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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--bearingnatural NO3- fertilizer imported from the Atacama Desert, Chile, but evidence for this has been largely circumstantial. Here we report ClO4- stable isotope data (delta(CI)-C-37, delta O-18, and Delta O-17), 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 decreasing application rates of Atacama NO3- fertilizers and/or decreasing ClO4- concentrations in Atacama NO3- fertilizers in recent years. Because ClO4-/NO3- ratios of Atacama NO3- fertilizers imported in the past (similar to 2 x 10(-3) mol mol(-1)) were much higher than the ClO4-/NO3- ratio of recommended drinking-water limits (7 X 10(-5) mol mol(-1) in New York), ClO4- could exceed drinking-water limits even where NO3- does not, and where Atacama NO3- was only a minor source of N. Groundwater ClO4- with distinctive isotopic composition was a sensitive indicator of past Atacama NO3- fertilizer use on Long Island and may be common in other areas that received NO3- fertilizers from the late 19th century through the 20th century.