DEGRADATION OF CHLORINATED SOLVENTS IN KARST–CONSTRAINTS AND EXAMPLES FROM AN ABANDONED LANDFILL PLUME

Although microbially-mediated degradation of chlorinated solvents has been firmly established as a mechanism in alluvial aquifers, well-documented examples of such degradation in karst aquifers are rare in the literature. Various workers have reported that degradation of chlorinated solvents in karst is unlikely due to inferred factors including 1) rapid dispersion of contaminants in karst systems, 2) lower bacterial abundances in bedrock, and 3) characteristics and motility of bacterial populations in bedrock.

In this study, the degradation of chlorinated solvents in a karst bedrock setting is examined, and illustrated with data from a contaminated site in Pennsylvania where reductive dechlorination of trichloroethylene (TCE) and its daughter products can be demonstrated. The characteristics of karst bedrock and groundwater that may control dechlorination reactions (e.g., pH, sulfate, magnesium and iron concentrations, and porosity) are discussed for their influence on both the dechlorination reactions and the geochemical parameters used for inferring microbially-mediated redox zones. The sensitivity of mass-balances calculations to these parameters is also examined.

Geochemical data (VOCs, sulfate, nitrate, dissolved Fe and Mn, methane, ethane, and ethane) from a monitoring well network at the site clearly document the degradation of TCE and the generation of daughter products cis-1,2 dichloroethene and vinyl chloride. The data define oxidation-reduction zones ranging from methanogenic in the source area, through sequentially more oxidizing conditions. Constraints on the geometry of the redox zones and contaminant destruction rates are discussed in light of a site-specific groundwater flow model, and the presence of organic contaminants that act as electron donors in the source area. It is concluded that degradation system is limited by electron donor concentrations in the source area and that this degradation has served to help stabilize the plume over time.