Evaluation of Recharge Mechanism of Confined Groundwater Using Long Term Tritium Concentration Records in the Metropolis of Tokyo.

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  • 東京都における被任地下水のかん養機能　　長期間のトリチウム濃度変化によるかん養機能評価
  • トウキョウト ニ オケル ヒアツ チカスイ ノ カンヨウキノウ チョウキカン ノ トリチウム ノウド ヘンカ ニ ヨル カンヨウキノウ ヒョウカ
  • Long Term Tritium Concentration Records in the Metropolis of Tokyo
  • 長期間のトリチウム濃度変化による涵養機能評価

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Long-term tritium concentration records for 1969 to 1997 were used to evaluate the recharge mechanism of confined groundwater in the Metropolis of Tokyo. Fluctuation patterns of time series of tritium concentrations are classified into the following four types: 1) Gently-sloping peak type (type A), 2) Sharp peak with irregularly decreasing curve type (type B), 3) Simple decreasing without peak type (type C), and 4) Low concentration with an average tritium concentration of 5 TU or less and monotonous changing type (type D). Type D is seemed to be the fluctuation pattern of pre-bomb tritium.<BR>Analysis of time series of tritium concentrations of types A, B and C by theconvolution integral model using the normal distribution function as a simple transit time distribution function shows that the difference of dispersion causes three types of fluctuation pattern. The depth of penetration of bomb tritiuminto aquifers was used to calculate the rate of groundwater recharge on the assumption of vertical downward infiltration of precipitation and groundwater. The rate of recharge of the unconfined groundwater was estimated to range from 75 mm/year to 140 mm/year, and that of confined groundwater to range from 493 mm/year to 1, 752 mm/year. However, the vertical distribution of the fluctuation patterns under the upland of Tama district suggests that the confined groundwater is not recharged from the unconfined groundwater with a vertical downward flow, and that the recharge of the confined groundwater is supplied from groundwater with a lateral flow recharged in the area where the confined aquifers outcrop near the surface of the earth. Therefore the rate of recharge of the confined groundwater appears to indicate the apparent rate of recharge.

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