Spatially intensive and weekly data on 227 pesticides (111 parents and 116 degradates) in wadeable streams are being collected by the U.S. Geological Survey’s National Water Quality Assessment (NAWQA) Project in a series of regional synoptic studies. Because complex mixtures are typically detected, mixtures are screened using aquatic-life benchmarks and the Pesticide Toxicity Index (PTI) to assess potential toxicity and to identify key contributing analytes. In the first regional study, done in the Midwest, 12 samples were collected from each of 100 sites (12 urban sites and 88 sites along an agricultural gradient) over 14 weeks (May–August, 2013). A total of 181 pesticide compounds were detected, and the median number of compounds detected was 25 per sample and 54 per site. Corn and soybean herbicides and their degradates were prevalent in agricultural-gradient streams, consistent with past studies. Urban streams had higher detection frequencies and concentrations of insecticides and most fungicides than agricultural-gradient streams. PTI analysis showed potential acute toxicity to invertebrates at 10% of sites, usually dominated by a single pesticide. However, 32% of sites had multiple pesticides at concentrations with potential chronic effects. Acute-plant benchmarks were exceeded at 75% of sites, and chronic-invertebrate benchmarks at 46% of sites. Temporal patterns in benchmark exceedances were different for urban and agricultural-gradient streams. In urban streams, chronic-invertebrate benchmark exceedances peaked in late July/early August, mostly due to the insecticides imidacloprid, fipronil, and organophosphates. In contrast, chronic-invertebrate exceedances in agricultural-gradient streams peaked during late May/early June (spring flush) and were mostly due to the herbicide metolachlor. Plant benchmark exceedance rates were higher in agricultural-gradient streams than urban streams, but peaked in all streams during late May/early June, mostly due to elevated atrazine concentrations. Urban pesticide signatures in Midwestern streams will be compared to those in other urban settings in the U.S.

Agricultural and urban pesticide signatures in wadeable streams from regional stream-quality studies

253rd National Meeting (2017)

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