Evaluations of Origins and Material Transports of Submarine Groundwater Discharge

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Three different approaches including (1)hydrological studies in the land,(2)coastal zone studies, and (3)oceanic studies, have been done to evaluate the SGD and seawater-fresh water interaction in the Suruga Bay, Japan. The study area is the Abe river basin including the coastal zone and the offshore of the Suruga Bay. Measurements have been done from 2001 to 2003. Analyses of stable isotopes (O-18 and D) and chemical components of groundwater in boreholes and seepaged waters have been made to evaluate the groundwater flow system in the basin and material transports from the land to the ocean. In the coastal zone, continuous measurements of SGD have been done by using automated seepage meters and piezometers, as well as resistivity measurements with tomography method to evaluate the interaction between fresh water and seawater. Offshore measurements in the Suruga Bay by R/V Tansei-maru, have been made by core boring to extract pore water below the seabed, CTD measurements above the seabed, and ROV measurements. The automated seepage meters which were installed along the coastal zone and offshore can tell us high resolutions of SGD in time, therefore, the tidal effects on SGD were clearly found. Inverse relationships between SGD rate and barometric pressure were also found. Resistivity measurements every three hours in the coastal zone showed the changes of the freshwater-saltwater interface due to the sea level change with the time delay. According to the continuous measurements of conductivity of the seepaged groundwater, 10 percent of the SGD was the fresh water, and the 90 percent of the water was the recirculated seawater.