STABLE ISOTOPE DIAGNOSTICS OF SOME COMMON CL-BEARING ENVIRONMENTAL CONTAMINANTS

Understanding the sources and environmental behaviors of environmental contaminants is crucial for determining apportionment of liability and for developing optimal remediation strategies. A number of laboratory and field investigations during the past decade have established the utility of stable isotope ratio measurements (H, C, O, Cl) for diagnostic investigations of some common Cl-bearing contaminants, including volatile chlorinated aliphatic hydrocarbons (CAHs), semivolatile chlorinated organic compounds (SVOC) and perchlorate. For CAHs, there are significant variations in isotopic composition of C and Cl in raw products depending on compound and manufacturer (van Warmerdam et al., Appl. Geochem. 10, 1995; Holt et al., Anal. Chem. 69, 1997; Jendrzejewski et al., Appl. Geochem. 16, 2001). Both evaporation and biodegradation have significant and contrasting kinetic isotope effects, allowing the relative extent of these processes to be evaluated in given field scenarios (Heraty et al., Org. Geochem. 25, 1999; Huang et al., Org. Geochem. 25, 1999). TCE produced by biodegradation of PCE has a distinct H isotope ratio that can distinguish it from raw TCE product (Shouakar-Stash et al., J. Cont. Hydrol. 60, 2003). SVOCs have a smaller range in C and Cl isotope ratios than volatile organic compounds, and have apparently smaller kinetic isotope effects during biodegradation (Drenzek et al., Org. Geochem. 33, 2002; Drenzek et al., Environ. Poll. 128, 2004). However, enzyme-catalyzed chlorination of an aromatic substrate has a fairly large (~11 per mil) KIE, indicating that naturally chlorinated SVOCs may be distinguishable from synthetic SVOCs on the basis of their Cl isotope ratios (Reddy et al., JACS 124, 2002). Synthetic perchlorate has a significant range in O isotope ratios, but a fairly narrow range in Cl isotope ratios, reflecting its synthesis from seawater-derived Cl salts, whereas natural perchlorate from the Atacama Desert (Chile) has a distinct isotopic composition, that is strongly depleted in 37-Cl and enriched in both 18-O and 17-O relative to synthetic perchlorate (Bao and Gu, ES&T 38, 2004, Beloso et al., Fall AGU, 2004). Microbial reduction of perchlorate has significant KIEs for both Cl (~15 per mil) and O (~30 per mil) (Sturchio et al., ES&T 37, 2003; Coleman et al., Geophys. Res. Abst. 7, 2005).