The Water Quality and isotopic composition on T, ²²²Rn, D and ¹⁸0 of ground water discharge in the adit for power station were investigated. Main obtained results are summarized as follows:
1) Investigation on the quality of ground water shows that the ground water is intermixed by comparable amount of surface water.
2 ) Investigation on the isotopic composition of ground water also shows the same result as inferred from water quality.
3) It is expectated that the characteristics of origin and intermixed surface water are cleared by detailed investigations on T, ²²²Km, D and ¹⁸0 isotopic composition of ground water discharge.

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It has been reported that the salt groundwater invade inland and the aquifers become contaminated with salt as a result of over discharges of fresh groundwater by the wells in coastal areas, and the drawdown of groundwater level by the shortage of precipitation. Consequently, it is significant to calculate the movement of salt groundwater.
In the present paper, for the study of preventation of salt water intrusion, the author discusses the formulation for a numerical analysis of the groundwater density flow with the boundary element method. Numerical analysis was carried out for the following two cases, (1) the steady problem in -unconfined groundwater, (2) the unsteady problem in confined groundwater.
The stability of unsteady numerical solution was obtained to calculate the real part of eigen values every time step. About the behavior of fresh and salt water interface, the numerical solutions show good agreement with the experimental results.
Also, the author estimated the effect of the impervious wall for countermeasure against salt water intrusion.

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