| Tetrachloroethene (PCE) is one of the most ubiquitous groundwater contaminants in the United States. Aquifers contaminated with PCE can be remediated under anaerobic conditions if the proper chloridogenic microorganisms are present and if they are provided with an appropriate electron donor. Liquid electron donor injection is labor and cost intensive, often fails to maintain proper reducing conditions, and is typically difficult to achieve in low permeability sediments. In this study, the ability of chitin from crab shells to serve as a long-term, in situ source of electrons for the bioremediation of chloroethenes was evaluated at both laboratory and field scales.; In continuous-flow columns, spatial and temporal correlations were found between the nutrient flux from chitin fermentation, PCE degradation, and the distribution of dechlorinating, fermenting, and methanogenic microbial communities. The dominant product of chitin fermentation was acetate, although significant concentrations of butyrate, propionate, and hydrogen were also detected. PCE was sequentially dechlorinated to vinyl chloride (VC) in the columns, and subsequent batch tests with column material showed chitin to support significant conversion of VC to ethene after extended incubation times at neutral pH. Using nested polymerase chain reaction (PCR) with genus specific primers, hydrogenotrophic Dehalococcoides strains were detected in the sand along the length of the columns, whereas acetotrophic Desulfuromonas strains were found clustered where acetate and PCE concentrations were highest. Molecular and kinetic analyses indicated the rapid migration of dechlorinating populations in response to local variations in substrate concentrations. Terminal-restriction length polymorphism (T-RFLP) analysis indicated that chitin can support a diverse, temporally variable microbial community, composed predominantly of fermenting Clostridium and methanogenic Methanosarcina strains.; In a pilot field test, chitin was delivered into low permeability sediments using hydraulic fracturing. Chitin produced fatty acids similar to laboratory experiments, and created reducing conditions conducive to reductive dechlorination. The longevity of chitin as an electron donor source and its dechlorination performance are currently being tested at the full scale.; This work indicates that chitin may have several advantages over conventional, liquid electron donor injection for the in situ bioremediation of chloroethenes, and raises interesting questions on microbial community structure and motility in groundwater systems. |
