Perchlorate-contaminated groundwater could be a widespread legacy of the U.S.'s agricultural past, according to researchers who have pioneered perchlorate forensics. The researchers, led by John Karl Bhlke of the U.S. Geological Survey, used isotopes and other geochemical tracers to identify perchlorate sources. The impact of the historic use of Chilean nitrate fertilizer from the Atacama Desert, which contains naturally occurring perchlorate, is emerging from studies such as one published recently in Environmental Science and Technology (DOI link).

The study, which identifies historic use of the fertilizer as the most likely cause of groundwater contamination in some areas of Long Island, New York, is one of the first published reports on the use of such forensics in the field. Similar studies are under way in California, Iowa, Arkansas, and New Jersey, but these are part of ongoing litigation, according to coauthor Neil Sturchio of the University of Illinois Chicago. The Long Island study "is a beautifully conceived and executed work that will be helpful to pinpoint sources in some other cases, as well," says analytical chemist Purnendu ("Sandy") Dasgupta of the University of Texas Arlington.

Perchlorate can come from many sources, including the manufacture and use of fireworks and road-flare runoff. But two sources - military-industrial sources and Atacama fertilizer - stand out because of the enormous quantities of perchlorate they probably released into the environment in the past, according to Dasgupta. Several years ago, he used historical records to estimate that Chilean nitrate fertilizer likely accounts for more low-level perchlorate contamination in the U.S. than military-industrial sources Environmental Science and Technology (DOI link).

Perchlorate from different natural and synthetic sources has distinctive chlorine and oxygen isotopic signatures, and the signature of Atacama fertilizer can be distinguished from other sources.

Groundwater is the sole drinking-water source for approximately 3 million people in Suffolk County on eastern Long Island. Some of the public water supplies there are being treated to remove perchlorate to comply with the state's drinking-water guidance level of 5 parts per billion (ppb), according to Suffolk County Water Authority CEO Stephen Jones.

Researchers found perchlorate at each of the three locations they sampled in the winter of 2006. The highest concentrations, 4340 and 355 ppb, were measured at different times at a former missile site. These samples had isotopic signatures typical of synthetic perchlorate, but fireworks, not missiles, are considered a likely source of this perchlorate, says Bhlke. He notes that a fireworks disposal pit sits nearby and that high concentrations of potassium, strontium, and antimony, which add color and brightness to fireworks, were found. This conclusion is supported by public records indicating that the missiles housed at the site used liquid fuel, rather than perchlorate-bearing solid rocket fuel.

At current and former agricultural sites, the researchers found deeper groundwater, more than 20 years old, containing perchlorate with Atacama's distinctive isotopic signature. Concentrations were lower than at the fireworks site, and data indicated that old fertilizer is likely the predominant source. "Perchlorate isotopes are still a subject of active research, and there is much yet to learn. Nevertheless, the good match between these Long Island perchlorate isotope ratios and the known Atacama data, combined with other geochemical and chronologic data from the groundwater, make this the simplest explanation for these occurrences," says Bhlke.

Jones believes that massive amounts of fertilizer were used in the area. "Suffolk County used to be the agricultural leader in New York," he says. "The fertilizer from South America would come up here in bulk in railroad cars, and it would be spilled all over."

In 2000, Chilean nitrate producers instituted processing changes that they say removed perchlorate from their product. But Atacama fertilizers used previously contained high concentrations of perchlorate in comparison to current drinking-water standards. For that reason, the legacy of fertilizer use could cause perchlorate to exceed standards in older groundwater, even where the fertilizer was only a minor source of nitrogen, the authors conclude. As a result, similar scenarios may be widespread in the U.S.