|Abstract:||Thirty pesticides or their degradates were detected in 315 samples collected from 10 streams in the Lake Erie-Lake St. Clair Basin between March 1996 and February 1998 as part of the U.S. Geological Survey?s National Water-Quality Assessment (NAWQA) Program. Atrazine was detected in every sample, and deethylatrazine, metolachlor, and simazine were detected in more than 90 percent of all samples. Atrazine and metolachlor, the most heavily applied pesticides in the Basin, had the highest detected concentrations (85 and 78 micrograms per liter, respectively). No annual average concentrations exceeded the U.S. Environmental Protection Agency Maximum Contaminant Level or health advisory level at any of the surface-water-sampling sites. Seasonally elevated pesticide concentrations, however, have economic consequences on water-treatment facilities required to remove pesticides in water to meet drinking-water standards. From May through July, when most pesticides are transported by runoff into streams, time-weighted average concentrations of atrazine exceeded the Maximum Contaminant Level at five row-crop sites, and time-weighted average concentrations of atrazine and cyanazine frequently exceeded lifetime adult health advisories at these same sites. For some heavily used herbicides such as atrazine, metolachlor, cyanazine, and acetochlor, elevated concentrations persisted 4 to 6 weeks after the initial maximum concentration in the row-crop streams was measured. Land use and physical processes can affect the occurrence and distribution of pesticides. Pesticides were detected at greater frequency and at higher concentrations in samples from streams in basins dominated by row-crop agriculture than in samples from streams in urban or pasture/forest areas. Maximum measured concentrations were higher in 1997 than in 1996 and probably were related to greater precipitation in 1997. Generally, the number of pesticides detected in a basin increased with basin size. Pesticide concentrations showed strong seasonal trends related to the timing and amount of pesticide application. Row-crop herbicides applied in the spring, such as atrazine, had maximum measured concentrations in the spring; pesticides typically applied in late summer and early fall, such as diazinon, had maximum measured concentrations then. The increased number of detections and maximum measured concentrations of acetochlor and the corresponding decrease in the number of detections and concentrations of alachlor reflected changes in the amount of pesticides applied during the sampling period. The percentage of the applied atrazine that was detected in streams, in general, increased when the percentage of impermeable soils within each basin increased.Loads and yields of selected pesticides were calculated. The highest loads calculated were those for atrazine and metolachlor in the Maumee River at Waterville, Ohio, with 47,000 and 44,000 pounds per year, respectively. Of the row-crop basins, either the St. Joseph River near Newville, Ind., or the Auglaize River near Fort Jennings, Ohio, had the highest yields for the herbicides acetochlor, alachlor, atrazine, cyanazine, metolachlor, and simazine. The Cuyahoga River at Cleveland, Ohio, had the highest yields for diazinon and prometon?pesticides that typically are applied heavily in urban areas. The percentage of the applied atrazine that was calculated in the stream was determined for each basin in 1997. The export of atrazine ranged from 0.10 percent at the River Raisin near2 Manchester, Mich., to 10.6 percent at the St. Joseph River near Newville, Ind.