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

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.