報告書番号

N10005

タイトル（和文）

地化学環境を考慮した14C地下水年代補正方法の検討-六ケ所サイトにおける水質形成機構の解明と地下水年代評価-

タイトル（英文）

Development of the correction method of 14C groundwater dating -Evaluating the groundwater evolution and dating at the Rokkasho site-

概要 （図表や脚注は「報告書全文」に掲載しております）

地下水溶存炭酸に含まれる14Cを用いた地下水年代測定には補正計算が必要であり，サイトの地化学環境の違いにより様々な補正計算手法が用いられる。本研究では日本原燃(株)六ケ所サイトを対象として地下水水質形成機構を検討し，地化学環境を考慮した地下水年代補正式を作成した。年代補正計算の結果，他の研究による三次元地下水流動解析による年代測定結果と概ね近い値の地下水年代を得ることができたことから，相補的に年代は確からしいということができ，補正式も確からしいと考えられた。

概要 （英文）

To estimate the 14C groundwater age, groundwater evolution of dissolved inorganic carbon (DIC) needs to be investigated at a site. Groundwater evolution depends on the geochemical setting at a site. This paper proposed a correction method of 14C groundwater dating by evaluating the DIC groundwater evolution at the site of low level radioactive waste disposal in Rokkasho, Aomori prefecture, Japan.
The d13C of DICs in the shallow part of the Rokkasho site (shallower than ~-100m elevation) showed inhomogeneous values. From the rock core chemical analysis, no inorganic carbon existed and concentrations of DIC were unchanged with depths in the shallow part. It indicates that carbonate mineral has no effect on d13C values in the shallow part but recharging system affected on the d13C of DIC. It is expected that both of open system and closed system or partially open and closed system recharging were taken place in shallower depth of the site. On the other hand, in the deep part of the site (deeper than ~ -100m elevation), there are shell fossils in aquifer consisting rocks. The shell fossils are dissolved based on the Na-(Ca, Mg) ion exchange reaction. It causes higher d13C and lower 14C in the groundwater DIC at the deep part.
The result of mass balance calculation of DIC concentrations and d13C of DIC showed that DIC concentrations and d13C was affected by some sort of reaction other than shell fossil dissolution in the deep part in the site.
Correction of 14C was conducted by the IAEA equation. The corrected 14C groundwater age in this study shows good correlation with the result of other dating study that derived by hydrological flow analysis compare to uncorrected age.

報告者

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