Sandy Subterranean Estuaries Minimize Groundwater Nitrogen Pollution Impacts on Coastal Waters

Abstract Widespread anthropogenic activities pollute groundwater that eventually seeps out to the coastal ocean. Here, we resolve nutrient transformations and fluxes in 11 sandy subterranean estuaries (STEs) with contrasting nutrient sources and development trajectories. Coastal groundwater nitrogen pollution stems from sewage discharge and land use change. Anthropogenically derived groundwater nutrient fluxes with high N/P ratios (∼170) accounted for 22%–61% of riverine inputs into China's coastal waters, providing an additional source of nutrients that can fuel coastal eutrophication and algal blooms. Sandy STEs remarkably attenuated ∼84% of nitrogen pollution, minimizing the impact of submarine groundwater discharge (SGD) on coastal water quality. Hence, STEs deliver an overlooked ecosystem service that is particularly important in highly polluted coastal aquifers. Protecting STEs and recognizing the integrated nature of groundwater and seawater is thus important in coastal water quality management initiatives.