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The Small Streams Toxicity/Pesticide Study is a study intended to assess the possible biological implications associated with the presence of pesticides in selected small streams in King County. This study was begun after recent studies detected the presence of pesticides in storm runoff and surface waters in King County and elsewhere (Davis 1993, Davis 1996, Davis 1998, Davis 2000, Voss and Embrey, 2000, Voss et al. 1999). While pesticides have been a concern in the surface waters draining agricultural areas, these studies have shown that small urban and suburban streams can contain high concentrations of a wide variety of pesticides during storm runoff periods. This has led to the hypothesis that chemicals applied to lawns and landscapes are consistently making their way into the aquatic environment through non-point runoff. Many of the pesticides present in these urban or suburban streams do not have water quality standards or guidelines. Therefore, knowledge of the presence and concentration of pesticides in streams alone does not provide a full understanding of the ecological consequences of these pesticides to aquatic life in these streams.

The Small Streams Toxicity/Pesticide Study included analysis of: approximately 150 different pesticides, dissolved and total metals, and other contaminants; toxicity using up to three different test species (Ceriodaphnia dubia, Lemna minor, and Selenastrum capricornutum); dissolved and total organic carbon and total suspended solids. Samples were collected during spring storms, summer baseflows, and fall storms. Sampling was conducted during four years: 1999, 2000, 2001, and 2002. In 1999, samples were collected from Lyon, Juanita, Lewis, and Rock Creeks. In 2000, Lyon, Swamp, and Little Bear, were sampled. Additionally, a small tributary draining into the Sammamish River near 124th street in the Sammamish Valley was tested one time during summer baseflow conditions. In 2001, Big Bear and Issaquah Creeks were sampled. And in 2002 North, Little Bear, and the 124th street Creeks were sampled. Rock Creek was tested for pesticides in 1999 and then it was used as the reference stream for toxicity testing during each year of the study. Additionally, a set of effects threshold levels were identified for all contaminants detected during this study that do not have water quality standards.