安定同位体を用いた足柄平野における地下水流動系の区分

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タイトル別名
  • Classification of Groundwater Flow Region in the Ashigara Plain by Stable Isotopes
  • アンテイ ドウイタイ オ モチイタ アシガラ ヘイヤ ニ オケル チカ ミズリュウドウケイ ノ クブン

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<p>Water quality and stable isotopes in groundwater, river water and precipitation in the Ashigara Plain were analyzed to consider the groundwater flow-pattern in the plain. Regression analysis between isotope ratio in precipitation and those in groundwater both in summer and winter suggested that the groundwater was recharged mainly by summer precipitations.</p><p>The Ashigara Plain could be classified into four regions by the movement patterns of the unconfined groundwater. In the first area, the δ18O values in groundwater was almost equal to that in the upper part of the Sakawa River, showing that the groundwater is recharged by infiltration of the irrigation water derived from the upper part of the Sakawa River. In the second area, the groundwater was judged to be local water which was unaffected by the Sakawa River, but mostly supplied from precipitation, because δD and δ18O values in the groundwater were different from those in the river water of the Sakawa River, but were almost equal to the weighted mean of the values in the precipitation. In the third area, the groundwater was also judged to be unaffected by the Sakawa River, but isotope values in the groundwater was isotopically heavier than the weighted mean of those in the precipitation. In contrast, in the fourth region, the EC and water quality of the groundwater were different from those in the other regions, because the groundwater was supplied from the foot part of Mt. Hakone.</p><p>The plain was also classified into three confined groundwater-flow regions. The first region locates, in the center of the Ashigara Plain, where the δ18O values of groundwater were equal to that in the river water of the upper part of the Sakawa River. This suggests that groundwater recharge from the upper part of the Sakawa River exists in this region. In the second region, i.e., the Kari River Basin, the EC in the confined groundwater was smaller than those in the other two areas. From this result and the geologic column, the groundwater recharge from the foot part of Mt. Hakone can be expected in this region. In the third region, i.e., in the southeast part of the Ashigara Plain, the δ18O values of groundwater were isotopically lighter than that in the unconfined groundwater in the same region, suggesting that the isotopically light groundwater, which is affected by the altitude isotope effect, is supplied from the Ooiso-uplands.</p>

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