報告書番号

U99062

タイトル（和文）

火山活動が高レベル処分のための天然バリアの地下水環境に及ぼす影響評価 --文献調査と岩手山における地下水調査--

タイトル（英文）

Evaluation on changes caused by volcanic activities in the groundwater environment as a natural barrier for the HLW disposal- Literature survey and groundwater observation conducted at Mt. Iwate

概要 （図表や脚注は「報告書全文」に掲載しております）

火山活動が高レベル処分のための天然バリア性能とりわけ地下化学環境に及ぼす影響を把握しておくことは候補地選定において重要である。一部地震を伴う火山活動が地下水環境に及ぼす影響の種類と範囲については文献調査を行い水温・地下水位・水圧、溶存ガス(ラドンやヘリウム同位体比)と塩素イオン濃度が変動することが明らかとなったが定量的な評価は十分ではない。そこで、現在活動が活発化している岩手山周辺で地下水位・地下水水質の変化を定期連続観測した結果、火山周辺の深部地下水よりも浅層地下水や湧水に活動の影響が早く現われる。火山活動の中心から5km以上離れると地下数100mから1000m程度の深部の地化学環境は安定している。この結果は、火山フロントの海溝側には地下のマグマの活動が直接及ばない地下水環境域が存在する可能性を示唆している。

概要 （英文）

It is very important in the site characterization for the HLW disposal to understand changes in geochemical performances caused by volcanic activities in the groundwater environment as the natural barrier. The various effects and its magnitude of changes were listed up and were filed from literature surveys on the correlation between volcanic activities and hydrological and geochemical changes (e.g. water temperature, water pressure, water level, dissolved gas concentration of He and Rn, isotopic ratio of He, and chloride concentration) in volcanic aquifer. However, it is difficult to evaluate the magnitude of impacts, which volcanic activities will give to the groundwater environment in the natural barrier, through only the literature surveys. We have started monitoring of groundwater level and changes in groundwater quality, since volcanic activities have enhanced at Mt. Iwate from June in 1998. Judging from variation of isotopic ratio of dissolved He in groundwater, a prompt and sharp signals indicating volcanic activities will easily be found in shallow groundwater and discharged ponds. On the other hands, geochemical conditions in deep groundwater surroundings from some 100 m to 1000 m deep will be very stable, if the area being more than 5 km apart from the volcanic active center. Consequently, our observed results suggest that the groundwater environment which is not directly disturbed by the underground magmatic activities spreads under the area that is connected to trench side of the volcanic front.

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