Geochemical tracers in submarine groundwater discharge research: Practice and challenges from a view of climate changes

Submarine groundwater discharge (SGD), the flux of porewater from permeable seabed or karst conduits to surface water bodies, delivers a significant quantity of land-borne solutes to coastal oceans. This input of land-derived solutes is frequently linked with eutrophication, harmful algae blooms, and benthic hypoxia, and hence has the potential to trigger great economic losses. Geophysical and geochemical tracers, including salinity, temperature, water stable isotopes, and radioactive elements, have been widely applied in SGD studies for more than 50 years to, amongst others, identify water sources, estimate residence times, and quantify discharge rates. Here we review advantages and shortcomings of these tracers in the study of SGD. Application requirements are outlined based on previous research and combined tracer approaches in karst environments, permeable coasts, and estuaries are illustrated under the view of climate changes. Current challenges with the use of geochemical tracers in SGD studies are highlighted and opportunities to develop these tracers for improved coastal management showcased.