Spatiotemporal Variability of Denitrification in Intertidal Mixing Zones: The Roles of Rainfall Recharge and Spring-Neap Tides

Submarine groundwater discharge (SGD) is a key pathway for transporting terrestrially derived nutrients, including elevated nitrate levels, from coastal groundwater to coastal waters, thereby impacting coastal water quality and ecosystem health. Tidally driven saltwater-freshwater mixing zones in coastal aquifers can promote denitrification, which attenuates terrestrially derived nitrate in groundwater before its discharge into coastal waters. However, the effect of rainfall recharge, which can significantly alter flow and mixing regimes in intertidal zones, on this mixing-dependent denitrification remains poorly understood. In this study, we employ a numerical variable-density groundwater flow and reactive transport model to evaluate how rainfall recharge interacts with spring-neap tides to shape denitrification spatially and temporally. We conduct a sensitivity analysis across various rainfall recharge patterns (uniform, random, extreme, and seasonal), recharge intensities, dissolved organic carbon (DOC) reactivity, and recharge-derived solutes. Our results show that rainfall recharge and spring-neap tides jointly regulate denitrification patterns. However, as DOC reactivity increases, the dominant driver shifts from rainfall recharge to tidal forcing. While different rainfall recharge patterns result in similar annual nitrate removal, they lead to substantial variability in daily removal rates. Increasing recharge intensity generally reduces overall nitrate removal unless additional nitrate is introduced via recharge. Additionally, in all scenarios, the percentage of nitrate removed relative to terrestrial inputs declines with increasing recharge intensity. These results underscore the interconnected hydrological and biogeochemical controls on denitrification in intertidal zones, offering important implications for estimating coastal nutrient fluxes and managing coastal water quality.