北海道幌延地域における深部地球化学環境の長期変遷シナリオの構築

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A methodology to estimate long-term hydrochemical evolution of deep underground is indispensable for the safe geological isolation of high-level radioactive waste (HLW). This study demonstrates the methodology by illustrating scenarios of past geological events, processes, and their interrelationships with present-day hydrochemical conditions. Besides, we infer long-term variations of groundwater chemistry at Horonobe, Hokkaido, Japan. The region is underlain mainly by Neogene to Quaternary marine sedimentary rocks, (the Wakkanai Formation (Fm) and the overlying Koetoi Fm: siliceous and diatomaceous mudstones). During various events in the geological past, such as deposition, compaction, uplift, and denudation, and the more recent Neotectonic activities in this area, highly permeable hydrogeological structures formed at depths not over than 400m below ground level in the Wakkanai Fm. The hydrogeological system can be subdivided into three hydrogeological sub-systems: (1) overlying, relatively low permeability Koetoi Fm., (2) highly permeable, upper Wakkanai Fm at depths less than 400m and (3) relatively low permeability Wakkanai Fm, at depths greater than 400m. The present-day hydrochemical conditions in each sub-system have been influenced by hydrogeological properties and hydraulic conditions over a long period. In subsystems 1 and 2, recharging with meteoric water flushed connate seawater during uplifting and denudation during the last 1Ma. In contrast, fossil seawater with one-third to one-half the salinity of present-day seawater has been preserved in subsystem 3. The relatively low permeability sequence in sub-system 3 was formed by the compaction of diatomaceous mudstone during subsidence prior to 1.0Ma. After that, changes of climatic conditions and geographical features would not have influenced groundwater flow. The groundwater chemistry evolved from seawater during long-term geochemical diagenesis in a relatively closed system. The long-term variations of salinity probably range from that of seawater to that of present-day groundwater. The occurrence of secondary minerals shows that the buffer reactions of carbonate and sulphide minerals have preserved the near neutral pH and reducing condition since initial diagenesis immediately after deposition of the rock formation.
2009-08-25

北海道幌延地域における深部地球化学環境の長期変遷シナリオの構築

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