Atacama Perchlorate as an Agricultural Contaminant in Groundwater: Isotopic and Chronologic Evidence from Long Island, New York

Perchlorate (ClO4−) is a common groundwater constituent with both synthetic and natural sources. A potentially important source of ClO4− is past agricultural application of ClO4−-bearing natural NO3− fertilizer imported from the Atacama Desert, Chile, but evidence for this has been largely circumstantial. Here we report ClO4− stable isotope data (δ37Cl, δ18O, and Δ17O), along with other supporting chemical and isotopic environmental tracer data, to document groundwater ClO4− contamination sources and history in parts of Long Island, New York. Sampled groundwaters were oxic and ClO4− apparently was not affected by biodegradation within the aquifers. Synthetic ClO4− was indicated by the isotopic method in groundwater near a fireworks disposal site at a former missile base. Atacama ClO4− was indicated in agricultural and urbanizing areas in groundwaters with apparent ages >20 years. In an agricultural area, ClO4− concentrations and ClO4−/NO3− ratios increased with groundwater age, possibly because of decreas...