Intensive anthropogenic activities drive dynamic surface water-groundwater interactions: Implications for contaminant pathways at a catchment scale

Interactions between surface water and groundwater (SW-GW) are critical interfaces controlling contaminant fate and transport. However, their dynamic behavior at the catchment scale under intensive anthropogenic stress remains poorly understood. This study integrates hydrogeochemical and isotopic analyses (δ¹⁸O, δ²H) to investigate the spatiotemporal patterns of SW-GW interactions and their implications for contaminant pathways in the Yiluo River Watershed, a major agricultural region in China. Three sampling campaigns were conducted across low-flow (Nov 2022, T1), normal-flow (May 2023, T2), and high-flow (Aug 2023, T3) periods. Results show that while waters are predominantly Ca–HCO₃ type, geochemical and isotopic signatures reveal highly dynamic exchange processes. Isotopic enrichment from upstream to downstream (δ¹⁸O from ∼ − 10 ‰ to ∼ − 6.5 ‰) indicates significant evaporation and mixing. Crucially, SW-GW interactions create fluctuating pathways for contaminants like nitrate (NO₃⁻). During the T1 dry/irrigation season, extensive groundwater abstraction for agriculture reverses local hydraulic gradients, reducing river-to-groundwater recharge and inducing high-nitrate groundwater discharge into the river, with groundwater NO₃⁻ concentrations reaching up to 42.5 mg/L. Correlation analysis reveals that the dominant controls on recharge ratios shift seasonally: from cropland (r = −0.53) and forest (r = 0.49) in the dry season to population density (r = −0.26) and bare land (r = 0.35) in the wet season. These findings reveal a conceptual framework where seasonal human activities (e.g., irrigation vs. urban runoff) actively switch the direction and magnitude of contaminant exchange pathways between surface and subsurface systems. This study highlights the necessity of integrated, dynamic management of SW-GW resources to mitigate contamination risks in anthropogenically-dominated agricultural catchments worldwide.