Global Geochemical Cycle of Perchlorate

The Great Lakes hydrologic system is one of the world's most precious natural resources, and over time its value will only rise with increasing fresh water demand of the fast-growing human population. Millions of people and a wide range of large industries both in the US and Canada depend on the Great Lakes for their water supply. The heavy industrialization of the Great Lakes region during the past century has considerably altered the biogeochemical balance of the Great Lakes ecosystem. Increased levels of a variety of contaminants have significantly affected water quality with potentially serious public health consequences for the millions of people who rely on the Great Lakes for their drinking water. Fortunately, the US and Canadian governments have realized the potential long-term catastrophic consequences of the contamination of the Great Lakes, and its implications for the large number of people living in the region. In the early 1970s, both countries made a commitment to restore and maintain the chemical, physical and biological integrity of the Great Lakes basin ecosystem. These efforts by the US and Canada have resulted in subsequent improvements of the Great Lakes water quality. Despite having some negative effects on the Great Lakes, the industrialization of this region has enormous economical benefits for both countries, thus making it implausible to ban all activities or compounds that could potentially degrade Great Lakes water quality. The key for sustainable future development is to maintain the economic vitality of the Great Lakes region without compromising the ecological balance and water quality in the Great Lakes basin, and preserving these valuable natural resources for the future generations. In order to sustain environmentally sound progress in the Great Lakes region it is essential to have a comprehensive understanding of the behavior and origin of contaminants having potentially adverse effects on the ecosystem. As a contribution to this understanding, we investigated perchlorate origin, transport, and biodegradation and uptake in all five lakes by using perchlorate concentrations, oxygen and chlorine isotope ratios (delta37Cl, delta18O, Delta 17O, 36Cl/Cl). In addition, we investigated the isotopic composition of chloride from the Great Lakes (delta37Cl, 36Cl/Cl) to provide a comparison of perchlorate with a relatively well-understood conservative contaminant.;The delta37Cl values of perchlorate are nearly constant in all five Great Lakes with an average of +3.4 +/- 0.4 ‰. We also analyzed the delta37Cl values of the chloride from the Great Lakes and these are similarly constant, averaging at 0.2 +/-0.1 ‰. In contrast, the delta18O values of perchlorate have a larger range, with the values increasing from Lake Superior (-4.1 ‰) to Lake Erie (+4.0 ‰). This large range in delta18O values is similar to the variability of the delta18O values in precipitation over the Great Lakes basin, indicating that this observed relatively large variability of the delta18O values of perchlorate could potentially reflect the isotopic variability of the source material for perchlorate formation. The Delta17O values in perchlorate from the Great Lakes show a mass-independent oxygen isotopic signature. The Delta17O values define two distinct groups, with a higher Delta17O value in Lake Superior (+2.7 ‰) and lower Delta17O values in the other four lakes (+1.6 ‰ to +1.8 ‰). However, the observed variations in oxygen isotope ratios may also indicate mixing with other sources, or possibly reactions such as biodegradation or oxygen exchange. Great Lakes perchlorate generally resembles that of indigenous natural perchlorate from the southwestern USA in terms of the delta18O, Delta17O and delta 37Cl values. Thus, the isotopic constraints along with the relatively low and nearly constant perchlorate concentrations observed across all five Great Lakes indicate that perchlorate has a predominantly natural origin in the Great Lakes.;Overall, this study has identified the dominant source of Great Lakes perchlorate and evaluated its biogeochemical cycle in a hydrological context. The combination of concentration and isotopic analyses, and simple numerical models quantify the perchlorate inputs from all potential sources and perchlorate outputs through outflow and other processes (e.g. biodegradation and uptake). We also quantified isotopic fractionation effects associated with the perchlorate loss in the Great Lakes basin. The numerical models also successfully explain the observed temporal evolution of 36Cl/Cl ratios in perchlorate and chloride from the Great Lakes over the last 70 years. The new insights obtained on the behavior of these conservative solutes may provide a useful reference for studies of the origin and behavior of other contaminants in the Great Lakes system. (Abstract shortened by UMI.).