Submarine Groundwater Discharge: Updates on Its Measurement Techniques, Geophysical Drivers, Magnitudes, and Effects

Water is a naturally circulating resource that is constantly recharged. Therefore, even though the stocks of water in natural and artificial reservoirs are helpful to increase the available water resources for human society, the flow of water should be the main focus in water resources assessments. The climate system puts an upper limit on the circulation rate of available renewable freshwater resources (RFWR). Although current global withdrawals are well below the upper limit, more than two billion people live in highly water-stressed areas because of the uneven distribution of RFWR in time and space. Climate change is expected to accelerate water cycles and thereby increase the available RFWR. This would slow down the increase of people living under water stress; however, changes in seasonal patterns and increasing probability of extreme events may offset this effect. Reducing current vulnerability will be the first step to prepare for such anticipated changes.

The exchange of groundwater between land and sea is a major component of the hydrological cycle. This exchange, called submarine groundwater discharge (SGD), is comprised of terrestrial water mixed with sea water that has infiltrated coastal aquifers. The composition of SGD differs from that predicted by simple mixing because biogeochemical reactions in the aquifer modify its chemistry. To emphasize the importance of mixing and chemical reaction, these coastal aquifers are called subterranean estuaries. Geologists recognize this mixing zone as a site of carbonate diagenesis and dolomite formation. Biologists have recognized that terrestrial inputs of nutrients to the coastal ocean may occur through subterranean processes. Further evidence of SGD comes from the distribution of chemical tracers in the coastal ocean. These tracers originate within coastal aquifers and reach the ocean through SGD. Tracer studies reveal that SGD provides globally important fluxes of nutrients, carbon, and metals to coastal waters.