Flowpath independent monitoring of reductive dechlorination potential in a fractured rock aquifer

The flowpath dependent approaches that are typically employed to assess biodegradation of chloroethene contaminants in unconsolidated aquifers are problematic in fractured rock settings, due to difficulties defining discrete groundwater flowpaths in such systems. In this study, the variation in the potential for chloroethene biodegradation with depth was evaluated in a fractured rock aquifer using two flowpath independent lines of field evidence: (1) the presence of the three biochemical prerequisites [electron donor(s), chloroethene electron acceptor(s), and chlororespiring microorganism(s)] for efficient chloroethene chlororespiration and (2) the in situ accumulation of chloroethene reductive dechlorination daughter products. The validity of this approach was assessed by comparing field results with the results of [1, 2‐¹⁴C] cis‐DCE microcosm experiments. Microcosms were prepared with depth‐specific core material, which was crushed and emplaced in discrete packer intervals for 1 year to allow colonization by the indigenous microbial community. Packer intervals characterized by significant electron donor concentrations, elevated numbers of chlororespiring microorganisms, and high reductive dechlorination product to parent contaminant ratios correlated well with the production of ¹⁴C‐labeled reductive dechlorination products in the microcosm experiments. These results indicate that, in the absence of information on discrete groundwater flowpaths, a modified approach emphasizing flowpath independent lines of evidence can provide insight into the temporal and spatial variability of contaminant biodegradation in fractured rock systems.