradation is becoming an increasingly important component of overall site remediation.
The methodology that the scientists used to estimate the TCE biodegradation rates in fractured rock is now available to be applied at other fractured bedrock sites. This is good news because contamination in fractured bedrock is widely regarded by environmental professionals as being particularly difficult to clean up. The development of the methodology described above was made possible primarily by the comprehensive dataset that has been collected over 20 years at the NAWC Research Site.
Chapelle, F.H., Lacombe, P.J., and Bradley, P.M., 2012, Estimated trichloroethene transformation rates due to naturally occurring biodegradation in a fractured rock aquifer: Remediation Journal, v. 22, no. 2, p. 7-20, doi:10.1002/rem.21307.