Groundwater of the Miyakejima Volcanic Island in the Izu Volcanic Chain, Japan

Abstract

The Miyakejima volcanic (basaltic) island is situated about 200 km south-southwest of Tokyo and has a round outline measuring about 8 km in diameter. This paper deals with the groundwater systems in this island. The results of our study can be summarized as follows : (1) Our hydro-geological study comprised the survey of geomorphology, geology, springs, wells, electric prospecting and test drilling in the island with the results shown in Figs. 4 to 8 and Table 1. (2) From these surveys, the groundwater of the island can be subdivided into main-groundwater system and sub-groundwater system. The distribution of the two types of groundwater system are shown in Fig. 9 with schematic profiles. (3) The main-groundwater system is considered to develop as Ghyben-Herzberg's lens within the main somma volcanic deposits along the coastal areas. It may not be distributed in inland area of the island owing to the presence of subsurface impermeable bed. The sub-groundwater system is comprised within shallow parts of somma deposits, the younger parasitic deposits, coastal gravels and sand deposits (Fig. 9). (4) Based on our study, new wells were drilled at four sites, where main-groundwater system was considered to be well developed. Total amount of fresh water measuring 45000 m³/day has been discharged by pumping from these new wells and has been used for water supply of the Miyakejima island. (5) At the time of the 1983 eruption, the phreatic explosions took place mostly underneath the main-groundwater system.