Published online 17 December 2008 | Nature | doi:10.1038/news.2008.1310

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Wide-ranging survey reveals low levels of some drugs and pesticides in US tap water.

The researchers found small traces of pesticides and drugs in US drinking water. Punchstock

The most comprehensive survey so far has found a slew of drugs, personal care products, pesticides and other contaminants in drinking water being delivered to millions of people across the United States. None of the compounds appeared at levels thought to be immediately harmful to human health. But the researchers were surprised to find widespread traces of a pesticide, used largely in corn (maize) growing, that has, at higher levels, been linked to cancer and other problems.

The researchers from the Southern Nevada Water Authority (SNWA) in Las Vegas tracked 51 compounds in a survey of 19 water utilities supplying more than 28 million people. Of the 20 drugs or drug metabolites on their list, most of those that the chemists detected were at concentrations of below a microgram per litre, in source water, treated water and tap water. Their findings are published in Environmental Science & Technology1.

The study unexpectedly revealed relatively high levels of the pesticide atrazine, a suspected endocrine disruptor used throughout the US corn belt (the American midwest) but banned by the European Union. The authors detected atrazine in water far from farm land and even in the source water of a plant located in the most arid part of the United States, where the pesticide is not used at all.

Atrazine could be getting into water through food and drink, the researchers suggest, with, for example, many soft drinks containing corn syrup helping the pesticide to spread through the water-treatment system. However, like the other contaminants found by the team, the levels were below the US Environmental Protection Agency's safe maximum. For atrazine, this is 3.0 micrograms per litre; the highest value recorded by the researchers was 930 nanograms (0.93 micrograms) per litre.

The atrazine findings underscore that "there's more contribution [to contaminant loads] from industrial chemicals than pharmaceuticals," says Jörg Drewes, an environmental engineer at the Colorado School of Mines in Golden. He points to the study's results on other endocrine disruptors: bisphenol A, nonylphenol and chlorinated flame retardants. "These could be leaching out of [household] plastics, even plastic pipes [used in] the treatment process," says Drewes.

The tiniest of traces

Scientists and regulators have tracked these compounds for years. But this team was able to detect contaminants at some of the lowest levels yet, using analytical methods generally beyond the means of most labs and utilities, says Stuart Krasner of the Metropolitan Water District of Southern California.

"As analytical chemists improve the sensitivity [of their techniques], they are ... going to see things we couldn't see before," Krasner says. Now, he adds, researchers must establish what risks are posed to human health from long-term, low-level exposure to such contaminants, as well as to mixtures of these chemicals.

The survey is part of a larger project by the Awwa Research Foundation in Denver, Colorado, that is set to be made public next year. Its aim is to help water utilities to find substances that they can monitor to check that their treatment processes are working. Water companies in the United States generally use oxidants — chlorine, ozone or both — to treat water.

"There are virtually no data" on what happens in water-distribution systems, adds Alexa Obolensky of the city of Philadelphia's Water Department. This study "provides that data and toxicological relevance," she says.

Odd abundances

The SNWA survey, says co-author Shane Snyder, found that for drugs "prescription volume did not translate into occurrence". For instance, the team detected the anticholesterol drug atorvastatin (Lipitor), the most prescribed drug in the United States in 2006 and 2007, only in source water and then only for three utilities.

But the epilepsy medicine carbamazepine, not even in the top 200 prescribed drugs in the United States, was one of the most frequently detected in source waters, and appeared at the tap on several occasions too. The drug is hard to break down and so may be more persistent than some other compounds.

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The team also found steroid hormones in source water, but never at the tap. "You have to look at what people are exposed to, not what's in the reservoir," when assessing human exposure risks, says Snyder.

Chlorination breaks down these endogenous steroid hormones in seconds, Snyder adds. And at least one report of hormones in tap water, for San Francisco, has since been found to be a false result.

The survey illustrates that cleaner source waters produce cleaner drinking water at the tap. "We should be going toward protecting water resources," says Obolensky. She believes preventing pollutants getting into the water supply in the first place is the most efficient way of tackling contamination — rather than putting money into advanced treatment at drinking water plants.