Any assessment of long-term repository safety will require development of a set of analyses and arguments to demonstrate the persistence of the key safety functions of the geological environment up to several hundred thousand years into the future. However, likely future global climatic and sea-level fluctuations and uplift/subsidence would result in a dramatic change in the location of the current coastline with a subsequent significant change to hydraulic and hydrochemical conditions at coastal sites. It is thus of great importance in the Japanese disposal programme to establish comprehensive techniques for coastal site characterisation. To this end, a systematic framework, which is known as a ‘Geosynthesis Data Flow Diagram’, has been formulated, which outlines a basic roadmap of the geosynthesis methodology for characterising temporal and spatial changes of various properties and processes at coastal sites, with particular focus on the palaeohydrogeology. A basic strategy for stepwise surface-based investigations has also been proposed, which incorporates the geosynthesis methodology in an effective manner. This technique has been introduced in an ongoing collaborative programme for characterising the coastal geological environment around Horonobe in northern Hokkaido, Japan, and now tested and optimised based on accumulated technical knowledge and experience during the progress of the investigations.

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