How often do mixtures occur?
Pesticides almost always occur as complex mixtures of multiple compounds, rather than individually, whether in water, bed sediment, or fish tissue. Throughout most of the year (more than 90 percent of the time), water samples from streams with agricultural, urban, or mixed-land-use watersheds contained 2 or more pesticides or degradates, and about 20 percent of the time they had 10 or more. Mixtures were less common in ground water. Nevertheless, about half of the shallow wells in agricultural areas and about a third of the shallow wells in urban areas contained 2 or more pesticides and degradates—less than 1 percent had 10 or more. The co-occurrence of pesticides included thousands of unique mixtures, some uncommon and some very common. The herbicides atrazine (and its degradate, deethylatrazine), simazine, metolachlor, and prometon were common in mixtures found in streams and ground water in agricultural areas. The insecticides diazinon, chlorpyrifos, carbaryl, and malathion were com
