Field and numerical investigation of groundwater flow and dissolved inorganic nitrogen (DIN) dynamics in a sandy nearshore aquifer

Nutrients delivered to the coastal ocean via submarine groundwater discharge (SGD) can have a major impact on marine ecosystems. The flux of nutrients delivered via this pathway can be modified by biogeochemical processes occurring in coastal nearshore aquifers. This study combines field investigations with numerical simulations to characterize patterns of groundwater flow, salinity and dissolved inorganic nitrogen (DIN) in a permeable nearshore aquifer and to quantify SGD and associated DIN fluxes to the ocean. At the field study site, an upper saline plume (USP) and saltwater wedge with low DIN concentrations were observed together with a fresh groundwater zone with high DIN concentrations. Seawater infiltration predominantly occurred in the intertidal zone, contributing 85% of the total infiltration, with the greatest infiltration occurring in the upper-middle intertidal zone. The simulations indicate that the USP expanded and contracted through the tidal cycle, and the width of the high-DIN freshwater discharge zone also varying. The freshwater discharge zone was narrower around the high tide period due to expansion of the overlying low-DIN zone associated with the USP. Over the simulated period, only approximately 30% of the DIN entering the nearshore aquifer ultimately discharged to the ocean. The findings of this combined field and numerical study are needed to inform future field and numerical investigations examining the fate of DIN in nearshore aquifers.