三宅島の陸水について

抄録

General characteristics of the inland water in Miyake Island were investigated as a part of the water conservation project of the island. Miyake Island, locating about 180 km south of Tokyo, is a volcanic island and covered with highly permeable volcanic ash or scoria that prevent the formation of surface water bodies except for two crater lakes.
The main groundwater system of the island is assumed to exist as a wedge of Ghyben-Herzberg's model. But, another form of groundwater also exists in the elevated regions as the under-ground compartment supposed by Ecker (1976).
Lake Tairo-ike is a crater lake created by the volcanic eruption about 2200 years B. P. The lake is considered as a part of main groundwater mentioned above. Salt intrusion is not observed in this lake, though the lake bottom reaches about 6 m below the sea surface.
Lake Shinmiyo was created by the eruption in 1763. The lake surface shows a clear tidal fluctuation, and its maximum amplitude reaches about 60 cm. The lake is a meromictic, i. e., its deep layer is occupied by salt water, the concentration of which increases gradually with the depth as expressed in Fig. 3 and Fig. 4. The vertical circulation is restricted to the layer shallower than about 10 m in depth. The residence time of the bottom water (30 m layer) was estimated as 29.7 years, based on concentration of tritium. The meromictic nature of the lake is explained by this low rate of water exchange. Salt water in the deep layer is considered to originate from the sea, as seen in the key-diagram of water quality (Fig. 5).