Perchlorate in the Great Lakes: Isotopic composition and origin

Perchlorate is a persistent and mobile contaminant in the environment with both natural and anthropogenic sources. Stable isotope ratios of oxygen (δ¹⁸O, δ¹⁷O) and chlorine (δ³⁷Cl) along with the abundance of the radioactive isotope ³⁶Cl were used to trace perchlorate sources and behavior in the Laurentian Great Lakes. These lakes were selected for study as a likely repository of recent atmospheric perchlorate deposition. Perchlorate concentrations in the Great Lakes range from 0.05 to 0.13 μg per liter. δ³⁷Cl values of perchlorate from the Great Lakes range from +3.0‰ (Lake Ontario) to +4.0‰ (Lake Superior), whereas δ¹⁸O values range from -4.1‰ (Lake Superior) to +4.0‰ (Lake Erie). Great Lakes perchlorate has mass-independent oxygen isotopic variations with positive δ¹⁷O values (+1.6‰ to +2.7‰) divided into two distinct groups: Lake Superior (+2.7‰) and the other four lakes (∼+1.7‰). The stable isotopic results indicate that perchlorate in the Great Lakes is dominantly of natural origin, having isotopic composition resembling that measured for indigenous perchlorate from preindustrial groundwaters of the western USA. The ³⁶Cl/Cl ratio of perchlorate varies widely from 7.4 × 10^-12 (Lake Ontario) to 6.7 × 10^-11 (Lake Superior). These ³⁶ClO₄^- abundances are consistent with an atmospheric origin of perchlorate in the Great Lakes. The relatively high ³⁶ClO₄^- abundances in the larger lakes (Lakes Superior and Michigan) could be explained by the presence of ³⁶Cl-enriched perchlorate deposited during the period of elevated atmospheric ³⁶Cl activity following thermonuclear bomb tests in the Pacific Ocean.