BIOREMEDIATION OF FRACTURED ROCK CONTAMINATED WITH CHLORINATED SOLVENTS: STATE-OF THE ART AND REMAINING CHALLENGES

Once thought to be devoid of life, fractures in bedrock are now known to support unattached (free-floating) and attached microbial communities. The bacteria and protists that inhabit macro and micro water-bearing fractures differ and their ecology and its relationship to mass transfer kinetics are not well understood. The ability of the communities to degrade chlorinated solvents is related to: the types of rock media and compounds (e.g., PCE, TCE, DCE, VC) present; the location of the solvents within the fractured rock environment (e.g., dissolved in the groundwater, sorbed onto or into the rock matrix, as free product); and the availability of electron donors and acceptors and nutrients as well as the other environmental parameters (e.g., pH, temperature) that affect microbial activity.

Biodegradation of chlorinated solvents can be accomplished by naturally-occurring microbial communities, if the appropriate environmental conditions exist, but in many cases injection of amendments such as electron donors or bacteria that can completely dechlorinate and mineralize the organic molecules to inorganic carbon are required. In unconsolidated subsurface environments, injections of amendments to enhance biodegradation of chlorinated solvents have been successful in numerous cases; however, achieving similar results in fractured rock has proven elusive. Even with recent advances in geophysics and geohydrology, hydraulic control in fractured rock is not readily achieved. The linkage between biodegradation and hydraulics is of paramount importance to chlorinated solvent remediation in fractured rock. This linkage and its relationship to bedrock microbial ecology, and the hurdles that need to be overcome to make naturally-occurring or engineered bioremediation of chlorinated solvents in fractured rock acceptable to regulators will be discussed.