PERCHLORATE IN THE GREAT LAKES: MASS INVENTORY AND ISOTOPIC CONSTRAINTS ON ITS ORIGIN AND BEHAVIOR

Perchlorate is a highly soluble contaminant found across much of the United States. It is produced naturally in the environment, and it is also manufactured by electrochemical oxidation of chloride for use as an oxidizer in rocket fuels, propellants, matches and flares. Imported Chilean nitrate fertilizers can contain relatively high concentrations of perchlorate as well. The EPA and other environmental organizations are currently highlighting perchlorate contamination because of its known health risks as a goiterogen. It can easily enter the body through ingestion, and it inhibits iodine uptake in the thyroid, causing disruption of thyroidal hormone production. The Great Lakes comprise one of the largest sources of fresh surface water on earth and are essential economic, navigational, and water sources for both the United States and Canada. Average perchlorate concentrations in rainwater and in the Great Lakes have been found to be ~0.014 ppb and 0.05--0.1 micrograms/liter, respectively. This indicates perchlorate is being enriched in Great Lakes waters either by natural processes or by contamination. We are measuring stable isotope ratios of both chlorine and oxygen to determine sources of perchlorate. Stable isotope ratios can distinguish natural and anthropogenic sources. Preliminary data show relatively constant depth profiles of perchlorate from seasonal lake turnover, and a gradual increase in perchlorate concentration from west to east. We hypothesize that the isotopic compositions of perchlorate are relatively natural in Lake Superior and become more synthetic eastward. We present a simple box model for the perchlorate cycle in the Great Lakes based on measured concentrations and isotopic compositions. [This work supported by the U.S. Dept. of Defense's ESTCP program].