In situ assessment of microbial sulfate reduction in a petroleum-contaminated aquifer using push-pull tests and stable sulfur isotope analyses

Anaerobic microbial activities such as sulfate reduction are important for the degradation of petroleum hydrocarbons (PHC) in contaminated aquifers. T he objective of this study was to evaluate the feasibility of single-well p ush-pull tests in combination with stable sulfur isotope analyses for the i n situ quantification of microbial sulfate reduction. A series of push-pull tests was performed in an existing monitoring well of a PHC-contaminated a quifer in Studen (Switzerland). Sulfate transport behavior was evaluated in a first test. In three subsequent tests, we injected anoxic test solutions (up to 1000 1), which contained 0.5 mM bromide (Br-) as conservative trace r and 1 mM sulfate (SO42-) as reactant. After an initial incubation period of 42.5 to 67.9 h, up to 1100 1 of test solution/groundwater mixture was ex tracted in each test from the same location. During the extraction phases, we measured concentrations of relevant species including, Br-, SO42- and su lfide (S(-II)), as well as stable sulfur isotope ratios (delta S-34) of ext racted, unconsumed SO42-, and extracted S(-II). Results indicated sulfate r eduction activity in the vicinity of the test well. Computed first-order ra te coefficients for sulfate reduction ranged from 0.043 +/- 0.013 to 0. 130 +/- 0.015 day Isotope enrichment factors (epsilon) computed from sulfur is otope fractionation of extracted, unconsumed SO42- ranged from 20.2 +/- 5.5 parts per thousand to 22.8 +/- 3.4 parts per thousand. To-ether with obser ved fractionation in extracted S(-II), isotope enrichment factors provided strong evidence for microbially mediated sulfate reduction. Thus, push-pull tests combined with stable sulfur isotope analyses proved useful for the i n situ quantification of microbial sulfate reduction in a PHC-contaminated aquifer. (C) 2001 Elsevier Science B.V. All rights reserved.