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March 11, 2021

Pesticide transformation products (TPs) may be a critical missing piece of the toxicity puzzle for aquatic life in small U.S. streams, according to a new study by the USGS.

The continental-scale study of pesticide TPs, the most geographically extensive to date, reveals the near-ubiquity of these chemicals in small U.S. streams. One or more TPs were detected in nearly 90% of 442 small U.S. streams investigated during 2013–2017. The most frequently detected TPs were degradates of nine herbicides, one insecticide, and two fungicides; the summed concentrations of these transformation products frequently exceeded the concentration of the parent pesticide.

The “disappearance” of a pesticide in the environment often is a result of incomplete degradation (transformation) that creates one or more pesticide TPs—new chemicals related to but different from the parent pesticide. Many TPs are less toxic to aquatic life than the parent, but some are more toxic. However, toxicity information is available for only about one quarter of the 117 TPs analyzed for this study.  A toxicity analysis using the assumption that TPs have the same toxicity as their parent pesticide indicates that potential aquatic effects of pesticide-related compounds could be underestimated by an order of magnitude or more.

TPs were commonly detected during baseflow conditions, implicating groundwater as an important source of TPs to streams. The presence of TPs in baseflow indicates chronic and potentially long-term environmental exposures of aquatic life to TPs.

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