報告書番号

N10036

タイトル（和文）

涵養温度を指標として用いる地下水年代評価法の提案

タイトル（英文）

Proposal of groundwater dating method by using recharge temperature

概要 （図表や脚注は「報告書全文」に掲載しております）

本研究では、地下水涵養時の温度を指標とした地下水年代測定を試みた。温度の指標となるクリプトン（Kr）濃度および酸素同位体比を採用し、評価対象地点でのそれらの分布を調べた。Kr濃度が極大値をとる地点と酸素同位体比が極小値をとる地点は一致しており、この地点の地下水が涵養されたときの温度が低かったことを示唆する結果となった。これは、最終氷期の最盛期である18000年に対応すると考えられ、この地点の地下水年代を18000年と決定することができた。また、Kr濃度や酸素同位体比を指標として、地下水中に蓄積するHeの増加速度を推定することができた。これによって評価地域では4He濃度を用いて地下水年代の正確な定量が可能となった。上記のように温度指標の極値の位置や、He濃度との比較により、地下水年代測定が可能であることを明らかにした。

概要 （英文）

The residence time of groundwater is one of the most important information to know migration of groundwater. 14C and 4He are powerful tools for dating groundwater with 10000 to 30000 years residence time. However, in some cases, 14C and 4He can not provide accurate residence time of groundwater because residence time estimated by 14C and 4He is affected by geochemical reaction or He flux. Thus another indicator have to be developed to estimate residence time such as 10000 to 30000 years, even in such cases.
In this study, temperature during groundwater recharge was used as a indicator of residence time of groundwater. Concentration of Krypton (Kr) and oxygen isotope ratio are used as indicators of temperature and depth profile of these indicators were investigated. Local maximum value of Kr concentration and minimal value of oxygen isotope were found at same depth indicating groundwater at this depth recharged in local lowest temperature. The residence time of groundwater at this depth can be estimated as 18000 years because this lowest temperature is considered to be related to LGM (the Last Glacial Maximum) that took place 18000 years ago. Thus extremal values of temperature indicator can indicate the groundwater that recharged 18000 years ago. Furthermore, the relationships between temperature indicator and 4He concentration were investigated. He accumulation rate in groundwater can be estimated by comparison between He concentration and local maximum value of Kr concentration. The residence time of groundwater can be by using 4He concentration and 4He accumulation rate calibrated by temperature indicator.
Thus by using extremal values of temperature indicator or by using 4He concentration and 4He accumulation rate calibrated by temperature indicator, residence time of groundwater can be estimated even in the cases 14C and 4He can not provide accurate residence time.

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