The Concentration of Naturally Formed Perchlorate, Nitrate, and Other Soluble Salts in the Mojave Desert, California, USA.

Perchlorate (ClO₄^-), a naturally occurring and manmade inorganic contaminant, has been detected in soil, groundwater, surface water, milk, and fresh produce. Perchlorate ingestion may adversely affect human health by competitively inhibiting iodide uptake in the thyroid gland, which may in turn lower the production of key hormones that are needed for proper growth and development. The hyper-arid region of the Atacama Desert, Chile was once believed to be the only environment world-wide where perchlorate salts formed naturally. The occurrence of naturally formed perchlorate has now been investigated in several semi-arid regions of the United States including Texas, New Mexico, Nevada, Utah, and California. This study identified 39 soil horizons containing reportable levels of perchlorate (MRL >165 µg/kg) with a maximum concentration of 23 mg kg^-1. A weak but significant correlation was also observed between perchlorate and nitrate, which could be indicative of similar concentration mechanisms. An investigation of the distribution of perchlorate among soil horizons revealed that over sixty percent of the samples containing perchlorate were from C horizons while only twenty percent of the samples were from B horizons and even fewer in the overlying A horizons. These results suggest that the perchlorate, nitrate and/or other soluble salts have either moved in a “top-down” manner where the salts were deposited on the soil surface and leached deeper in the profiles with time. The other possibility, a “bottom-up” approach, is that the salts were deposited in strata through geological time and have since been redistributed and evapo-concentrated into the upper soil horizons by capillary rise. Soil chemical, morphologic, and geologic explorations of the soils provide evidence for the latter hypothesis although more field measurements should be made to further support this research.