Thirty five water samples were collected from the Obama pond in 1983 and 1984, when the waterlevel of the pond decreased gradually to almost dryness. The pond water is fed by the groundwater system originated from Mt. Fuji. The chemical and isotopic compositions in these samples were analyzed to investigate groundwater flow sysems in and around Mishima city.
They can be divided into four groundwater flow systems. The main water system originates from Mt. Fuji along the Mishima lava flow.
Complexity of the groundwater flow system arises from the Mishima lava, bed which is composed of several layers. Groundwater in each layer is hardly mixed.
Tritium concentration in the Mishima spring zone was low. The low tritium comcentration probably reflects existence of a large groundwater aquifer flowing extremely slowly into the Mishima lava. This interpretation agrees with the geology around the Mishima city.