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August 9, 2019

A new analysis of USGS National Water Quality Laboratory (NWQL) quality-control protocols finds that “false-positive” and “false negative” results—reporting that a compound is present in a sample when it is not or that a compound is not present when it is—are minimal.

The study also concludes that historical protocols used by the NWQL to identify detections in environmental samples were robust and produced results that are consistent with current identification protocols.

The NWQL measures concentrations of hundreds of pesticides in water and quantifies pesticide occurrence at environmentally relevant concentrations while maintaining a high level of quality control. More than 30,000 pesticide results for 21 pesticide compounds over a 15-year period were reviewed, making this the most comprehensive USGS pesticide quality-control study to date.

The report describes the steps taken by the laboratory to generate pesticide results, including qualitative and quantitative protocols to identify compounds, assessment of potential laboratory contamination, and other considerations related to the generation of pesticide analytical data. This is the first time that internal NWQL quality-control datasets related to pesticide data have been collated, made publicly available, or used to provide a context for interpreting environmental data. The quality-control datasets are published in an accompanying data release.


Medalie, L., Sandstrom, M.W., Toccalino, P.L., Foreman, W.T., ReVello, R.C., Bexfield, L.M., and Riskin, M.L., 2019, Use of set blanks in reporting pesticide results at the U.S. Geological Survey National Water Quality Laboratory, 2001–15: U.S. Geological Survey Scientific Investigations Report 2019–5055, 147 p.,


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