Origin and Mixing of Groundwater Estimated by Oxygen and Hydrogen Isotope Tracers Technique in Yachi landslide Area, Akita, Japan.

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  • 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
  • サンソ スイソ アンテイ ドウイタイヒ オ リヨウ シタ アキタケン ヤチ ジスベリチ ノ チカスイ ノ コンゴウ ト キゲン ニ ツイテ

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Abstract

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‰, δ18O=-9.9‰ through the year, and the other shows large variations of isotopic ratios, δD=-47--70, δ18O=-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.

Journal

  • Landslides

    Landslides 36 (4), 48-55, 2000

    The Japan Landslide Society

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