Macropores and burial of dissolved organic matter affect nitrate removal in intertidal aquifers

Coastal ecosystems are sensitive to the input of chemicals originating from submarine groundwater discharge (SGD). The mixing zone between freshwater and saltwater in beach aquifers promotes biogeochemical transformations affecting nutrient fluxes into the coastal ocean. Macropores affect groundwater flow, leading to a more complicated mixing process and greatly promoting nitrogen transformations. Laboratory-scale experiments and numerical modeling were employed in this study to investigate how macropores affect freshwater-saltwater mixing and terrestrial solute transport. A numerical investigation is presented demonstrating the influence of macropores on the transport and transformation of nitrate (NO3-) in a homogeneous unconfined nearshore aquifer under spring-neap tide action using COMSOL. The results indicate that macropores modify the groundwater discharge pathway of terrestrial NO3- by changing nearshore groundwater flow dynamics. The denitrification zone expanded from the low tide mark to the high tide mark, extended downward to the base of the upper saline plume (USP), and expanded with increasing macropore depth and quantity. Macropores increased NO3- removal by creating larger mixing zones, which increased with the macropore depth and quantity. Lower freshwater levels enhanced denitrification and weakened the impact of macropores on denitrification. The deep burial dissolved organic matter (DB-DOM) simulation results demonstrated that dissolved organic matter (DOM) could stimulate denitrifiers to increase the denitrification zone by fully consuming O-2 along longer flow paths, resulting in up to 85% higher NO3- removal. The findings provide notable implications for management design engineering to protect coastal ecosystems and decrease NO3- fluxes into the ocean.