Screening for the Pesticides Atrazine, Chlorpyrifos, Diazinon, Metolachlor, and Simazine in Selected Michigan Streams, March-November 2005

From March through November 2005, the U.S. Geological Survey, in cooperation with the Michigan Department of Environmental Quality (MDEQ), did a statewide screening to aid in understanding the occurrence and distribution of selected pesticides in Michigan streams. Stream-water samples were collected from 23 sites throughout Michigan. In all, 320 water samples were analyzed by use of rapid immunoassay methods for the herbicides atrazine, metolachlor, and simazine and the insecticides chlorpyrifos and diazinon. On one occasion (June, 2005), atrazine concentrations exceeded the Michigan water-quality value (7.3 micrograms per liter) at the Black River in St. Clair County. Neither chlorpyrifos nor diazinon was detected during April through September. MDEQ detected chlorpyrifos in streams throughout the state in November. Herbicide concentrations were highest in samples influenced by intensive agriculture; however, median herbicide concentrations were similar among agricultural and urban sites. Concentrations of herbicides were very low to undetected in undeveloped areas. Seasonal patterns were also evident during the sampling period. Increased concentrations generally occurred in late spring to early summer. At 11 sites, daily sampling was done every day for 5 days following a rainfall after herbicide application in the area. Substantial changes in concentrations of herbicides - greater than tenfold from the previous day - were observed during the daily sampling. No consistent relation was found between concentration and streamflow. Results of this study may be used to aid in the development of a more comprehensive pesticide monitoring study for the State of Michigan.