Perchlorate seems to be everywhere these days. It is in a lot of water supplies, in fruits and vegetables, in Chile's Atacama Desert, and even on Mars.
That is good and bad news for the Defense Department and military contractors battling accusations that they have polluted groundwater with perchlorate, a primary component of rocket fuel.
The fact that perchlorate -- a salt comprising a chlorine atom and four oxygen atoms -- occurs naturally makes it difficult to draw simple conclusions about whether to regulate it or remove it from the drinking-water supplies of at least 35 states and the District of Columbia.
On the other hand, that perchlorate is more widespread than previously thought suggests to proponents of federal water regulations that there is all the more need to regulate its presence in drinking water.
The debate over perchlorate now moves to U.S. EPA, which requested public comments this week on possible regulation of perchlorate in drinking water. EPA is asking whether there are alternative ways to evaluate if perchlorate occurs at a frequency and at levels to cause health concerns and whether a national drinking-water regulation would lead to a "meaningful opportunity for health risk reduction" (E&ENews PM, Aug. 5, 2009).
The agency said it is particularly concerned about the possible impact of perchlorate on the health of very young children. Perchlorate has been known to inhibit the thyroid gland's iodine uptake and interfere with fetal development at high doses.
"It is critically important to protect sensitive populations, particularly infants and young children, from perchlorate in drinking water," EPA Administrator Lisa Jackson said in a statement. "As we re-evaluate the science around perchlorate, we will seek public input before making a regulatory determination based on the best science."
But the science behind perchlorate has grown complicated in recent years. Notably, researchers were stunned last year when they discovered perchlorate on Mars.
"The last thing anyone expected was to have almost all of the chloride in Martian soil as perchlorate," said Dave Stonestrom, a research hydrologist with the U.S. Geological Survey.
The Martian perchlorate was formed naturally, Stonestrom said, but no one can say whether Martian perchlorate is found all around the planet or whether its formation is ongoing or is a relic from past conditions.
New technology sparks regulatory debate
The debate over perchlorate began after EPA began noticing it in groundwater at certain California Superfund sites in the 1980s, said Kevin Mayer, the Superfund project manager at the EPA Region 9 office in San Francisco.
There were huge gaps then in what was known about the chemical's toxicology and detection.
But in 1997, scientists developed a way of detecting concentrations of the chemical at 4 parts per billion, a level far lower than levels detectable previously. Almost overnight, it seemed, perchlorate was turning up everywhere, including the Colorado River, which provides water for 20 million people.
"It was pretty clear by the middle of 1997 that Region 9 had a big issue on its hands, bigger than the Superfund program was set up to deal with on a site-by-site basis," Mayer said. "We needed some really good science."
Science was critical because DOD and other parties that were being accused of polluting water and soil with perchlorate were arguing that they were not responsible for contamination nor for the extremely expensive cleanups.
"A lot of people like to know whose perchlorate it is when they find perchlorate where it isn't supposed to be," said Andrew Jackson, an engineering professor and perchlorate researcher at Texas Tech University.
The military has been using manufactured perchlorate since the early 1900s. It is best known for military uses, Mayer said, because so many contaminated sites, especially in California, are linked to defense operations. In other cases, though, there is no clear source for the presence of perchlorate.
To distinguish natural from man-made perchlorate, scientists began using isotopic signatures. They discovered perchlorate atoms that formed naturally have a different number of neutrons than manufactured ones, so the atomic masses differ. The technology is still largely used in research, Jackson said.
Key questions facing EPA and others are where perchlorate is located and how toxic it is, Mayer said. Over the past decade, scientists have been working on those questions, and they still have not found satisfactory answers.
Perchlorate is a salt that accumulates underground just below the soil surface.
In wetter areas, the salts get flushed out, but they accumulate in deserts. The Atacama Desert in Chile has some of the highest levels of the chemical, and Chilean fertilizer, which is exported, has had historically high perchlorate concentrations.
Perchlorate remained in desert areas like the Atacama and throughout the southwest United States until farmers began tilling the soil. As it got stirred around, perchlorate entered groundwater in some areas, Texas Tech's Jackson said.
But how the perchlorate got there continues to puzzle researchers, who have not learned exactly what causes perchlorate to form, Jackson said. It is probably produced in the atmosphere and deposited on the ground, he said, but this process is not well understood.
