Isotopic tracing of perchlorate sources in groundwater from Pomona,California

The groundwater of Pomona, California, is contaminated with perchlorate (${\text{ClO}}_4^{-}$). This water is treated to reduce the ${\text{ClO}}_4^{-}$ concentration to less than 6 μg L⁻¹ for compliance with California Department of Public Health drinking water regulations. A study of the isotopic composition of oxygen and chlorine in ${\text{ClO}}_4^{-}$ has been conducted to determine the source of the contamination. Isotopic compositions were measured for ${\text{ClO}}_4^{-}$ samples extracted from 14 wells, yielding ranges of δ¹⁸O values from −10.8‰ to −8.0‰, Δ¹⁷O values from +4.6‰ to +7.5‰, and δ³⁷Cl values from −12.8‰ to −8.9‰. Evaluation of mixing proportions using published isotopic data for three ${\text{ClO}}_4^{-}$ end-members (synthetic, Atacama, and indigenous natural ${\text{ClO}}_4^{-}$) indicates that contamination is dominantly (85–89%) Atacama ${\text{ClO}}_4^{-}$ derived from past use of imported Chilean nitrate fertilizer in citrus cultivation. This interpretation is consistent with (1) aerial photography archives showing extensive citrus fields surrounding Pomona in the early- to mid-20th century, (2) mass-balance estimates for ${\text{ClO}}_4^{-}$, and (3) numerical hydrologic models yielding travel-times for ${\text{ClO}}_4^{-}$ from fields to wells that are in the range of 15 to >100 years. The hydrologic models predict that ${\text{ClO}}_4^{-}$ contamination of Pomona groundwater will persist for decades into the future.