Modeling biogeochemical processes in subterranean estuaries Effect of flow dynamics and redox conditions on submarine groundwater discharge of nutrients

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A two‐dimensional density‐dependent reactive transport model, which couples groundwater flow and biogeochemical reactions, is used to investigate the fate of nutrients (NO3−, NH4+, and PO4) in idealized subterranean estuaries representing four end‐members of oxic/anoxic aquifer and seawater redox conditions. Results from the simplified model representations show that the prevalent flow characteristics and redox conditions in the freshwater‐seawater mixing zone determine the extent of nutrient removal and the input of nitrogen and phosphorus to coastal waters. At low to moderate groundwater velocities, simultaneous nitrification and denitrification can lead to a reversal in the depth of freshwater NO3− and NH4+‐PO4 plumes, compared to their original positions at the landward source. Model results suggest that autotrophic denitrification pathways with Fe2+ or FeS2 may provide an important, often overlooked link between nitrogen and phosphorus biogeochemistry through the precipitation of iron oxides and subsequent binding of phosphorus. Simulations also highlight that deviations of nutrient data from conservative mixing curves do not necessarily indicate nutrient removal.