酸素・水素安定同位体比を利用した秋田県谷地地すべり地の地下水の混合と起源について Origin and Mixing of Groundwater Estimated by Oxygen and Hydrogen Isotope Tracers Technique in Yachi landslide Area, Akita, Japan

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地すべりが活動する誘因の1つに地すべり崩積土中の間隙水圧の増加が挙げられる。間隙水圧の増加は集中豪雨や融雪水などに起因する場合が多く, 降雨 (雪) 水の地下への浸透や地下水との混合のプロセスを解明することは, 地すべりの防止や予知に重要な情報を提供することが期待される。本論文では様々な水の挙動を把握するために有効な追跡子である酸素・水素安定同位体比を用い, 降水が地下に浸透し涵養域に至るまでの過程を検討した。<br>調査地域として, 地質状況が把握され, 水抜き用集水井が数多く設置されている秋田県・谷地地すべりを選び, 集水井から排水される地下水や周辺の地表水の同位体比を測定した。その結果, 集水井ごとに岡位体比の変化幅がそれぞれ異なることが判明した。このことは降水が地下に浸透してからの地質状況や浸透距離 (時聞) を反映しているものと推定される。そこで降水の季節変動を利用したモデル計算で平均滞留時間を試算したところ, 短期間の地下水で2ヶ月程度, 長期間の地下水で1年程度で流入から排水に至ることが判明した。

In Yachi landslide area at south-eastern part of Akita Prefecture, many drainage wells of groundwater have been installed. There, groundwater from the drainage wells as well as the surface waters were monthly collected, and analyzed for the oxygen and hydrogen isotopic ratios, to discuss origin of the groundwaters and mixing process in the groundwater reservoirs.<br>One of two cold springs has almost constant isotopic ratios of δD=-57‰, δ<sup>18</sup>O=-9.9‰ through the year, and the other shows large variations of isotopic ratios, δD=-47--70, δ<sup>18</sup>O=-8.7--11.2‰, of which seasonal patterns are similar to those of precipitation in Akita City. It is suggested that the former is precipitation well mixed for long duration enough to smooth its seasonal variation of isotopic ratios, while the latter, precipitation not well mixed. The groundwater from drainage wells shows three different degrees of mixing, Groundwater drained after infiltration of precipitation about 300m in shale layer shows small seasonal variation similar to the cold spring having almost constant isotopic ratios, while groundwaters drained in 30m distance from the shale outcrop shows seasonal variation of which extent is about half of the cold spring having large isotopic variation. Groundwater in colluvial soils shows intermediate variation of above 2 types of the groundwaters. These differences are able to explain by the difference in mean residence time of groundwater. As the result of a model calculation based on well mixed reservoir in a steady state, mean residence times of groundwaters were estimated as; about one year for the groundwater of almost constant isotopic ratio from shale layer, about six months for the those from colluvial soils, and about two months for those drained near the shale outcrop, respectively. As for the groundwater near the shale outcrop, also, certain discord of seasonal variation phase was observed between the model calculation and the observation. This suggests that the infiltration of precipitation includes some piston flow like process.

収録刊行物

  • 地すべり

    地すべり 36(4), 48-55, 1999-03-15

    公益社団法人 日本地すべり学会

参考文献:  14件中 1-14件 を表示

被引用文献:  7件中 1-7件 を表示

各種コード

  • NII論文ID(NAID)
    10013006318
  • NII書誌ID(NCID)
    AN00332562
  • 本文言語コード
    JPN
  • 資料種別
    ART
  • ISSN
    02852926
  • NDL 記事登録ID
    6353995
  • NDL 雑誌分類
    ZM49(科学技術--地球科学--地質)
  • NDL 請求記号
    Z15-405
  • データ提供元
    CJP書誌  CJP引用  NDL  J-STAGE 
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