Not-so-simple patterns of neonicotinoids and diamides in small Prairie streams: implications for assessing risk and understanding pesticide dynamics

Year-round neonicotinoids detections in waterways pose a threat to aquatic ecosystems and drinking water supplies. Neonicotinoids, and increasingly diamides, are being used in the Canadian Prairies, but there is a paucity of detection and concentration data in streams and rivers. We report on neonicotinoids and diamides in 16 streams in southern Saskatchewan, Canada between 2017 and 2019. Approximately half of all samples had measurable levels of at least one insecticide, generally below guidelines. Thiamethoxam was most frequently detected across sites (42%), followed by clothianidin (18%) and imidacloprid (9%), while diamide detections differed with location. Most samples with detections contained at least one of thiamethoxam, clothianidin, or imidacloprid (98%). About 15% of samples between 2018 and 2019 detected diamides, reflecting their increasing use in Canada. While thiamethoxam and clothianidin concentrations were similar between rain events and snowmelt, their average daily loads were greatest during snowmelt (p < 0.05); suggesting overwintering and spring freshet as a significant source to streams. Generally, agriculturally intensive subwatersheds dominated by canola and cereals had higher neonicotinoid concentrations, yet crop cover and sites explained a small proportion of the variance. Neither site, crop, flow, or year considerably accounted for the large variation in detections, suggesting a complexity of factors. Based on probability distributions, exceedances of 7% to 15% were observed for thiamethoxam, clothianidin, and imidacloprid when compared to a highly protective chronic predicted no-effect concentration guideline value, suggesting limited acute or chronic risk in these systems. The variation of insecticide concentrations reflects crop practices, precipitation, prairie hydrology, agricultural practices, and environmental conditions, and highlights the need for improved monitoring across Canada to better understand processes affecting their distribution and risk.