J.K. Bohlke Neil C. Sturchio John A. Izbicki Nicholas F. Teague Paul B. Hatzinger 2018 <p><span>Perchlorate&nbsp;(ClO</span>₄⁻<span>) in groundwater can be from synthetic or natural sources. Natural sources include ClO</span>₄⁻<span>&nbsp;associated with historical application of imported natural nitrate fertilizer from the Atacama Desert of Chile, and indigenous ClO</span>₄⁻<span>&nbsp;that accumulates locally in arid regions from&nbsp;atmospheric deposition. The Rialto-Colton groundwater subbasin, 80 km east of Los Angeles, California, includes two mapped ClO</span>₄⁻<span>&nbsp;plumes from known military/industrial sources. Larger areas downgradient from those plumes, and in the Chino subbasin to the southwest, also contain ClO</span>₄⁻<span>. Perchlorate from wells was analyzed for&nbsp;chlorine&nbsp;and oxygen&nbsp;stable isotope&nbsp;ratios (δ</span>³⁷<span>Cl, δ</span>¹⁸<span>O, Δ</span>¹⁷<span>O) and radioactive&nbsp;chlorine-36(</span>³⁶<span>Cl) isotopic abundance, along with other geochemical, isotopic, and hydrogeologic data. Isotopic data show that synthetic ClO</span>₄⁻<span>&nbsp;was the dominant source within the mapped plumes. Downgradient from the mapped plumes, and in the Chino subbasin, the dominant source of ClO</span>₄⁻<span>&nbsp;was related to past agricultural use of Chilean (Atacama) nitrate fertilizer. The&nbsp;</span>³⁶<span>Cl and δ</span>¹⁸<span>O data indicate that wells having predominantly synthetic or Atacama ClO</span>₄⁻<span>&nbsp;also contained small fractions of indigenous ClO</span>₄⁻<span>. Little or no differences were observed in&nbsp;isotopic composition&nbsp;or ClO</span>₄⁻<span>&nbsp;source with depth in depth-dependent data from selected wells. Indigenous ClO</span>₄⁻<span>&nbsp;was most evident in upgradient wells having ClO</span>₄⁻<span>&nbsp;concentrations &lt;1 μg/L, consistent with its occurrence as a background constituent throughout the region. Stable isotope ratios of chlorine and oxygen and&nbsp;</span>³⁶<span>Cl isotopic abundance data provided relatively unambiguous discrimination of synthetic and Atacama sources in most wells having ClO</span>₄⁻<span>&nbsp;concentrations greater than 1 μg/L.</span></p> application/pdf 10.1016/j.apgeochem.2018.08.020 en Elsevier Four-dimensional isotopic approach to identify perchlorate sources in groundwater: Application to the Rialto-Colton and Chino subbasins, southern California (USA) article