The Mishima lava flow is located within the length of 40 km in the valley between Mt. Ashitaka and Hakone which is at the southern east slope of Mt. Fuji. There are rich fissures and caves in the lava flow and they make a large-scall subterranean stream. There are many springs of with yield of about 1.5 million m3 per day at the end of lava flow.
The groundwater in Mishima lava flow is not completely investigated. That is because there is no drop in the groundwater level even if the pumping test is carried out, so that the specific permeability is not obtained.
The actual velocity v of the groundwater in lava flow and the porosity P of lava were measured by using the isotope flow meter, which was designed by the author (Fig.6) and γ -γ logging in 6 measuring holes of 2 sections of lava flow are shown in Fig.3.
Amount of groundwater flow Q was obtained by the following formula;
The flow velocity distribution of groundwater in lava flow is shown in Fig.7. In the figure, the flow velocity indicates 700 m to 800 m per day in the central shallow layer and O.01 per day which is the minimum value in the outer part of lava. The porosity ratio of lava is shown in Fig.4. The porosity ratio shows less than 10 % in compact lava,20 % to 40 % in porous lava and 60 % to 90 % in the lava cave. However, the flow velocity of groundwater is not alwaays fast in the lava cave. The flow distribution of groundwater depends on the movement law in the flow condition on the ground surface.
The flow discharge of groundwater obtained by Formula (1) is shown in Table 3. According to the table, the discharge was estimated 1,620,000 m3 in the upper stream section and about 1,680,000 m3 per day in downstream section.