Global Geochemical Cycle of Perchlorate

The Great Lakes hydrologic system is one of the world’s most valuable 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 United States and Canada depend on the Great Lakes for their water supply. 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. 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 (δ18O, Δ17O, δ37Cl, 36Cl/Cl). In addition, we investigated the isotopic composition of chloride from the Great Lakes (δ37Cl, 36Cl/Cl) to provide a comparison of perchlorate with a relatively well-understood conservative contaminant. 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 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 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.