Spatial and temporal trends in pesticide mixtures within channelized agricultural headwater streams in central Ohio

Background/Question/Methods

Channelized agricultural headwater streams are streams that have been created or modified for agricultural drainage.  Previous research within these modified streams has documented the periodic occurrence of elevated concentrations and seasonal changes in concentrations of individual pesticides.  However, the majority of this research has focused on single pesticides of interest and has not evaluated the spatial and temporal trends exhibited by pesticide mixtures.  Recent ecotoxicology findings suggest that pesticide mixtures may have a greater effect on aquatic animals and plants than individual pesticides.  Documenting the spatial and temporal trends of pesticide mixtures is a first step towards understanding the risks of pesticide mixtures in channelized agricultural headwater streams.  We measured concentrations of 13 pesticides in water samples collected from seven agricultural headwater streams in central Ohio over a four year period to address the following research questions:  1) what type of pesticide mixtures occur in channelized agricultural headwater streams? 2) does watershed size influence pesticide mixtures?; and 3) do pesticide mixtures change seasonally and/or annually?

Results/Conclusions

Preliminary analysis of the first two years of data documented that atrazine, simazine, metolachlor, metalaxyl, and chlorothalonil were the five most frequently occurring pesticides within all water samples.  The other eight pesticides measured occurred in less than 10% of all water samples.  No differences in mean number of pesticides (P > 0.05) occurred between streams with different watershed sizes.  Channelized streams with watershed sizes averaging 4 km² contained pesticide mixtures with greater percent herbicide concentrations (P < 0.05) and lesser percent insecticide concentrations (P < 0.05) than smaller channelized streams with watershed sizes averaging 1.3 km².   Additionally, pesticide mixtures from May and June contained the greatest number of herbicides (P < 0.05), while pesticide mixtures from the July and August contained the greatest number of fungicides (P < 0.05).  Our preliminary analyses suggest that the composition of pesticide mixtures within channelized agricultural headwater streams in central Ohio differs with watershed size and season.