Pesticides and transformation products in surface waters of western Montérégie, Canada: occurrence, spatial distribution and ecotoxicological risks†

The objective of this study was to investigate the occurrence, spatial distribution, and ecotoxicological risk of pesticides and transformation products in surface waters of western Montérégie (Quebec, Canada). A total of 29 samples were collected from 11 rivers during the summers of 2019 and 2021, and the samples were analyzed for 48 pesticides and 8 transformation products. The downstream data were used to assess the ecotoxicological risks based on Quebec's acute or chronic aquatic life criterion (AALC or CALC). Overall, 9 herbicides (glyphosate, S-metolachlor, 2,4-D, metribuzin, atrazine, MCPA, prometryn, dimethenamid, simazine), 3 insecticides (clothianidin, imidacloprid, chlorantraniliprole), and 4 fungicides (azoxystrobin, fluxapyroxad, tebuconazole, carbendazim) were detected at all sampling sites, demonstrating their widespread use in western Montérégie. Glyphosate (87–4095 ng L⁻¹), S-metolachlor (6–2519 ng L⁻¹), and 2,4-D (6–1094 ng L⁻¹) were identified as the most abundant pesticides in surface water. Furthermore, 6 pesticide transformation products (metolachlor ESA, AMPA, metolachlor OA, desethylatrazine, atrazine-2-hydroxy, desisopropylatrazine) were detected at all sampling sites. The concentration of transformation products accounted for 51% on average of the total concentration, demonstrating the abundance of transformation products in surface waters. Neonicotinoids exhibited the highest ecotoxicological risk in the surface water samples with an average CALC risk quotient of 28 for 2019 and 12 for 2021, respectively. The present study offers insights into pesticides occurrence and their ecological impacts on surface waters of western Montérégie and allows for supporting future pesticide management and ecotoxicological risk mitigation strategies.