Perchlorate-contaminated groundwater is an issue of national concern. Perchlorate (ClO₄^-) has now been detected in groundwater or drinking water in 35 states. In the past, it was assumed that historical use and disposal practices by the military and military contractors, perchlorate manufacturers, and several commercial industries were the sole contributors of perchlorate contamination in the environment. However, a number of sites now have been identified for which military or commercial sources of perchlorate contamination are unlikely, including nearly 40,000 square miles in western Texas. Recent studies suggest that natural perchlorate from both Chilean fertilizers and indigenous sources represents a potentially large source of perchlorate in groundwater and drinking water in the United States.

The key objectives of this project were as follows: (1) to validate the combined use of Cl and O stable isotope ratio analysis as a forensic tool to distinguish sources of perchlorate in groundwater, and (2) to demonstrate the isotopic techniques as a method to verify perchlorate biodegradation in the field.

Stable isotope ratio analysis of chlorine and oxygen in the perchlorate molecule were employed as the primary forensic tool to identify perchlorate from different sources. This technique was applied to a wide variety of solid perchlorate samples of differing origin to develop a broad database of isotopic signatures among natural and man-made materials. Once this database was established, groundwater samples were collected from various locations, including known military sites and sites suspected to have natural contamination. Isotopic signatures of perchlorate in these waters were then determined. These data were evaluated with other geochemical variables, including iodine and tritium levels, to firmly establish perchlorate origin. Isotopic evaluation of perchlorate found in other environments, including soils, surface water, rainwater, and plant matter, also was performed for source identification. In addition to forensics, recent data suggests that isotopic analysis is a sensitive technique to document and confirm perchlorate biodegradation during in situ remediation efforts. The utility of this technique for monitoring perchlorate in the field also was validated in this project.

Forensic studies at four separate sites were conducted, and the technique for documenting in situ biodegradation was applied at one field location. Moreover, this project has resulted in the description and isotopic characterization of natural perchlorate occurring in various regions of the United States. This information provides a foundation for understanding both the possible mechanisms of formation of natural perchlorate, and the processes that may impact its fate and distribution in soils and groundwater. A summary of the results and conclusions from this project is provided below:

1. Perchlorate has both synthetic and natural sources, each of which contributes to its occurrence in soils and groundwater. Typical analytical methods for determining perchlorate concentrations do not provide direct information on its potential origin.
2. A new forensic approach for perchlorate has been developed, based on measurements of the stable isotopes of chlorine (³⁷Cl and ³⁵Cl) and oxygen (¹⁸O, ¹⁷O, and ¹⁶O), and a radioactive chlorine isotope (³⁶Cl) in perchlorate.
3. The data gained from these isotopic analyses can be used to distinguish natural from synthetic perchlorate in both source materials and environmental samples.
4. Experiments to date indicate that post depositional modification by biodegradation causes a reproducible fractionation factor ratio between O and Cl isotopes in perchlorate (ε¹⁸O/ε³⁷Cl = 2.5) that is roughly perpendicular to the area in which mixtures of synthetic and Chilean ClO₄^- plot in dual isotope plots. Thus, biodegradation will not obscure differences between these two major sources, when considered with the large differences in ³⁶Cl/Cl and/or Δ¹⁷O values (which will not change appreciably during biodegradation). Biodegradation is also unlikely to lead to errors in source delineation among indigenous U.S. sources and synthetic or Chilean sources of ClO₄^- when all relevant parameters are considered.
5. In addition to Cl and O isotopes of perchlorate, there are a large number of supporting methods/analyses available as forensic lines of evidence to help identify sources of perchlorate (or other contaminants) in a groundwater environment. These analyses include basic field parameters, stable H, O, N, and S isotopes in water (H₂O), nitrate (NO₃^-), and sulfate (SO₄^2-), concentrations of anions, dissolved gases, and trace elements, and groundwater dating. The use of these methods in conjunction with Cl and O isotopic analyses of perchlorate is recommended in studies to identify the origin of perchlorate in groundwater.
6. The currently defined ranges of isotopic compositions characteristic of different perchlorate sources may evolve as more samples are analyzed, but these data already have proven useful for identifying perchlorate sources in a number of groundwater studies and additional data is expected to further enhance the value of this approach for perchlorate forensic applications.

This project provided the Department of Defense (DoD) with a scientific assessment of the contribution of natural perchlorate to environmental contamination, a validated forensic technique for distinguishing military from natural perchlorate, and a new method to document perchlorate biodegradation in the field. The results can be valuable in determining DoD's appropriate liability for perchlorate contamination and in verifying perchlorate bioremediation.