Fingerprinting perchlorate sources
A new technique identifies the origins of perchlorate even after microbes break it down.
Perchlorate, found nearly everywhere, has become a fiery topic over the past few years. The chemical, which occurs both naturally and as a residue from explosives and rocket fuel, interferes with thyroid function and is particularly dangerous to children. In research published today on ES&T’s Research ASAP website (DOI: 10.1021/es0621849), a team led by Neil Sturchio of the University of Illinois Chicago reports on advances in perchlorate fingerprinting (75KB PDF) that could help resolve some hot debates surrounding the chemical’s origin.
Communities with perchlorate contamination want to know where it came from and who is responsible for cleaning it up, and isotopes can help answer those questions. But perchlorate can also linger from decades-old sources, such as cold war rocket-fuel production, or the historical use of nitrate fertilizers from Chile’s Atacama Desert, which naturally contain perchlorate. “These change with degradation in the environment, and we need to know if those isotopic signatures are preserved. We need to know if the Atacama signature would degrade to look like man-made, or vice versa,” says Andrew Jackson of Texas Tech University. “This work is important to society,” he adds. “Millions of dollars are spent cleaning up perchlorate, and we need to understand what we’re cleaning up.”
In the new study, Sturchio’s group used isotopes of chlorine and oxygen to determine how biodegradation affects perchlorate’s isotopic composition. They found large effects on isotopes and a strong correlation between effects on chlorine and oxygen. The new work goes beyond the team’s previous research, in which they studied biodegradation of chlorine isotopes in perchlorate.
With both chlorine and oxygen isotopes, the team saw similar effects of biodegradation by two bacterial strains at different temperatures. “You can essentially ‘see through’ any dilution or dispersion of perchlorate, such as mixing with perchlorate-free water” in aquifers, using the combined isotope signatures, Sturchio says. By comparing the results with known signatures, “you can get some idea if perchlorate is biodegrading and [by] how much,” he continues. More important, “you can extrapolate backwards [to an undegraded signature] to get an idea of the perchlorate’s source.”
The approach might help the town of Morgan Hill, Calif. Perchlorate contamination there became a complex mystery, after a national monitoring program turned up perchlorate in about 800 private wells in 2002.
Because a former road-flare manufacturer is located in town, Morgan Hill officials thought at first that the issue of responsibility would be straightforward. But the area used to be covered in orchards and farms, which may have used Chilean fertilizer. And a maker of solid rocket fuel was located to the north. “It really became puzzling in Morgan Hill that we found perchlorate upgradient from the old flare plant,” says Tom Mohr, a hydrogeologist who is the perchlorate project manager for the Santa Clara Valley Water District.
Because water carrying perchlorate should not flow uphill, the road-flare maker, Olin Corp., is only willing to pay for downgradient cleanup. So residents are paying a 15% surcharge on water bills until the matter is resolved—possibly by Sturchio and co-workers’ tests. “Our goal is to see if we can distinguish perchlorate from the manufacture of flares from perchlorate in rocket fuel or fireworks,” Mohr says. The district plans to send samples to Sturchio’s lab within the next few months.
Eventually, Sturchio’s team hopes to be able to pinpoint a particular manufacturer. “They’re not there yet, but this is a critical step,” says Jackson.
The Morgan Hill case won’t be the first time the dual-isotope method has been put to work. On January 31, NASA’s Jet Propulsion Laboratory (JPL) in Pasadena, Calif., released an analysis of perchlorate (817KB PDF) by Sturchio and others. “Pasadena measured significant perchlorate levels in drinking water starting 10 years ago, and pointed the finger at JPL from work they did in the 1940s. But imported Colorado River water may turn out to be much of the source,” Sturchio says.
The team is also thinking beyond water supplies, because perchlorate has been found in the U.S. food supply and in human breast milk and urine. “It’s conceivable that by coupling our methods with studies of food grown in certain areas, we might be able to pinpoint sources of perchlorate in humans,” Sturchio says.
