Denitrification hotspots in intertidal mixing zones linked to geologic heterogeneity

The mixing between fresh and saline groundwater in beach aquifers promotes biogeochemical transformations that affect nutrient fluxes to the coastal ocean. We performed variable-density groundwater flow and reactive transport simulations with geostatistical representations of sedimentary structure to understand the influence of heterogeneity on groundwater dynamics and denitrification in intertidal mixing zones. Ensemble-averaged simulation results show that heterogeneity can enhance mixing between fresh and saline groundwater and increase residence time, resulting in up to 80% higher nitrate removal relative to equivalent effective homogeneous aquifer sediment. Denitrification hotspots form in high permeability structures where DOC and nitrate are readily supplied by convergent flow. The results provide a physical explanation for the formation of denitrification hotspots observed in beach aquifers and illustrate for the first time the influence of sediment heterogeneity on rates and spatial patterns of biogeochemical processes in intertidal aquifers that are critical mediators of land-sea solute fluxes along world coastlines.