The study, a collaboration between USGS and the U.S. Environmental Protection Agency Office of Pesticide Programs, collected daily and weekly samples from seven streams in the Midwest and seven streams in the Southeast in agricultural or urban areas. In the Midwest, 141 different pesticides were detected at least once in daily samples and 70 were detected in weekly samples. In the Southeast, 73 pesticides were detected in daily samples and 32 in weekly samples.

Some insecticides measured in the daily samples were at concentrations that could be acutely toxic to stream-dwelling organisms but those concentration peaks were not reflected in the weekly samples. Daily sampling revealed that pesticides occurred at concentrations potentially toxic to bottom-dwelling organisms at least once at 11 of the 14 sites, but weekly samples did not indicate potential toxicity at any sites.

The occurrence of pesticides—especially insecticides—may be degrading the communities of organisms living in those streams. The more days that the concentration of the mix of pesticides present exceeded a chronic toxicity threshold, the fewer the number of mayfly, stonefly, and caddisfly species present. Mayflies, stoneflies, and caddisflies are sensitive species of stream insects and are often used to assess stream water quality.

Citation: Daily stream samples reveal highly complex pesticide occurrence and potential toxicity to aquatic life. Norman, J.E., Mahler, B.J., Nowell, L.H., Van Metre, P.C., Sandstrom, M.W., Corbin, M.A., Qian, Y., Pankow, J.F., Luo, W., Fitzgerald, N.B., Asher, W.E., McWhirter, K.J. Science of the Total Environment. Vol. 715: 136795. https://www.sciencedirect.com/science/article/pii/S0048969720303053?via…