Perchlorate is widespread in U.S. Southwest
The widespread occurrence of natural perchlorate in unsaturated zones of the southwestern U.S. approaches that of other sources.
Natural atmospheric perchlorate deposits occur in deep, dry soils across the southwestern U.S., at levels that could rival the amount known to come from military sources and natural fertilizer, according to new research published today online in ES&T (doi 10.1021/es062853i). The finding has some concerned about the implications for human exposures.
Perchlorate, a potent oxidizer that disrupts thyroid function by inhibiting the uptake of iodide, is best known for its use in rocket fuel but also occurs naturally in the environment. Since the beginning of the last century, Chilean nitrate fertilizers that contained naturally produced perchlorate were applied to U.S. agricultural fields. Today, perchlorate hot spots occur, some of which seem to extend farther than what is predicted from known contamination sources. Researchers have recently turned to isotopic analysis to help pinpoint the origins of perchlorate, because manufactured and atmospherically deposited perchlorate have very different isotope signatures.
Andrew Jackson of Texas Tech University and colleagues took a different tack to examine atmospherically derived perchlorate in the Southwest’s dry unsaturated or vadose zones. They collected 10–20 grams of soil to get enough perchlorate from 11 sites across seven ecosystems. Isotopic analyses would have required tons of soil.
Previous research led by coauthor Michelle Walvoord of the U.S. Geological Survey (USGS) examined many of the same sites for desert salts in the vadose zone. Using that data, the team correlated perchlorate concentrations in their soil columns with chloride deposits (and in some cases with nitrate, which has a more variable occurrence because of microbial and other activity). The results suggest that the perchlorate arrived in tandem with the chloride and was deposited atmospherically and ubiquitously in the vadose zone across different desert environments—but at very low concentrations.
One studied site sat above the High Plains aquifer in Texas, a highly cultivated region where farmers and water utilities have mined the aquifer. The new ES&T research suggests why the groundwater there now contains relatively high concentrations of perchlorate: pulling out the groundwater and then reapplying it, so it could trickle down through the unsaturated zone, could have “flushed a long-term reservoir into the drinking water,” Jackson says.
This process could also deliver perchlorate to food grown in such dry areas through groundwater used for irrigation. At one Mojave Desert study site, the perchlorate salts migrated 6 meters in less than a decade.
The team estimates that the deserts in the southwestern U.S. hold ~4–103 million kilograms (kg) spread out over 1 million square kilometers. Estimates of Chilean nitrate fertilizer imported into the U.S. amount to ~81 million kg in the past century, and total U.S. production of perchlorate in the past half century has reportedly hit 5 billion kg. The amounts the team found are “significant,” says Sandy Dasgupta of the University of Texas, Arlington. “[The] potential for contaminating groundwater to levels that can easily exceed the currently recommended safe-dosage level is therefore of considerable concern, if this water were to be used as drinking water. . . . Now it would appear people living in arid and semi-arid lands [which is a large part of the U.S.] need to be particularly aware of their iodine nutrition.”
The potential for migration with changes in water-usage patterns also complicates remediation efforts for perchlorate-contaminated military and manufacturing sites. “They think they know where a plume is,” Jackson says of the cleanup efforts by the Department of Defense (DOD), “and then they get a ‘hit’ upstream. It’s becoming a big problem for them: ‘Is this our perchlorate or [is it from] another source?’ they have to ask.”
Shannon Cunniff, director of DOD’s Emerging Contaminants Office, says the new work is “increasing the body of evidence that perchlorate levels that are relevant to public health can exist naturally. Especially in these arid and semiarid regions, one can no longer assume that DOD activities are the source of perchlorate.”
Researchers agree that issues related to the origins of perchlorate are far from settled. The new research “clearly is an important contribution, and the collection of data from these sites presents a very convincing story,” says JK Böhlke of USGS, and “the search for other sources of perchlorate is still on.”
