ENVIRONMENTAL ISOTOPE FORENSICS OF PERCHLORATE CONTAMINATION

Widespread occurrence of perchlorate has been reported in recent years in surface water bodies, ground water aquifers, and agricultural products across the U.S. This perchlorate, which may have both anthropogenic and natural sources, can be problematic because the highly soluble perchlorate anion is relatively inert and difficult to remove by conventional water treatment methods. Because several toxicological studies indicate that trace amounts of perchlorate can cause thyroid cancer and child developmental problems, there is increasing interest in characterizing acceptable contamination levels as well as the ultimate sources of perchlorate in the environment. The stable isotope ratios of Cl and O can potentially be used to distinguish the source(s) of perchlorate in a given location and to evaluate the extent of biodegradation.

The stable isotope ratios of Cl and O in anthropogenic perchlorate salts can now be readily measured in milligram amounts, but it is difficult to extract and recover isotopically measurable amounts of perchlorate from natural waters, in which perchlorate concentrations may be of the order of ppb. The recent development of a new class of bifunctional anion exchange resins for efficient sorption and removal of perchlorate, along with a new resin regeneration technique for recovering sorbed perchlorate, both developed at Oak Ridge National Laboratory, provides a means for nearly quantitative recovery of perchlorate from water samples collected in the field for isotopic analysis. Using these innovative separation-recovery technologies, we have begun a systematic characterization of the stable isotope ratios of Cl and O in perchlorate from different sources including anthropogenic perchlorate reagents, natural perchlorate-bearing salt deposits, salt-derived fertilizers, and waters from military and industrial contamination sites. Our preliminary results indicate that there are isotopic differences between various anthropogenic and natural perchlorate sources. When the data set of isotopically characterized perchlorate sources becomes sufficiently representative, stable isotope forensics of perchlorate may become useful for issues of source apportionment, natural attenuation, or monitoring of remediation efforts.