Chlorine isotope fractionation during microbial reduction of perchlorate.

Perchlorate contamination of surface water and groundwater is an emerging public health problem that has adversely affected the drinking water supplies of millions of people in the western United States. Microbial reduction has shown promise as a cost-effective means for in situ bioremediation of perchlorate-contaminated water. Measurements of stable isotope ratios of light elements (H, C, N, O, S, Cl) can often be used to distinguish biodegradation of organic and inorganic molecules from abiotic loss mechanisms such as adsorption, dispersion, or volatilization because of the relatively large kinetic isotope effects accompanying biodegradation. We quantified chlorine isotope fractionation during perchlorate biodegradation by a common perchlorate-reducing bacterium, Dechlorosoma suillum, initially isolated from a perchlorate-contaminated groundwater source in southern California. The values of the chlorine isotopic fractionation factor alpha derived from two microcosm experiments were alpha = 0.9834 +/- 0.0001 (R2 = 0.9999) and alpha = 0.9871 +/- 0.0008 (R2 = 0.9832). These alpha values indicate that the rate of the 35ClO4 reduction is approximately 1.3-1.7% faster than that of the 37ClO4 reduction. This relatively large kinetic isotope effect indicates that chlorine isotope analysis provides a sensitive technique by which to document in situ bioremediation of perchlorate in groundwater.