Pesticide breakdown products found in hundreds of US streams

When toxic pesticides break down in the environment, the threat they may pose to aquatic life does not simply disappear—instead, they spawn a deluge of derivatives known as transformation products, which can be just as toxic as their parent molecules.

An extensive environmental survey of pesticide transformation products has now shown that these derivatives are almost ubiquitous in small streams across the US and could be contributing a lot more to the overall toxic burden of some pesticides than researchers previously thought (Environ. Sci. Technol. 2021, DOI: 10.1021/acs.est.0c06625).

The findings come from a 5-year study by the US Geological Survey (USGS), which gathered more than 3,700 water samples from 442 small streams in urban and agricultural areas across five regions spanning the US. The study identified 108 parent pesticides and 116 transformation products in the samples. About 95% of the streams contained at least one pesticide, while 90% contained at least one transformation product. “What we’re finding is that wherever you use a pesticide, you do find its transformation products,” says Barbara J. Mahler, an environmental geochemist at the USGS, who led the research.

“A lot of scientists, a lot of regulatory organizations, tend to ignore the transformation products, so once the parent compound has disappeared they assume everything’s OK,” says Alistair B. A. Boxall, an environmental chemist at the University of York, who was not involved in the study. Previously, Boxall had found that about 30% of pesticide transformation products are actually more toxic than their parents (Environ. Sci. Technol. 2003, DOI: 10.1021/es030038m).

Mahler’s team analyzed the stream water samples using liquid chromatography-tandem mass spectrometry and found picomolar to micromolar concentrations of transformation products formed by a range of chemical reactions, such as oxidation, reduction, and hydrolysis. In many cases, the total concentration of the transformation products was greater than that of the parent pesticide.

Then the researchers compared the measurements with data on the biological effects of these molecules contained in US Environmental Protection Agency databases, including ToxCast. The data included the concentrations that would kill half of the invertebrates or aquatic plants exposed to the compounds as well as information about their sublethal effects, such as changes in gene regulation.

Some transformation products had a significant impact on top of the parent pesticide’s toxicity. For example, in some streams the transformation products of the insecticide fipronil increased the overall toxicity to invertebrates by tens or hundreds of times, compared with fipronil alone. Since the researchers detected fipronil and its transformation product fipronil sulfone in over 20% of all the streams they sampled, “that certainly is of concern,” Mahler says.

ToxCast contains data on sublethal effects for 90% of the parent pesticides found in the study, but only 21% of the transformation products, highlighting important data gaps. When the researchers estimated, as a worst-case scenario, that each transformation product was as toxic as its parent, the number of streams that exceeded the threshold of concern for sublethal effects nearly doubled from 35 to 68.

The team also found that groundwater was the main source of pesticide transformation products, rather than seasonal surface runoff, indicating that aquatic organisms were likely to be exposed to the compounds year-round. This exposure could result in long-term effects such as changes in reproduction rate, the size of offspring, or a creature’s ability to swim, Mahler says.

The study is “quite unique,” Boxall says. “It’s just the sheer scale of the number of sites that they monitored, the number of samples that they took from those sites, and the number of transformation products they looked for.”

Mahler says that researchers should now try to fill the gaps on transformation products in the ToxCast database and develop toxicity tests that are more representative of the organisms living in streams. “We have new pesticides that are being introduced to the market every year, and each of those active ingredients has transformation products,” she says. “It’s a challenge to keep up.”