Annual and seasonal differences in pesticide mixtures within channelized agricultural headwater streams in central Ohio

Only a limited amount of information on pesticide mixtures within agricultural headwater streams is available. A greater understanding of the characteristics of pesticide mixtures and their spatial and temporal trends within agricultural headwater streams is needed to evaluate the risks of pesticide mixtures. We measured concentrations of 13 pesticides within seven channelized agricultural headwater streams in central Ohio for 5 years to document what types of pesticide mixtures occur, trends in pesticide mixtures with increasing watershed size, and annual and seasonal trends in pesticide mixtures. Pesticide mixtures consisted mostly of combinations of herbicides or herbicides and fungicides. Only one of 14 response variables differed between watershed sizes. Seasonal trends in the percent occurrence, complexity, concentration, and types of pesticide mixtures differed annually. Our results suggest channelized agricultural headwater streams with watershed sizes ranging from 0.7 to 4.5 km2 will exhibit greater temporal variation than spatial variation in pesticide mixtures.