A Comparative Study of the Co-Occurrence of Nitrate and Naturally Formed Perchlorate in the Mojave Desert, California, USA and the Atacama Desert, Chile.

Perchlorate is both a naturally occurring and manmade contaminant that has been identified in soil, groundwater and surface water. Perchlorate directly affects human health by interfering with iodide uptake in the thyroid gland, which may in turn lower the production of key hormones that are needed for proper growth and development. Until recently, the Atacama Desert, Chile was thought to be the only location where perchlorate salts formed naturally. Recent work has documented the occurrence of these salts in several semi-arid regions of the United States. The focus of this study was to identify putatively natural sources of perchlorate in the Mojave Desert of California with specific emphasis on the Death Valley region. Soil samples were collected from six field sites varying in geologic age. Of the 189 samples analyzed, 31 contained reportable levels of perchlorate (MRL >160 µg/kg).  The co-occurrence of perchlorate and nitrate in caliches from the Atacama Desert and soils from the Mojave Desert has also been observed.  Although the former are richer in NO₃^-, near-ore-grade (~5%) deposits occur in the vicinity of Death Valley National Park.  Weak but significant correlations exist between ClO₄^- and NO₃^- at both locations, but the perchlorate levels are much higher (up to 800 mg/kg) in the Chilean samples than in California (<25 mg/kg). These results may reflect geologic differences in landform age and stability between the two localities and or variation in the origins of nitrate. The oxygen isotopes in the nitrate are currently under investigation in an attempt to better understand the sources of these anions in both environments.