Submarine groundwater discharge in Osaka Bay, Japan

Submarine groundwater discharge (SGD) rates in Osaka Bay were continuously measured and analyzed to evaluate seawater–groundwater interactions. Fast Fourier transfer and power spectrum density methods were applied to analyze the dominant periods of the SGD variations. Diurnal and semidiurnal periods of SGD variation were found, and they were caused by tidal effects. According to the separation of SGD into fresh and recirculated water components using automated seepage meter measurements and terrestrial groundwater flow analyses, the fresh groundwater component in SGD was evaluated to be in the range 4%–29% at Tannowa, Osaka. Therefore, SGD rates depend mainly on the volume of recirculated seawater. Correlation analyses between SGD and sea level show that SGD is delayed by 4h after sea level changes.