Most samples with a detectable pesticide contained mixtures of two
or more detectable pesticides. These mixtures differ in agricultural
and urban areas and in relation to crops grown and agricultural
pests. For example, simazine and prometon were in the most commonly occurring mixtures of two or more compounds in urban areas, whereas atrazine, DEA, and metolachlor were the most common compounds in mixtures found in agricultural areas. A distinctive
feature of urban streams was the common occurrence of mixtures
of herbicides and insecticides. Urban streams and rivers with mixed
land-use influences generally contained mixtures of greater numbers of compounds than did agricultural streams. More than 10% of
urban stream samples contained a mixture of at least four herbicides, plus diazinon and chlorpyrifos.
