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Widespread Accumulations of Natural Perchlorate in Southwestern Soils

USGS scientists in a shallow trench studying the unsaturated zone.
Soil sampling for naturally occurring perchlorate in Amargosa Desert, Nevada, soils. Just beneath the soil layer with active plant roots the scientists found a salt-rich zone containing perchlorate.

Just below the active root zone in  deserts and other dry regions in the southwestern United States are salt-rich layers with substantial quantities of perchlorate that occurs naturally, according to a paper published in Environmental Science and Technology by U.S. Geological Survey (USGS) scientists and their colleagues. The amount of perchlorate they found—up to hundreds of grams per hectare—is surprising because the aggregate amount of naturally occurring perchlorate may exceed the total amount manufactured to date. The environmental and health community is concerned about perchlorate because:

  • High concentrations of perchlorate in drinking water can cause human health problems related to hypothyroidism.
  • The presence of natural perchlorate could complicate investigations at contamination sites. Distinguishing between natural and man-made perchlorate may become necessary. It is important to consider the possibility of natural sources when evaluating perchlorate levels in water resources, and deciding on potential management actions to protect ecosystem and human health.
  • Irrigation of desert and semi-desert areas, a shift to higher rainfall, disposal of liquid wastes, or construction of dams could mobilize accumulated perchlorate to ground water, which in turn could threaten drinking-water supplies.

The perchlorate the scientists found is present with high concentrations of naturally occurring chloride and other salts, and represents thousands of years of atmospheric deposition and concentration through evaporation. The new study is an outgrowth of previous work showing that nitrate frequently occurs in arid-region soils, as well.


Rao, B., Anderson, T.A., Orris, G.J., Rainwater, K.A., Rajagopalan, S., Sandvig, R.M., Scanlon, B.R., Stonestrom, D.A., Walvoord, M.A., and Jackson, W.A., 2007, Widespread natural perchlorate in unsaturated zones of the southwest United States: Environmental Science and Technology, doi:10.1021/es062853i.

What is Perchlorate?

Perchlorate is an anion consisting of one chlorine atom bonded to four atoms of oxygen, giving it a net charge of minus one (ClO4 -). It readily forms salts with ammonium, potassium, and sodium. Perchlorate is a powerful and easily manufactured oxidant that is commonly used for flares, rocket fuel, and explosives. In addition to manufactured sources, perchlorate occurs naturally as a minor constituent of fertilizers mined from Chilean nitrate ores in the Atacama Desert. Natural perchlorate occurs elsewhere, including evaporite deposits and sub-root-zone accumulations of salty pore water in arid and semi-arid climates (see photo above). Perchlorate is a health concern because it can block iodine uptake by the thyroid gland, disrupting hormonal regulation of neurological development, metabolism, and growth in humans and other complex animals.

Perchlorate Information and References

Orris, G.J., Harvey, G.J., Tsui, D.T., and Eldrige, J.E., 2002, Preliminary analyses for perchlorate in selected natural materials and their derivative products: U.S. Geological Survey Open-File Report 03-314, 6 p.

Ericksen, G. E., 1981, Geology and origin of the Chilean nitrate deposits: U.S. Geological Survey Professional Paper 1188, 37 p.

U.S. Environmental Protection Agency, 2005, Perchlorate and perchlorate salts: Integrated Risk Information System (on-line database).

U.S. Environmental Protection Agency, 2007, Contaminant Focus—Perchlorate: CLU-IN.ORG, Technology Innovation Program, Web page.

Böhlke, J.-K., Sturchio, N.C., Gu, B., Horita, J., Brown, G.M., Jackson, W.A., Batista, J.R., and Hatzinger, P.B., 2005, Perchlorate isotope forensics: Analytical Chemistry, v. 77, no. 23, p. 7838-7842, doi:10.1021/ac051360d.

Plummer, L.N., Böhlke, J.K., and Doughten, M.W., 2006, Perchlorate in Pleistocene and Holocene groundwater in north-central New Mexico: Environmental Science and Technology, v. 40, no. 6, p. 1757-1763, doi:10.1021/es051739h.

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