Yoron-jima is situated in the southernmost part of Kagoshima Prefecture, Kyushu. Geologically, Most of this island is mainly composed of the relatively thick reef limestone or calcareous sandstone about 20-50m in thickness which is called the Ryuku Group, and its basements, the Ritcho Formation, occurs in the southern part. Around the island, there are several groups of marine terraces which indicate the existance of the former high sea level.
Groundwaters in the island are controlled by the stratigrafic sequence and the structure of basements, and classified roughly into five types. Among the rest, the Nama Formation, which contain the groundwater of the stratum water type will be the best aquifer for developing groundwater in the near furture.
