Perchlorate abundant in Nevada desert

New research has characterized for the first time the distribution of natural perchlorate in a terrestrial ecosystem. The results should allow better assessments of the risks perchlorate pose to ecosystems and human health, as well as the potential movement of natural perchlorate contamination into groundwater.

Three scientists sampling desert soil

Perchlorate, which is found in rocket fuel and fireworks, also forms naturally in the atmosphere and is then deposited onto the land surface. However, while perchlorate contamination near former military and industrial sites has been well documented, the natural background levels of the chemical are poorly understood.

Scientists at the U.S. Geological Survey (USGS) and Texas Tech University quantified natural levels of perchlorate in desert soil, plants, and rain and other air-borne materials, and identified what controls the chemical’s cycling and accumulation. Knowing these background levels and how perchlorate participates in soil-plant-atmospheric processes should help scientists and land managers identify, interpret, and treat sites contaminated by human uses of perchlorate.

The study published online in the Journal of Environmental Quality on Mar. 21, 2014.

The research is important because perchlorate has emerged as an environmental contaminant in drinking water and food. Medical research has shown its ingestion can disrupt human thyroid function. Fetuses and infants are at the greatest risk because of the role thyroid hormones play in early growth and brain development.

The researchers collected and analyzed soil, leaves from shrubs, and rain, dust, and other materials from the atmosphere at the USGS Amargosa Desert Research Site in Nevada, USA. Shallow soils in the research area contained high levels of perchlorate—10 to 20 grams per hectare (about one- to three-tenths of an ounce per acre) in the top 30 centimeters (one foot).

That amount of highly soluble perchlorate, if flushed to groundwater, would be enough to contaminate roughly a quarter million gallons of water per acre at California’s maximum contaminant level. Although there is no federal drinking water standard for perchlorate, California’s maximum contaminant level for perchlorate in drinking water is six micrograms per liter (or one ounce of perchlorate per 125 million gallons of water).

Two scientists in desert

The study also identified previously-unrecognized sinks for perchlorate in desert ecosystems. In particular, the creosote bush canopy was found to be a reservoir for the chemical. Creosote bush—the dominant species in the desert plant community studied by the researchers—contained about four times the annual average atmospheric deposition of perchlorate, or about one to two grams of perchlorate per hectare.

Creosote bush normally keeps its leaves year round, but drops them—along with their accumulated perchlorate load—during extended periods of drought. Creosote bush is also browsed by many small mammals, while other desert shrubs, such as shadscale and Mormon tea, are eaten by livestock and wildlife.

What this all suggests, say the scientists, is that desert vegetation plays a key role in regulating the accumulation, retention, and release of perchlorate across the land surface. Its transfer from soil, through plants, and to higher organisms may also lead to perchlorate exposure in desert ecosystems.

The team also measured atmospheric deposition rates for perchlorate that are about 10 times higher than previously reported. Atmospheric deposition occurs both from rain—which washes compounds out of the atmosphere—and the settling out of dry, airborne materials, including dust. Previous atmospheric deposition rates for perchlorate were reported for wet atmospheric deposition only. Dry deposition is normally excluded from atmospheric deposition network collectors, but this study showed that dry deposition is an important contributor of perchlorate to the land surface.

Funding for this research was provided by the USGS programs, Toxic Substances Hydrology, National Research, Groundwater Resources, and National Water Quality Assessment; and by the U.S. Department of Defense Strategic Environmental Research and Development Program.

Source: USGS news release

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