Perchlorate levels in samples of sodium nitrate fertilizer derived from Chilean caliche

Paleogeochemical deposits in northern Chile are a rich source of naturally occurring sodium nitrate (Chile saltpeter). These ores are mined to isolate NaNO3 (16-0-0) for use as fertilizer. Coincidentally, these very same depo sits are a natural source of perchlorate anion (ClO4-). At sufficiently hig h concentrations, perchlorate interferes with iodide uptake in the thyroid gland and has been used medicinally for this purpose. In 1997, perchlorate contamination was discovered in a number of US water supplies, including La ke Mead and the Colorado River. Subsequently, the Environmental Protection Agency added this species to the Contaminant Candidate List for drinking wa ter and will begin assessing occurrence via the Unregulated Contaminants Mo nitoring Rule in 2001. Effective risk assessment requires characterizing po ssible sources, including fertilizer. Samples were analyzed by ion chromato graphy and confirmed by complexation electrospray ionization mass spectrome try. within a lot, distribution of perchlorate is nearly homogeneous, presu mably due to the manufacturing process. Two different lots we analyzed diff ered by 15%, containing an average of either 1.5 or 1.8 mg g(-1). Inadequat e sample size can lead to incorrect estimations; 100-g samples gave suffici ently consistent and reproducible results. At present, information on natur al attenuation, plant uptake, use/application, and dilution is too limited to evaluate the significance of these findings, and further research is nee ded in these areas. Published by Elsevier Science Ltd.