The most commonly detected pesticides in streams were among the most heavily used in the basin. The most frequently detected pesticide in streams was 2,4-D, the most heavily used herbicide in the Puget Sound Basin. Other commonly detected and heavily used pesticides were the herbicide dicamba and the insecticide diazinon.
Pesticide concentrations generally were small. None of the detected pesticides in streams exceeded existing State or Federal freshwater aquatic life criteria; however, criteria have been established for only two of the pesticides detected. Diazinon, mevinphos, malathion, and diuron were found in streams exceeding maximum concentrations recommended by the National Academy of Sciences for the protection of aquatic life (National Academy of Sciences, 1973). None of the detected pesticides exceeded the USEPA's standards or guidelines for drinking water.
Pesticides that are currently banned in the U. S. were found in streambed sediments. The pesticide most commonly found in streambed sediments was the fungicide PCP (pentachlorophenol); the insecticides DDT and chlordane were also detected. These compounds are organochlorine pesticides that were heavily used in the past, but are now banned from use in the U. S. However, detection of organochlorine pesticides is common in sediments across the U. S. because these compounds degrade slowly and bind strongly to soil particles. No State freshwater sediment-quality criteria exist for these compounds, but the USEPA has proposed guidelines for DDT and chlordane to protect benthic organisms.