Texas chemists show evidence for a natural atmospheric origin of perchlorate. For many, perchlorate is almost synonymous with the rocket fuel that has made it into U.S. groundwater and surface water and subsequently into foods such as lettuce and milk. In the human body, perchlorate inhibits the transport of iodide, which could lead to hypothyroidism.
When Andrew Jackson and Purnendu K. Dasgupta, professors of chemistry and environmental engineering at Texas Tech University, and coworkers sampled the groundwaters of a 60,000-sq-mile region in Texas and New Mexico last year, however, they found unexpectedly high levels of perchlorate even in samples that were unlikely ever to have been exposed to synthetic contamination.
Perchlorate levels in many of the Texas samples were more than 20 ppb, and some were as high as 60 ppb. EPA has recommended a regulatory level of 1 ppb for drinking water, but a recent review released by the National Research Council concludes that perchlorate is less hazardous than EPA estimated (C&EN, Jan. 17, page 13).
Now, the Texas group's follow-up research shows that the Texas perchlorate concentrations best correlate with the concentration of iodate, believed to come from the atmosphere [Environ. Sci. Tech., published online Jan. 29, http://dx.doi.org/10.1021/es048612x].
After also finding perchlorate in Texas rain and snow, Dasgupta's group modeled a few plausible scenarios for atmospheric formation. They took sea salt aerosol, exposed it to an electrical discharge to simulate lightning, and formed perchlorate. Jackson also observed perchlorate formation in a simulated desert water basin, which evaporates and fills up regularly in the presence of ozone.
"There are a multitude of pathways that might make it," Dasgupta says. He believes that both synthetic and natural sources of perchlorate should be monitored. Dasgupta is currently trying to study whether the rather high rates of hypothyroidism in the region surveyed could be attributed to perchlorate.