Movement of the seawater/freshwater interface and changes in groundwater flow in sedimentary soft rocks by pumping at a coastal site

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  • 短報 揚水に伴う沿岸域堆積軟岩中の塩淡境界形状と地下水流動の変化
  • 揚水に伴う沿岸域堆積軟岩中の塩淡境界形状と地下水流動の変化
  • ヨウスイ ニ トモナウ エンガンイキ タイセキ ナンガンチュウ ノ エンタン キョウカイ ケイジョウ ト チカスイ リュウドウ ノ ヘンカ

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Abstract

The seawater/freshwater interface at depth is influenced by fossil waters and other parameters, making its analysis more difficult than for shallow aquifers. Since the dynamics of the deep seawater/freshwater interface is poorly understood, an evaluation method combining both in-situ observations and numerical analysis is necessary (e. g. this is specially true in projects like geological disposal). The movement of the seawater/freshwater interface was observed to a depth of 200 m in the sedimentary soft rocks of the Quaternary and Tertiary of Tokai Village, Ibaraki prefecture (japan). Seawater intruded during pumping (average of 10,000 m3/d) related to the construction of a ring-like tunnel, as a part of the construction of a proton accelerator complex. Initially, the seawater/freshwater interface was simulated under original conditions. Based on these results, four monitoring wells were drilled during the pumping period. Electric conductivity and temperature were continuously, _ measured. Differences in the seawater intrusion patterns within the Quaternary and Tertiary sedimentary soft rocks were as follows. (1) Electric conductivity show ed that seawater intruded into the Quaternary during the 13-22th months of pumping, but there was no salinization in the Tertiary. The vertical migration of salts was prevented by the aquitard below the intruded aquifer. The permeability of this barrier was 1/10-1/100 of the upper aquifer. (2)Rises and drops of temperature were observed at various depths during pumping. For example, temperatures drops were observed at the seawater intrusion depth during the 11-12th months of pumping, and on the 18th month at the boundary between the Quaternary and the Tertiary. Salinity at the Tertiary might be related to other sources aside of present seawater intrusion. (3)The seawater/freshwater interface in the Quaternary and the Tertiary were simulated without pumping. The position of seawater/freshwater interface was in good agreement with the monitoring data, but it was difficult to calculate the salts concentration in the Tertiary sediments. It is thought that under static boundary conditions, even if the position of the simulated seawater/freshwater interface is valid in the shallow area, it may not be adequate for simulations at depth.

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