In 2006, the U.S. Geological Survey completed a study in cooperation with the Wyoming Department of Agriculture to determine the occurrence of pesticides in four major rivers within the Bighorn and North Platte River Basins in Wyoming. Surface-water samples were collected at five sites during three different times of the year and detectable concentrations of pesticides were measured in samples collected during all three sampling events. In 2009–10, the USGS, in cooperation with the WDA, resampled three of the sites from the 2006 study and three additional sites located in areas of interest to the State of Wyoming to further describe the occurrence of pesticides in Wyoming’s rivers. The change was made in order to include sites located near cities and towns. Results from the 2009–10 sampling along with comparisons to data collected in 2006 are presented in this fact sheet.

First posted February 28, 2011