Characterization of redox conditions in a petroleum contaminated aquifer: Implications for bioremediation potential

Currently, the application of bioremediation requires extensive and costly monitoring due to limited understanding of the terminal electron accepting processes (TEAPs) that control biodegradation, which impairs the accurate quantification of contaminant mass loss. The measurement of redox conditions and evaluation of TEAPs are critical in assessing the capacity for bioremediation at any site. A series of batch microcosm experiments, using sediment collected from a gasoline-contaminated aquifer at Fort McCoy, Wisconsin, was designed to: 1) evaluate the role of Fe(III) in the development of TEAPs during biodegradation of benzene, toluene, ethylbenzene, and the xylenes (BTEX); 2) examine the biodegradation potential in different portions of the plume; and 3) compare methods of TEAP characterization. In general, the presence of Fe-oxides in microcosms inhibited methanogenesis. Although Fe-reducers did not actively degrade BTEX, Fe-reduction did occur, and most probable number (MPN) counts showed that added Fe(III) increased numbers of Fe-reducers in the microcosms. Methane production in microcosms constructed from sediment near the source area was ~5 times lower than levels produced by the mid-plume sediment. No Fe-reduction occurred in microcosms containing sediment from the source area. These results suggest that the source area has much lower biological activity than the mid-plume.

TEAP characterization was conducted using a variety of methods, including geochemical indicators, redox dyes, MPN, and hydrogen concentrations. Monitoring of CH4 concentration yielded useful information in delineation of redox processes; Fe(II) monitoring was unreliable as a geochemical indicator. Redox dyes supplied basic information on reducing environments. MPN counts estimated microbial populations in lieu of faulty geochemical indicators, i.e., Fe(II). The measurement of H2 proved to be one of the more simple and reliable methods for TEAP identification. Results of this study indicate that TEAP characterization should include use of multiple methods; relying on geochemical indicators alone is not sufficient.