DIFFUSIVE TRANSPORT AND BIODEGRADATION OF CHLORINATED ETHENES IN GROUNDWATER AT A FRACTURED ROCK SITE

The purpose of our study was to quantify the rates of diffusion and biodegradation of chlorinated ethenes in a fractured rock aquifer-aquitard system. The study was conducted at the former Naval Air Warfare Center near Trenton, New Jersey where the groundwater was previously contaminated with trichloroethene (TCE) used as a coolant during jet engine testing.

Prior to our tests, the rock matrix adjacent to the uncased borehole was in contact with dissolved TCE, cis-1,2-dichloroethene (DCE) and vinyl chloride (VC) in the borehole groundwater. The dissolved ethene concentrations were monitored for about a decade prior to our tests as part of the sitewide monitoring program. During most of this period, the TCE concentration was about five times greater than the DCE concentration, and only trace VC was present in the borehole groundwater. To conduct a test: we isolated a borehole section of unfractured rock using a custom fabricated dual packer system; we replaced the borehole solution with groundwater with the chlorinated ethenes and other volatiles removed; we added tracers; and we monitored the tracer and chlorinated ethene concentrations for about 2.5 months. We fit a numerical model to the historical record (pretest borehole concentrations) and test data to estimate transport parameters; the model simulated retarded diffusion in the rock matrix coupled to biodegradation in the borehole water.

Following a lag phase, TCE biodegradation to cDCE within the borehole was rapid (up to 0.60 µM/day) but cDCE biodegradation to VC was slow, leading to cDCE concentrations much larger than TCE concentrations. A small amount of VC was produced. Modeling showed that TCE was stored in the rock matrix within a few cm of the rock-water interface. The TCE was not readily biodegraded while it was sorbed. This experiment illustrates that while TCE is the primary contaminant at the site and stored in the rock matrix, cDCE is often found at concentrations greater than TCE in groundwater extracted from boreholes across the site.

This work was completed in collaboration with the following current or former USGS staff: Daniel Goode, Paul Hsieh, Michelle Lorah, Allen Shapiro, Claire Tiedeman, Thomas Imbrigiotta, Pierre Lacombe, and Alex Fiore.