Scientists are now examining mechanisms that could be causing perchlorate formation. Jackson said atmospheric ozone is certainly involved in some cases, while lightning could also play a role. Another possible explanation is surface oxidation, by which chlorine on the soil's surface might react with oxygen to form perchlorate.
"There are probably various mechanisms -- all high-energy -- but we don't have a good handle on that," USGS's Stonestrom said. "It's still an area of active research."
Perchlorate's effects on human health are no mystery.
It has been known since the 1950s that high doses of perchlorate can disrupt thyroid function and hormone production. Perchlorate was used then to treat overactive thyroids, which cause neurological and behavioral problems in children.
But the questions that remain about what levels of exposure cause problems vex health experts and regulators.
A 2006 study by the Centers for Disease Control and Prevention found a statistical association between low amounts of perchlorate and observed changes in thyroid function in women with low iodine levels. The thyroid gland absorbs iodide from the blood to make and release hormones. The study did not establish a cause-and-effect relationship but pointed to a need for more research, said Benjamin Blount, the lead author and head of CDC's perchlorate biomonitoring lab.
"We need to do studies to explore the perchlorate exposure levels in the general population, especially in more sensitive subgroups," including pregnant and nursing women and infants, Blount said.
Expressing surprise at the results of the CDC study, Thomas Zoeller, a perchlorate expert at the University of Massachusetts, asked, "The question is, does a high-dose, short-term experiment predict a long-term, low-dose effect?"
The questions for EPA are whether those people could be hurt by perchlorate in drinking water and at what concentrations damage begins. EPA has set an interim health advisory level of 15 ppb for drinking water, the highest level that the most sensitive populations -- pregnant women, in this case -- should ingest.
But some CDC studies suggest the thyroid sometimes reacts to low levels -- between 1 and 20 parts per billion (ppb) -- of perchlorate, EPA's Mayer said.
"Is that an adverse effect?" Mayer asked. "We're putting a lot of pieces together to come up with what might be a threat and at what levels it might be a threat."
How to regulate?
Uncertainty about perchlorate's origins -- and by extension, how people are exposed to it -- makes it difficult to regulate.
"Obviously, you need to get it out of drinking water, but universally there's obviously some other source," said Richard Wiles, executive director of the Environmental Working Group. "We have internal debates on the appropriate policy response, and it's really tricky."
Environmentalists and health groups have been pushing EPA to set a more stringent standard for drinking water, arguing that it would quickly reduce exposure levels for the largest number of people.
"This is a very widespread contaminant," Wiles said. "While it's true that some sources of contamination are difficult to pin down, when you have one you can, you should definitely address it."
In January, EPA said it would delay its final decision on perchlorate regulation until the National Academy of Sciences studied the matter. That announcement followed an agency determination that it would not limit perchlorate in drinking water, finding that there was no meaningful opportunity for reducing health risks through regulation (E&ENews PM, Oct. 3, 2008).
But the Obama administration has opted to review decisions on perchlorate made by President George W. Bush's EPA.
The problem: Even if the government imposes strict drinking-water standards, people will still be exposed from a variety of sources. Perchlorate has been found in leafy vegetables and fruit, breast milk and infant formula.
Wiles suggested releasing a health advisory, similar to those issued by the Food and Drug Administration about high mercury-containing fish. That raises its own problems, because a healthy diet should include fruit and leafy vegetables.
But industry groups argue that exposure to perchlorate at low levels is not harmful.
Bill Romanelli of the Perchlorate Information Bureau said perchlorate has been studied for more than five decades and there is no evidence that low doses harm people.
"We have a real chasm of debate here about whether there is any health effect at all," Romanelli said. "It's not just about public health at that point. There are costs associated with regulating any kind of chemical or compound, and there are only a finite amount of resources to do it with."
The debate is far from over -- the Government Accountability Office is now starting a new study to look at the science surrounding perchlorate in drinking water and EPA's progress so far on regulating it, a GAO spokeswoman said. The report will also address another emerging contaminant, trichloroethylene.
Wiles, from the Environmental Working Group, said, "What do you do? There seems to be some source -- the old Chilean argument, maybe. I don't think people know. There's not enough research, although there's some."
But clues are emerging, and scientists are looking for answers everywhere -- even on Mars.