Uncertainties are inherent in performance assessments of the geological disposal of long-lived solid radioactive waste. Therefore, to attain “reasonable assurance” in the validity of these assessments' outcomes, in each investigation phase it is necessary to evaluate uncertainties associated with understanding and / or long-term estimation of a geological environment's heterogeneous characteristics. It is also necessary to investigate the geological environment effectively so as to reduce the uncertainties in subsequent investigation phases.
The authors have developed an “uncertainty analysis methodolo gy associated with the characterization of the geological environment”. In addition to conceptual models and parameter values for conventional modelling, this methodology can take into account conceptual models and parameter values that, although unlikely, cannot be completely eliminated. The aim was to contribute to a firm scientific and technological basis for the research and development of the highlevel radioactive waste geological disposal. This paper will describe the methodology's application, using the example of faults in the Tono area. It will be shown that, by means of sensitivity analyses, it is possible to quantify uncertainties in groundwater flow and at the same time suggest issues that should be investigated in order to reduce uncertainties.

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It is known that electrical methods for s u rface geophysical survey might be capable to locate saline water in a coastal unconfined aquifer. This is the result of the increase in bulk conductivity with increasing in pore water conductivity. The relation between the depth of saltwater intrusion estimated by electrical method and the electric conductivity (EC) of pore water corresponding to its depth, however, is not verified quantitatively. This study revealed the quantitative relation that the estimated depth by Wenner method corresponds to EC of 3 mS/cm (Cl^- = 450 mg/l) of groundwater. Considering that such depth is the upper part of the mixing zone, electrical method will be the powerful measure for monitoring the saltwater intrusion in a wide farming area along the coast.

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