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Perchlorate (ClO4-) in groundwater can be from synthetic or natural sources. Natural sources include ClO4 associated with historical application of imported natural nitrate fertilizer from the Atacama Desert of Chile, and indigenous ClO4- that accumulates locally in arid regions from atmospheric deposition. The Rialto-Colton groundwater subbasin, 80 km east of Los Angeles, California, includes two mapped ClO4-plumes from known military/industrial sources. Larger areas downgradient from those plumes, and in the Chino subbasin to the southwest, also contain ClO4-. Perchlorate from wells was analyzed for chlorine and oxygen stable isotope ratios (delta Cl-37, delta O-18,Delta O-17) and radioactive chlorine-36 (Cl-36) isotopic abundance, along with other geochemical, isotopic, and hydrogeologic data. Isotopic data show that synthetic ClO4-was the dominant source within the mapped plumes. Downgradient from the mapped plumes, and in the Chino subbasin, the dominant source of ClO4- was related to past agricultural use of Chilean (Atacama) nitrate fertilizer. The Cl-36 and delta O-18 data indicate that wells having predominantly synthetic or Atacama ClO4- also contained small fractions of indigenous ClO4-. Little or no differences were observed in isotopic composition or ClO4- source with depth in depthdependent data from selected wells. Indigenous ClO4- was most evident in upgradient wells having ClO4 concentrations< 1 mu g/L, consistent with its occurrence as a background constituent throughout the region. Stable isotope ratios of chlorine and oxygen and Cl-36 isotopic abundance data provided relatively unambiguous discrimination of synthetic and Atacama sources in most wells having ClO4- concentrations greater than 1 mu g/L.