ISOTOPIC TRACING OF NATURAL AND SYNTHETIC PERCHLORATE IN THE ENVIRONMENT

Widespread occurrence of perchlorate in surface water bodies, vadose zone pore waters, and ground water aquifers has been recognized in recent years. This perchlorate, which may have both synthetic and natural sources, is problematic from the public health perspective because the highly soluble perchlorate anion is relatively inert and difficult to remove by conventional water treatment methods. We have developed methods to measure the stable isotope ratios of Cl and O in perchlorate, as well as methods to extract and recover measurable amounts of perchlorate from natural waters in which perchlorate concentrations may be in the ppb range. Stable isotope ratios of Cl and O in perchlorate from different sources have been measured, including synthetic perchlorate reagents, natural perchlorate-bearing salt deposits, and perchlorate extracted from various ground water aquifers. These data can clearly distinguish the source(s) of perchlorate in many locations, and can be used to evaluate the extent of in situ microbial perchlorate reduction because of the large and systematic kinetic isotope effects on both O and Cl that have been measured in laboratory cultures. Additional studies are in progress to understand the isotopic diversity of natural perchlorate, which appears to vary geographically.