Mechanism of differential regulation of hydrogeological parameters (V and K) on denitrification (DEN) and dissimilatory nitrate reduction to ammonium (DNRA) under groundwater-surface water interaction

Due to the differences in hydrological and environmental characteristics, the interaction between groundwater (GW) and surface water (SW) causes the complexity of material migration and transformation in the interaction zone. However, the specific mechanisms by which flow velocity (V) and hydraulic conductivity (K) differentially regulate denitrification (DEN) and dissimilatory nitrate reduction to ammonium (DNRA) through hydraulic retention time remain unclear. This study conducted a series of one-dimensional soil column experiments, through changing the GW-SW interaction mode, V and K, the influence of hydraulic conditions on nitrogen-nitrate migration and transformation was evaluated. Light Gradient Boosting Machine (LightGBM) and SHapley Additive exPlanations (SHAP) methods were applied to study influencing factors of nitrogen migration and transformation. Results showed that during the process of GW recharge SW, higher V and lower K led to an accelerated increase of the growth rate of c(NO₃–-N) in the sediment layer and base soil layer, the equilibrium concentration of c(NO₃–-N) in the sediment layer also increased. c(TN) showed the similar trend and decreased in the reaction equilibrium stage. c(NO₂–-N) and c(NH₄+-N) showed the reverse trend as V decreased and K increased. During the process of SW recharge GW, the decreasing rate of nitrogen concentration accelerated as V increased and K decreased. SHAP analysis identified EC, DOC, pH, Eh and T as the most influential environmental factors governing nitrogen speciation. c(NO₃–-N), c(NO₂–-N) and c(NH₄+-N) was the main factor affecting c(TN). Our key innovation is the systematic decoupling of how V and K differentially regulate DEN and DNRA through hydraulic retention time. Results provided important references for hydrological cycling, nitrogen pollution and control and ecological environmental effects of GW-SW interaction.