Enhanced Bioremediation for the Treatment of Chlorinated Solvent Residual Source Areas

Chlorinated solvents are the most common and one of the most challenging classes of contaminants in groundwater. One innovative technology for remediating chlorinated solvent contamination is enhanced in situ bioremediation (ISB) through anaerobic reductive dechlorination. However, enhanced ISB has not generally been considered applicable to sites with DNAPL source areas because microbial degradation occurs only in the aqueous phase. That convention is now changing as recent studies have demonstrated the occurrence of enhanced mass transfer of chlorinated solvents from the nonaqueous phase to the aqueous phase during bioremediation. In addition to two previously identified mechanisms for enhanced mass transfer, a third mechanism has been observed during studies at the Idaho National Engineering and Environmental Laboratory. Laboratory measurements demonstrated, for example, that sodium lactate (used as an electron donor for biostimulation) can decrease interfacial tension with nonaqueous TCE by 26% when the concentration is increased from 0.1% to 3% by weight, and by 47% when the concentration is increased to 30%. A field study showed sodium lactate injection resulted in a factor of 23 increase in trichloroethene (TCE) concentrations due to enhanced mass transfer. The TCE was then stoichiometrically converted to cis-1,2-dichloroethene and ultimately was completely biologically reduced to ethene. This presentation provides an overview of the bioremediation mechanisms that can contribute to enhanced bioavailability and mass removal in DNAPL source areas, as well as a case study of a field site with nearly 4 years of bioremediation operations in a TCE source area. These data suggest that bioremediation is emerging as a viable approach for cleanup of residual DNAPL source areas either alone, or as a follow-on to more aggressive mass removal technologies that are not 100% effective.