Isotopic discrimination of natural and anthropogenic perchlorate sources in groundwater in a semi-arid region of northeastern Oregon (USA)

Paul B. Hatzinger, J. K. Böhlke, W. Andrew Jackson, Baohua Gu, Stanley J. Mroczkowski, Neil C. Sturchio

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Perchlorate (ClO4) has synthetic and natural sources. Synthetic ClO4 is released to the environment from its use as an oxidant in military and aerospace applications, and from its presence in a variety of common commercial products, such as safety flares, chlorate herbicides, and fireworks. Natural sources of ClO4 in the environment include imported nitrate fertilizers derived from salt deposits in the Atacama Desert of Chile and indigenous natural ClO4 that accumulates in unsaturated soils and groundwaters in other arid and semi-arid environments, largely from atmospheric deposition. The stable isotope ratios of chlorine (37Cl/35Cl) and oxygen (18O/16O, 17O/16O) and the isotopic abundance of radioactive 36Cl in ClO4 can be used to discriminate these different sources. Perchlorate was previously detected at relatively high concentrations (3.8–34.7 μg/L) in groundwater from many wells in the Boardman-Umatilla area near the Columbia River in northeastern Oregon, which is a semi-arid, highly agricultural, heavily irrigated area that includes several past and current military installations. Eight representative groundwater wells were sampled throughout this region and isotopic characteristics of ClO4 collected from each well were measured along with other chemical and isotopic parameters including tritium and other groundwater age indicators. Isotopic data indicate that indigenous natural ClO4 was present in groundwater from all sampled wells and was the predominant source in five of the wells. Synthetic ClO4 was present in the three remaining wells with natural ClO4, and a minor fraction of Atacama-fertilizer-derived ClO4 was indicated in one of the wells. Data from this study expand the geographic area of the USA in which indigenous natural ClO4 has been detected to include the semi-arid northwest. This study also illustrates the role of irrigation recharge as a mechanism for producing relatively high concentrations of indigenous natural ClO4 in groundwater by flushing accumulated salts from the unsaturated zone.

Original languageEnglish
Article number105232
JournalApplied Geochemistry
StatePublished - Apr 2022


  • Atacama
  • Chile
  • Fertilizer
  • Groundwater
  • Irrigation
  • Isotope
  • Nitrate
  • Oregon
  • Perchlorate
  • Umatilla


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