Chlorine-36 as a tracer of perchlorate origin

Perchlorate (ClO₄⁻) is ubiquitous in the environment. It is produced naturally by atmospheric photochemical reactions, and also is synthesized in large quantities for military, aerospace, and industrial applications. Nitrate-enriched salt deposits of the Atacama Desert (Chile) contain high concentrations of natural ClO₄⁻, and have been exported worldwide since the mid-1800s for use in agriculture. The widespread introduction of synthetic and agricultural ClO₄⁻ into the environment has contaminated numerous municipal water supplies. Stable isotope ratio measurements of Cl and O have been applied for discrimination of different ClO₄⁻ sources in the environment. This study explores the potential of ³⁶Cl measurements for further improving the discrimination of ClO₄⁻ sources. Groundwater and desert soil samples from the southwestern United States (U.S.) contain ClO₄⁻ having high ³⁶Cl abundances (³⁶Cl/Cl = 3100 × 10⁻¹⁵ to 28,800 × 10⁻¹⁵), compared with those from the Atacama Desert (³⁶Cl/Cl = 0.9 × 10⁻¹⁵ to 590 × 10⁻¹⁵) and synthetic ClO₄⁻reagents and products (³⁶Cl/Cl = 0.0 × 10⁻¹⁵ to 40 × 10⁻¹⁵). In conjunction with stable Cl and O isotope ratios, ³⁶Cl data provide a clear distinction among three principal ClO₄⁻ source types in the environment of the southwestern U.S.