Unheralded Submarine Groundwater Discharge

Land meets the ocean in the coast zone, which is of great importance for aquaculture, fisheries, tourism and transportation, among other things. As fifty percent of the world population lives within 100 km of the coast, human activities have put tremendous pressure on the coastal environment. For instance, excess nutrients that are generated by agriculture and present in domestic waste have been discharged into the oceans by rivers, making many coastal regions hypoxic or even anoxic. Most people do not realize that a substantial fraction of the freshwater on land enters the oceans directly from the seabed, unseen by human eyes. This process is called submarine groundwater discharge (SGD) [15]. Burnett and coworkers [6] were among the first to provide a ballpark estimate that the amount of SGD is 0.2-10% of global river discharge. Since groundwater has been frequently in contact with soil and bedrock for hundreds of years, if not longer, some of the dissolved organic carbon (DOC) in the groundwater as well as particulate organic carbon and CaCO3 in the soil and bedrock are expected to have decomposed or dissolved.