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

•Information on pesticide mixtures within agricultural headwater streams is limited.•Pesticide mixtures were documented in channelized agricultural headwater streams in Ohio for 5 years.•Pesticide mixtures consisted mostly of herbicide or herbicide–fungicide mixtures.•Seasonal trends in pesticide mixtures differed annually.•Pesticide mixtures did not differ among watershed sizes ranging from 0.7 to 4.5km2. 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 4.5km2 will exhibit greater temporal variation than spatial variation in pesticide mixtures.