Tritium concentrations in river water and in discharged water through tunnels, horizontal and vertical boreholes have been measured in the Rokko mountains scince 1968. River water has always relatively high values of tritium concentration which have a tendency to decrease gradually from 69 TU in 1969 to 40 TU in 1974, while water discharged through tunnel or borehole from formation has various values of tritium concentration less than those in river water.
A displacement flow-mixing model is considered for interpretation of these data. The model consists of two processes: the first process is a displacement flow in an aquifer with a constant thickness receiving uniform recharge; the second is a mixing process near a discharge point such as river or tunnel. Turnover time is given by the combination of these processes, and the response function of tritium concentration to an input is equivallent to mixing-cell model.
Calculated variations of the tritium concentration give a well-fitting to the observed data of the river water when the turnover time is about 30 years under the condition of recharge from precipitation.
On the other hand, the results calculated in the same way as river water under the assumption that water in fractured formation is replenished from shallow groundwater system are compared with the observed data of discharged waters through tunnels and boreholes. The following results on the fractured medium are obtained: (1) the turn over time is about 40 years, (2)the mean particle velocity of downward flow is about 10 to 17 m/y, (3) the porosity of the formation is about 0.01 to 0.06, and (4) the vertical extent of the flow system is about 900 m.

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From the engineering geological, hydrogeological and geochemical points of view, the characteristics of water quality in groundwater (partly stream water) from the Rokko mountain area is summarized as follows; 1) water quality in groundwater 'through Nunobiki granodiorite shows Ca- Na-HCO₃ type, and Na-HCO₃ type is greater than Ca-HCO₃ type in groundwater from Rokko granite, 2) the groundwater is characte-rized by higher alkalinity, higher pH and being rich in fluoride content than the stream water, 3) mineral and common water exist irregularly and alternatively within fault-sheared zone, 4) chondrite-normalized REE pattern in groundwater is regarded as an in-dex of water-rock interaction in deep underground, 5) from measurements of tritium, the turnover time of groundwater in a fractured system is about 40 years and the mean velocity of downward displacement flow about 10 to 17m/y, 6) some amount of stream water is supplied into groundwater contained in fractures or faults in base rock which. are crossing a stream, usually in a certain small amount, but during heavy rains dis- charge of supplied water increases suddenly to a great extent, and 7) a crustal deforma-tion occurs at or around the Rokko mountain area, increasing the underground supply of carbon dioxide gas and leading to very active water-rock interaction.

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It is very important to clear up relations between variation of groundwater and soil mass deformation in disaster of mountain area, so far as the mechanism is elucidated. As an existing condition of groundwater in mountain area is very complex, there are some fairly difficult problems to make clear the relation between the existing condition of groundwater and the disaster.
Judging from conditions of eruption or loss circulation of water during bore hole drilling and from geological condition and fissure condition in bore holes in landslide area. It seems that some layers with groundwater exist in landslide area, and it is conjectured that each groundwater shows different behavior and gives different effects to the deformation of soil mass. We considered that a all-strainer-type bore hole did not apply to make clear the behavior of each groundwater.
So we carried out investigations emphasizing on the behavior of each groundwater in a landslide area in Kobe Group.
The results are shown as follows.
There are roughly four types of groundwater, that is.
1. shallow residential groundwater in detritas and terrace deposits(A);
2. shallow flowing groundwater in detritus and terrace deposits(A');
3. middle groundwater in the Kobe Group(B); and
4. deep groundwater in the Arima Group(C).
The existing conditions of these groundwater are as follows; A is free groundwater

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Many aquifers are hydrogeologically classified as two types of soil and rock.
The pore and permeability of fractured rocks are larger than those of soil. In permeable zones of fractured rock, ground water is very easy to flow, and water level rises rapidly under rainfall.
The lava-flows in Fuji region with cooling joints, fractures, tubes, vesicles have the highest transmissivity. According to the observation data of wells in the lava-flows, the cyclic fluctuations of the water level reflect recharge from seasonal rainfall. The serious salt-water intrusion into the lava-flows since 1960 occurred as the result of overdraft in the region.
The management of ground-water resources was started in 1967, and the net effect of the regulation of pumping is the rapid rising of the regional water level and the recovery of fresh water in the salt-water contaminated lava-flows after several years.

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