Simulation of pesticide transport in 70-m-thick soil profiles in response to large water applications

Global groundwater depletion is a pressing issue, particularly in regions dependent on groundwater for agriculture. Agricultural Managed Aquifer Recharge (Ag-MAR), where farm fields are used as spreading grounds for flood water, is a promising strategy to replenish groundwater, but it raises concerns about pesticide leaching into aquifers, posing risks to both drinking water quality and ecosystems. This study employs a physically based unsaturated flow model, a Bayesian probabilistic approach and novel towed transient electromagnetic (tTEM) data to determine the fate and transport, especially the maximum transport depths (MTDs) of four pesticide residues (Imidacloprid, Thiamethoxam, Chlorantraniliprole, and Methoxyfenozide) in three 70-m-thick unsaturated zones (P1, P2, P3) of California's Central Valley alluvial aquifer. The results show that Ag-MAR significantly increased MTDs across all profiles for all pesticides and with higher variability in pesticide transport depths compared to the natural rainfall scenario. Profile P2, with the highest sand content exhibited the deepest MTDs under Ag-MAR, indicating a strong influence of soil texture on pesticide transport. While natural capillary barriers at the depth of 2.5-20 m impede water flow under natural rainfall conditions, the high-pressure infiltration during Ag-MAR overcomes these barriers, leading to deeper water and pesticide movement. Among various evaluated pesticides, Methoxyfenozide exhibited the smallest absolute MTDs but the largest relative increases in MTDs (RMTDs) under Ag-MAR due to its persistence and low mobility, posing a higher risk of deep transport during intensive recharge events. In contrast, Thiamethoxam showed the largest MTDs under both scenarios but smaller RMTDs due to its high mobility, suggesting a more consistent transport behavior regardless of recharge practices. The findings highlight the importance of understanding both site-specific and pesticide-specific behaviors to mitigate groundwater contamination risks during large water applications.