ISOTOPIC IDENTIFICATION OF NATURAL AND ANTHROPOGENIC PERCHLORATE AND NITRATE IN GROUNDWATER

Perchlorate and nitrate are common groundwater contaminants whose occurrence may be linked in some environmental settings. Perchlorate and nitrate both are formed in the atmosphere as a result of photochemical reactions that impart unusual isotopic characteristics including mass-independent signatures. Both species are unusually abundant in the hyper-arid Atacama Desert in Chile because of natural accumulation of salts, and both are present in sodium nitrate fertilizers imported from Chile. Both compounds also have well-known anthropogenic sources that overwhelm the natural sources locally. Nitrate stable isotopes (¹⁵N:¹⁴N and ¹⁸O:¹⁷O:¹⁶O) can provide information about the relative importance of different sources such as atmospheric deposition, nitrification of natural and anthropogenic sources of nitrogen in soils and fertilizers, or local contamination by waste disposal, but the data from both contaminated and uncontaminated settings commonly indicate substantial biogeochemical modification. Perchlorate may be present in groundwater as a result of various combinations of (1) natural production with natural distribution (e.g., local precipitation concentrated by evapotranspiration), (2) natural production with artificial distribution (e.g., application of Chilean nitrate fertilizers containing perchlorate), or (3) artificial production with artificial distribution (e.g., military and commercial releases of synthetic materials). Comprehensive stable isotope analyses (³⁷Cl:³⁵Cl and ¹⁸O:¹⁷O:¹⁶O) of perchlorate from known synthetic and natural sources reveal systematic differences in isotopic characteristics that are related to the formation mechanisms. New isotopic data from several different sites in the western United States prove that both naturally-produced and synthetic sources of perchlorate are present in groundwater at concentrations exceeding 10 µg/L. More work is needed on the association between perchlorate, nitrate, and other oxyanions in natural and agricultural settings, and the biogeochemical processes that can affect their isotopic compositions.