Atmospheric processes may create perchlorate
Traces of perchlorate are found in rain and snow and can be created in lab experiments simulating tropospheric processes, according to research posted to ES&T’s Research ASAP website. These observations suggest that there is a natural flux of atmospheric perchlorate to the earth and a natural perchlorate background level, says corresponding author Purnendu Dasgupta from Texas Tech University.
Facilities that manufacture or use perchlorate-containing rocket fuel appear to be the source of most contamination plumes. For several years, controversy has swirled around estimates of the health risks posed by perchlorate in drinking water. The U.S. EPA and environmental groups contend that only very low concentrations are safe, while the U.S. Department of Defense and its contractors support higher levels. A National Academy of Sciences committee concluded in January that perchlorate is about 23 times less hazardous than EPA had estimated.
However, as analytical methods improve, researchers are finding low levels of perchlorate everywhere, which suggests a natural flux, says Dasgupta. His lab was the first to publish a study of perchlorate in milk, but Dasgupta hastens to add that finding a natural source of perchlorate should not lessen concerns about anthropogenic contamination, which can range up to parts-per-million levels.
Dasgupta’s group embarked on the new study, which they funded themselves, in an attempt to explain vast swaths of perchlorate-contaminated groundwater in the southern high plains desert of the Texas panhandle. Although there is no evidence of anthropogenic sources in this region, water perchlorate values of 20–60 parts per billion (ppb) were measured (Environ. Sci. Technol. 2003, 37, 376A–377A). Instead, they find that groundwater levels of perchlorate best correlate with levels of iodate, which is known to be of atmospheric origin.
In their newest study, the researchers analyzed 21 rain and 4 snow samples collected mostly in Lubbock, Texas, in 2003 and 2004. They found perchlorate in 70% of the samples at concentrations ranging from 0.02 to 1.6 ppb. Rain collected at Cocoa Beach, Fla., from Hurricane Frances contained 0.6 ppb. The researchers don’t have enough information yet to quantify the flux. “The data are highly variable,” explains co-author Andrew Jackson. “They go from levels too low for us to detect to parts per billion, but we don’t know why.”
The group also passed a sodium chloride aerosol through an electric discharge, which simulated lightning, and detected perchlorate each time, with a ratio of perchlorate to chloride that was two orders of magnitude greater than the control. Perchlorate even formed when salt solutions were exposed to ozone and UV light, simulating desert conditions. Chemist Glen Miller at the University of Nevada, Reno, has found that photochemical oxidation on soil can also generate perchlorate.
The combination of a desert area and recent intensive irrigation may be what it takes to put natural perchlorate into groundwater. In West Texas and the Chilean Atacama Desert, which is the source of perchlorate-containing nitrate fertilizer, the harsh environment may well concentrate the low-level flux, says Dasgupta. Then, activity—mining in Chile or the recent onset of water-intensive farming in West Texas—mobilizes the perchlorate quickly. A natural source may also explain perchlorate contamination in part of eastern Oregon, where arid land has recently moved to intensive crop production, says Kevin Mayer with EPA Region 9.
The Texas group is collaborating with the U.S. Geological Survey to look for perchlorate in rain samples taken across the country and planning further lab experiments to pin down atmospheric mechanisms.
