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Everyone knows that rivers carry freshwater into the ocean, but not everyone knows that rivers are not the oceans' only source of freshwater. Geochemist Willard Moore of the University of South Carolina has performed studies and analyses which indicate that a large volume of groundwater directly enters the seas.
Moore's research focuses chiefly on a process known as tidal pumping. Groundwater typically seeps into tiny pores in layers of rock. At high tide, salt water also seeps into the rock layers, mixing with the fresh groundwater, and at low tide, the salt/freshwater mixture gets pulled into the ocean. Then more fresh groundwater is allowed to seep into the rock, and the tidal pumping cycle begins again.
Seawater along the coastline is diluted by freshwater and contains the radioactive isotope radium 226, which erodes off sediment encountered by rivers and groundwater. Since the radium concentrations in rivers are well established, unusually high radium levels in coastal waters indicated to Moore that there had to be another source of freshwater. To determine just how much groundwater was flowing into the ocean, Moore examined the chemistry of the freshwater entering the ocean along 200 miles of the South Carolina coast.
Rivers and groundwater have different chemical make-ups. The greater oxygen content in river water means that metals in the water, like iron, become oxidized and bind to other water elements; thus, they get trapped in sediment or sink to the bottom of rivers and oceans. Furthermore, river water generally passes through estuaries where sediment and pollutants are filtered out by plants, animals and plankton. Groundwater, on the other hand, contains greater concentrations of dissolved trace elements because there is not enough oxygen to oxidize them, and since it is not filtered before entering the ocean, its chemical components enter along with it.
From measuring concentrations of groundwater components along the South Carolina coast, Moore concluded that as much as 8 billion gallons of groundwater may flow into the coastal waters every day. That's almost half the volume of freshwater deposited by rivers in South Carolina.
Part of the significance of Moore's findings is the prospect that groundwater could serve as a source of ocean contamination which could impact marine plants and animals. As a result of his research, scientists will likely undertake studies to specifically measure groundwater pollutants. "I'm raising a flag," says Moore, conceding that much more research is needed to fully understand the impact of groundwater flowing into the sea.