Further biogeochemical characterization of a trichloroethene-contaminated fractured dolomite aquifer: Electron source and microbial communities involved in reductive dechlorination

A recent article presented geochemical and microbial evidence establishing metabolic adaptation to and in-situ reductive dechlorination of trichloroet hene (TICE) in a fractured dolomite aquifer. This study was designed to fur ther explore site conditions and microbial populations and to explain previ ously reported enhancement of reductive dechlorination by the addition of p ulverized dolomite to laboratory microcosms. A survey of groundwater geoche mical parameters (chlorinated ethenes, ethene, H-2, CH4, DIC, DOC, and delt a C-13 values for CH4, DIC, and DOC) indicated that in situ reductive dechl orination was ongoing and that an unidentified pool of organic carbon was c ontributing, likely via microbial respiration, to the large and relatively light onsite DIC pool. Petroleum hydrocarbons associated with the dolomite rock were analyzed by GC/MS and featured a characteristically low delta C-1 3 value. Straight chain hydrocarbons were extracted from the dolomite previ ously found to stimulate reductive dechlorination; these were particularly depleted in hexadecane (HD). Thus, we hypothesized that HD and related hydr ocarbons might be anaerobically respired and serve both as the source of on site DIC and support reductive dechlorination of TCE. Microcosms amended wi th pulverized dolomite demonstrated reductive dechlorination, whereas a com busted dolomite amendment did not. HD-amended microcosms were also inactive . Therefore, the stimulatory factor in the pulverized dolomite was heat lab ile, but that component was not HD. Amplified Ribosomal DNA Restriction Ana lysis (ARDRA) of the microbial populations in well waters indicated that a relatively low diversity, sulfur-transforming community outside the plume w as shifted toward a high diversity community including Dehalococcoides ethe nogenes-type microorganisms inside the zone of contamination. These observa tions illustrate biogeochemical intricacies of in situ reductive dechlorina tion reactions.