A New Focus on Groundwater–Seawater Interactions (Proceedings of Symposium HS1001 at IUGG2007, Perugia, July 2007). IAHS Publ. 312, 2007. Copyright © 2007 IAHS Press 60 Investigation of submarine groundwater discharge using several methods in the inter-tidal zone TOMOTOSHI ISHITOBI 1 , MAKOTO TANIGUCHI 1 , YU UMEZAWA 1 , SHIGERU KASAHARA 2 , SHIN-ICHI ONODERA 3 , MASAKI HAYASHI 4 , KUNIHIDE MIYAOKA 5 & MITSURU HAYASHI 6 1 Research Institute for Humanity and Nature, 457-4 Motoyama, Kamigamo, Kita-ku, Kyoto 603-8047, Japan tomotoshi@chikyu.ac.jp 2 Sohgoh kagaku Inc., 1-4-8 Minami-Shin-Machi, Chuo-ku, Osaka 540-0024, Japan 3 Faculty of Integrated Arts and Sciences, Hiroshima University, 1-7-1 Kagamiyama, Higashi-Hiroshima 739-8521, Japan 4 Graduate School of Biosphere Science, Hiroshima University, 1-7-1 Kagamiyama, Higashi-Hiroshima 739-8521, Japan 5 Faculty of Education, Mie University, 1577 Kurimamachiya-cho, Tsu 514-8507, Japan 6 Research Center for Inland Seas, Kobe University, 5-1-1 Fukae-Minami-Machi, Higashi-nada-ku, Kobe 658-0022, Japan Abstract To accurately estimate the flux of terrestrial groundwater discharge into the sea, a study using several methods was done in the coastal zone of Osaka Bay, Japan. The seepage-meter method and the measurement of temperature near the seabed were applied based on the hypothesis that seawater temperature in summer would decrease, reflecting the extent of active mixing with colder fresh groundwater. As a result, it was confirmed from the seepage-meter method that submarine groundwater discharge rates decreased with the distance from the coast. Evaluations of groundwater discharge rates from seabed temperature showed similar values to the results using the seepage meter, which means that the values were reasonable. Finally, the total groundwater discharge flux from this beach was estimated at 36.7% of the river discharge rate. Key words submarine groundwater discharge; inter-tidal zone; seepage meter; seabed temperature INTRODUCTION Recognition of the importance of submarine groundwater discharge (SGD) is increasing in the studies of water and dissolved-material transport from the land to the sea. According to recent studies from hydrology and coastal oceanography, SGD consists of both terrestrial groundwater and recirculated seawater (Taniguchi et al., 2002; Burnett et al., 2003). However, there are many uncertainties about SGD, because it is invisible and difficult to evaluate quantitatively. Therefore, further study is needed to better estimate submarine fresh–groundwater discharge (SFGD) in coastal areas. Our study area is located in the coastal zone of Omaehama, Osaka Bay, Nishinomiya city, Japan (Fig. 1). Osaka Bay is located next to Osaka city, the second capital of Japan. The annual precipitation and air temperature are about 1300 mm/year and 16.5°, respectively. Several rivers flow into the Osaka Bay and the largest of them