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Groundwater aquifers worldwide experience unsustainable depletion, compounded by population growth, economic development, and climate forcing. Managed aquifer recharge provides one tool to alleviate flood risk and replenish groundwater. However, concerns grow that intentional flooding of farmland for groundwater recharge, a practice known as Ag-MAR, may increase the leaching of pesticides and other chemicals into groundwater. This study employs a physically based unsaturated flow model to determine the fate and transport of residues of four pesticide in three vadose zone profiles characterized by differing fractions of sand (41 %, 61 %, and 84 %) in California's Central Valley. Here, we show that the complex heterogeneity of alternating coarse and fine-grain hydrogeologic units controls the transit times of pesticides and their adsorption and degradation rates. Unsaturated zones that contain a higher fraction of sand are more prone to support preferential flow, higher recharge rates (+8 %), and faster (42 %) water flow and pesticide transport, more flooding-induced pesticide leaching (about 22 %), as well as more salt leaching correlating with increased risks of groundwater contamination. Interestingly, considering preferential flow predicted higher degradation and retention rates despite shorter travel times, attributed to the trapping of pesticides in immobile zones where they degrade more effectively. The findings underscore the importance of considering soil texture and structure in Ag-MAR practices to minimize environmental risks while enhancing groundwater recharge. The study also highlights that selecting less mobile pesticides can reduce leaching risks in sandy areas., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests/personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2024 The Author(s). Published by Elsevier Ltd.. All rights reserved.)

Agricultural managed aquifer recharge (Ag-MAR, on-farm recharge), where farmland is flooded with excess surface water to intentionally recharge groundwater, has received increasing attention by policy makers and researchers in recent years. However, there remain concerns about the potential for Ag-MAR to exacerbate nitrate (NO 3 - ) contamination of groundwater, and additional risks, such as greenhouse gas emissions and crop tolerance to prolonged flooding. Here, we conducted a large-scale, replicated winter groundwater recharge experiment to quantify the effect of Ag-MAR on soil N biogeochemical transformations, potential NO 3 - leaching to groundwater, soil physico-chemical conditions, and crop yield. The field experiment was conducted in two grapevine vineyards in the Central Valley of California, which were each flooded for 2 weeks and 4 weeks, respectively, with 1.31 and 1.32 m 3 m -2 of water. Hydrologic, geochemical, and microbial results indicate that NO 3 - leaching from the first 1 m of the vadose zone was the dominant N loss pathway during flooding. Based on pore water sample and N 2 O emission data, denitrification played a lesser role in decreasing NO 3 - in the root zone but prolonged anoxic conditions resulted in a significant 29 % yield decrease in the 4-week flooded vineyard. The results from this research, combined with data from previous studies, are summarized in a new conceptual model for integrated water-N dynamics under Ag-MAR. The proposed model can be used to determine best Ag-MAR management practices., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2022 The Authors. Published by Elsevier B.V. All rights reserved.)