石灰岩地域秋吉台における水循環と地下水水質の形成

抄録

The drainage basins in the Akiyoshi-dai Plateau limestone area (Yamaguchi Prefecture, Western Japan) are divided into six areas, on the basis of the topographic divide and the existence of two groundwater systems: Hirotani (where Akiyoshi-do Cave is located), Kanoide, Kuroiwa, Ono, Narutaki and Sowa. In this limestone area, groundwater flows out from 4 springs and 5 caves at the foot of the plateau. The origin of the dissolved components in the groundwater was estimated by using the selfconsistent least squares method proposed by Tsurumi (1982). The contents of the dissolved components for all spring waters could be explained only by the mixing of three source waters: one characterized by a high contribution of limestone components, another by that of non-carbonate rock components, and the other by calcium sulfate probably originating from fallout and sulfur-oxidation in the soil in the plateau.
The drainage basin of Akiyoshi-do Cave, located in Hirotani Polje, occupies about a half of Akiyoshi-dai Plateau, where the annual mean precipitation is 1,974mm, and the average run-off from Akiyoshi-do Cave is. estimated to be 955mm. The calcium concentration of the baseflow showed seasonal fluctuations, which were followed by changes in CO₂ partial pressure in the soil. Measured soil CO₂varied from a minimum of 0.15 % at a soil temperature of 7.2°C, and a maximum of 1.7% at 19.0°C in the humus soil, whereas in the meadows which cover most of the area, it ranged from 0.08% at 3.8°C to 1.2% at 20.8°C. The calcium concentration in groundwater issuing from Akiyoshi-da i Plateau is controlled by water-limestone dissolution equilibrium, under open system conditions depending on the meadow's soil CO₂ contents.