Distribution of dissolved pesticides and other water quality constituents in small streams, and their relation to land use, in the Willamette River Basin, Oregon, 1996

Water quality samples were collected at sites in 16 randomly selected agricultural and 4 urban subbasins as part of Phase III of the Willamette River Basin Water Quality Study in Oregon during 1996. Ninety-five samples were collected and analyzed for suspended sediment, conventional constituents (temperature, dissolved oxygen, pH, specific conductance, nutrients, biochemical oxygen demand, and bacteria) and a suite of 86 dissolved pesticides. The data were collected to characterize the distribution of dissolved pesticide concentrations in small streams (drainage areas 2.6? 13 square miles) throughout the basin, to document exceedances of water quality standards and guidelines, and to identify the relative importance of several upstream land use categories (urban, agricultural, percent agricultural land, percent of land in grass seed crops, crop diversity) and seasonality in affecting these distributions. A total of 36 pesticides (29 herbicides and 7 insecticides) were detected basinwide. The five most frequently detected compounds were the herbicides atrazine (99% of samples), desethylatrazine (93%), simazine (85%), metolachlor (85%), and diuron (73%). Fifteen compounds were detected in 12?35% of samples, and 16 compounds were detected in 1?9% of samples. Water quality standards or criteria were exceeded more frequently for conventional constituents than for pesticides. State of Oregon water quality standards were exceeded at all but one site for the indicator bacteria E. coli, 3 sites for nitrate, 10 sites for water temperature, 4 sites for dissolved oxygen, and 1 site for pH. Pesticide concentrations, which were usually less than 1 part per billion, exceeded State of Oregon or U.S. Environmental Protection Agency aquatic life toxicity criteria only for chlorpyrifos, in three samples from one site; such criteria have been established for only two other detected pesticides. However, a large number of unusually high concentrations (1?90 parts per billion) were detected, indicating that pesticides in the runoff sampled in these small streams were more highly concentrated than in the larger streams sampled in previous studies. These pulses could have had short term toxicological implications for the affected streams; however, additional toxicological assessment of the detected pesticides was limited because of a lack of available information on the response of aquatic life to the observed pesticide concentrations. Six pesticides, including atrazine, diuron, and metolachlor, had significantly higher (p