The hydrogeological survey was executed in a small granite island for the construction of underground oil storage caverns. In spite of many cracks and fissures, the permeability of the bedrock was relatively low (10−4 to 10−6 cm/sec.) and thus the level of groundwater table was very high. As for the groundwater flow, the authors measured the groundwater pressure in the bore holes and described the potential distribution. The measuring result of environmental tritium concentration in groundwater also supported the existence of the groundwater flow from center to circumference of the island. Furthermore, the potential distribution of groundwater was calculated by the numerical analysis of finite element method using the measured permeability distribution and the estimated rate of recharge from the water balance calculation, and the reasonable groundwater flow enough to account the actual measurement was reproduced. The authors also investigated the environmental influence of the drawdown of groundwater in excavating of the caverns. In case that there exists the relatively permeable layer near the ground surface such as weathered zone of the present study and also that the rate of recharge is relatively large in a humid climate like Japan, the influence of the drawdown by excavation is seemed to be very little.

INTRODUCTION

The conservation of an oil material in underground caverns using its surrounding groundwater has been carrying out for long period. In this case, the role of groundwater in the bedrock is very important but there is almost no systematic measurement regarding the movement of this groundwater and a number of unclarified problems are still existed.

In the bed rock, there exist many cracks and fissures originated from crust diastrophism. The groundwater is stored in them or moves downwards through them by a gravity. Cracks and fissures in the bedrock have very different shapes such as tubular or plate-type, and they cross, expand and connect each other making a very complex structure. Nevertheless from a macroscopic point of view the network of these cracks and fissures compose the storage zone or pervious layer.

In case that many irregular and non-homogeneous fissures exist in the bedrock and make a network of fissures, it may be reasonable to consider this bedrock hydraulically homogeneous. As for the groundwater flow in isotropy and homogeneous media, Hubbert (1940) solved the groundwater flow analytically. After then, Toth (1962) and Freeze and his co-workers (1967) tried to estimate the flow in the more complex pervious layer using computer simulation. However, the actual measurement of the groundwater flow was observed only that by Meyboom (1966) in an aluvium deposit, and the groundwater flow in the bedrock has not been measured by now.

The purpose of the present study is to understand the regime of the groundwater in the bedrock by the actual measurement and mathematical modeling, and also to estimate the influence on the environment by excavation. In the present study, the authors investigated the flow characteristics of the groundwater in bedrock using the potential distribution measured by double packered groundwater pressure measuring apparatus, and the concentration of environmental tritium containing in the groundwater. study is shown in Fig. 1.

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