In 1976, the U.S. Geological Survey, in cooperation with the Wyoming Department of Agriculture, started a monitoring program to determine pesticide concentrations in Wyoming streams. This program was incorporated into the water-quality data-collection system already in operation. Samples were collected at 20 sites for analysis of various insecticides, herbicides, polychlorinated biphenyls, and polychlorinated napthalenes.\
The results through 1978 revealed small concentrations of pesticides in water and bottom-material samples were DDE (39 percent of the concentrations equal to or greater than the minimum reported concentrations of the analytical methods), DDD (20 percent), dieldrin (21 percent), and polychlorinated biphenyls (29 percent). The herbicides most commonly found in water samples were 2,4-D (29 percent of the concentrations equal to or greater than the minimum reported concentrations of the analytical method) and picloram (23 percent). Most concentrations were significantly less than concentrations thought to be harmful to freshwater aquatic life based on available toxicity data. However for some pesticides, U.S. Environmental Protection Agency water-quality criteria for freshwater aquatic life are based on bioaccumulation factors that result in criteria concentrations less than the minimum reported concentrations of the analytical methods. It is not known if certain pesticides were present at concentrations less than the minimum reported concentrations that exceeded these criteria.
