Pesticide presence and distribution in a cold region agricultural basin

Variable precipitation patterns in cold climates, worsening due to climate change, raise concerns regarding pesticide mobility and mixture toxicity, potentially amplifying drinking water contamination and adverse effects on aquatic and subsurface ecosystems. Currently, there is limited understanding of pesticide transport in cold-region watersheds characterized by alternating dry and wet conditions. This study investigates pesticide occurrence and distribution in the Mauvais Coulee Basin (MCB) across three hydrologically distinct years (2021-2023), using biannual sampling of streams and groundwater following snowmelt and major rainfall events. We hypothesized that cold-region hydroclimate influenced pesticide concentration, predicting lower concentrations during wet years and elevated levels during dry years. We also hypothesized that soil clay and organic matter influence vertical transport. Cumulative pesticide concentration was 7.87 μg/L across 102 analytes. Herbicides and their metabolites exhibited seasonal spikes, particularly Pyrasulfotole (0.22 μg/L), Metolachlor ESA (0.19 μg/L), 2,4-D (0.35 μg/L), and Tebuconazole (0.35 μg/L), while neonicotinoid detections were limited. A Random Forest Regression (RFR) model examining 2,4-D concentrations in groundwater against soil and well characteristics such as sand and clay content, pH, organic matter, and bulk density, identified pH (24.6 % importance) and sand percentage (21.1 % importance) as the primary influences. Compared with sandy, shallow aquifer systems reported elsewhere, mobility of neonicotinoids was limited due to clay content and deeper groundwater. Snowmelt volume and post-thaw rainfall alternately diluted (wet year) or concentrated (dry year) pesticide loads. Water pH and sand content, indicated by RFR modeling, governed leaching of 2,4-D to groundwater; clay and organic matter were not the primary influences.