Isotopic tracing of perchlorate sources in groundwater from Pomona, California

The groundwater of Pomona, California, is contaminated with perchlorate ( ClO 4 - ). This water is treated to reduce the 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 ClO 4 - has been conducted to determine the source of the contamination. Isotopic compositions were measured for 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 ClO 4 - end-members (synthetic, Atacama, and indigenous natural ClO 4 - ) indicates that contamination is dominantly (85–89%) Atacama 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 ClO 4 - , and (3) numerical hydrologic models yielding travel-times for ClO 4 - from fields to wells that are in the range of 15 to >100 years. The hydrologic models predict that ClO 4 - contamination of Pomona groundwater will persist for decades into the future.