Four-dimensional isotopic approach to identify perchlorate sources in groundwater: Application to the Rialto-Colton and Chino subbasins, southern California (USA)

Abstract

Perchlorate (ClO 4 − ) in groundwater can be from synthetic or natural sources. Natural sources include ClO 4 − associated with historical application of imported natural nitrate fertilizer from the Atacama Desert of Chile, and indigenous ClO 4 − that accumulates locally in arid regions from atmospheric deposition. The Rialto-Colton groundwater subbasin, 80 km east of Los Angeles, California, includes two mapped ClO 4 − plumes from known military/industrial sources. Larger areas downgradient from those plumes, and in the Chino subbasin to the southwest, also contain ClO 4 − . Perchlorate from wells was analyzed for chlorine and oxygen stable isotope ratios (δ 37 Cl, δ 18 O, Δ 17 O) and radioactive chlorine-36( 36 Cl) isotopic abundance, along with other geochemical, isotopic, and hydrogeologic data. Isotopic data show that synthetic ClO 4 − was the dominant source within the mapped plumes. Downgradient from the mapped plumes, and in the Chino subbasin, the dominant source of ClO 4 − was related to past agricultural use of Chilean (Atacama) nitrate fertilizer. The 36 Cl and δ 18 O data indicate that wells having predominantly synthetic or Atacama ClO 4 − also contained small fractions of indigenous ClO 4 − . Little or no differences were observed in isotopic composition or ClO 4 − source with depth in depth-dependent data from selected wells. Indigenous ClO 4 − was most evident in upgradient wells having ClO 4 − concentrations <1 μg/L, consistent with its occurrence as a background constituent throughout the region. Stable isotope ratios of chlorine and oxygen and 36 Cl isotopic abundance data provided relatively unambiguous discrimination of synthetic and Atacama sources in most wells having ClO 4 − concentrations greater than 1 μg/L.