Complex mixtures of dissolved pesticides show potential aquatic toxicity in a synoptic study of Midwestern U.S. streams

The authors sampled 100 streams from May-August 2013 weekly to characterize their pesticide loads. A median of 25 compounds was detected per sample, with 54 per site. Potential acute toxicity to invertebrates was predicted in 12% of streams, with 53% of streams showing chronic toxicity for 21 days or longer (though the authors note that weekly sampling underestimates acute exposures). Effects on aquatic plants that were likely reversible were predicted in 75% of streams, with longer-term impacts in 9%. A few of the pesticides identified in water were major contributors to toxicity, including imidacloprid. Agricultural streams had higher detections of herbicides, while urban streams had more frequent detections of insecticides and fungicides and higher potential for invertebrate toxicity. Imidacloprid was the most frequently detected insecticide across all sites, and was a major contributor to toxicity to invertebrates. Urban imidacloprid concentrations peaked during late July to early August. The authors note that neonicotinoid toxicity may be underestimated because only imidacloprid was analyzed, while other studies have found clothianidin and thiamethoxam more frequently than imidacloprid in Midwest agricultural streams.