Perchlorate is a widespread environmental contaminanthaving both anthropogenic and natural sources. Stable isotoperatios of O and Cl in a given sample of perchlorate maybe used to distinguish its source(s). Isotopic ratios may alsobe useful for identifying the extent of biodegradation ofperchlorate, which is critical for assessing natural attenuationof this contaminant in groundwater. For this approach tobe useful, however, the kinetic isotopic fractionations of Oand Cl during perchlorate biodegradation must first bedetermined as a function of environmental variables suchas temperature and bacterial species. A laboratorystudy was performed in which the O and Cl isotope ratiosof perchlorate were monitored as a function of degradationby two separate bacterial strains (
Azospira suillum JPLRNDand
Dechlorospirillum sp. FBR2) at both 10
C and 22
Cwith acetate as the electron donor. Perchlorate wascompletely reduced by both strains within 280 h at 22
Cand 615 h at 10
C. Measured values of isotopic fractionationfactors were
18O = -36.6 to -29.0 and
37Cl = -14.5to -11.5, and these showed no apparent systematicvariation with either temperature or bacterial strain. Anexperiment using
18O-enriched water (
18O = +198) gaveresults indistinguishable from those observed in theisotopically normal water (
18O = -8.1) used in theother experiments, indicating negligible isotope exchangebetween perchlorate and water during biodegradation.The fractionation factor ratio
18O/
37Cl was nearly invariantin all experiments at 2.50 ± 0.04. These data indicatethat isotope ratio analysis will be useful for documentingperchlorate biodegradation in soils and groundwater. Theestablishment of a microbial fractionation factor ratio (
18O/
37Cl) also has significant implications for forensicstudies.