Coupled salt and nitrogen dynamics in coastal reservoir-adjacent aquifer systems under extreme rainfall event

Coastal reservoir-adjacent aquifer systems are highly sensitive to extreme rainfall events, which are becoming more frequent and intense with global climate change. These events greatly alter coupled hydrodynamic and biogeochemical processes, yet their impacts on salt and nitrogen dynamics remain poorly understood. This study presented the comprehensive modeling analysis of how extreme rainfall events affected water exchange, salt transport, and nitrogen cycling in coastal reservoir-adjacent aquifer systems. Model reliability was validated against previous field and laboratory experimental data. Results revealed rapid hydrodynamic responses, with flow patterns and water exchange peaking within 12 h of extreme rainfall and generating a 133 % increase in total water flux across the reservoir-aquifer interface. Extreme rainfall also contributed to a decrease in salinity and caused a downward and seaward shift in the distributions of NO₃^- and NH₄⁺, thereby enhancing nitrification and denitrification near the seawater boundary. Notably, nitrogen dynamics exhibited pronounced spatial heterogeneity compared to the relatively uniform desalination response. This differential behavior resulted in asymmetric pollution reduction effects: maximum decreases of 2.46 % for NO₃^- and 2.21 % for NH₄⁺ pollution versus 14.1 % for salinization. The study demonstrates that extreme rainfall impacts on coastal reservoir-adjacent aquifer systems are highly non-linear and species-specific, emphasizing the need for differentiated approaches to controlling salinization versus nitrogen pollution in these climatically sensitive hydrogeological systems. These findings provide critical insights for adaptive management strategies affecting approximately 40 % of the global population residing in coastal regions.