MATSUOKA Shiro Kyushu University,College of General Education, Research Associate, 教養部, 助手 (10219404)
INOKURA Youji Kyushu University,Faculty of Agriculture, Research Associate, 農学部, 助手 (60203270)
On the basis of the topographic divide and the existence of two groundwater systems, the catchment basins in the Akiyoshi-dai Plateau limestone area(Yamaguchi Prefecture, Western Japan)are divided into six areas: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 self-consistent least squares method proposed by Tsurumi(1982). The contents of the dissolved components for all spring waters could be explained by the mixing of only three sources: one rich in limestone components,another in non-carbonate rock components,and the other in calcium sulfate probably originating from fallout and sulfur-oxidation in the soil in the plateau.
With an area of 18.5km^2, the catchment basin of Akiyoshi-do Cave occupies about half of the Akiyoshi-dai Plateau, where the annual mean precipitati
on is 1,974mm,and the average run-off from Akiyoshi-do Cave is estimated to be 955 mm. The calcium concentration of the baseflow showed seasonal fluctuations,and followed changes in the soil's CO_2 partial pressure. Measured soil CO_2 in the humus soil varied from 0.15% at a soil temperature of 7.2ﾟC to1.7% at 19.0 ﾟC, whereas in the meadows which cover most of the area,it ranged from 0.08% at 3.8ﾟC to 1.2% at 20.8ﾟC. The calcium concentration in groundwater issuing from the Akiyoshi-dai Plateau is controlled by the water-limestone dissolution equilibrium,under open system conditions depending on the meadow's soil CO_2 contents. A yearly average of 2,100tons of limestone was dissolved in groundwater issuing from Akiyoshi-do Cave,about 5% of which,of 110tons, was dissolved by acidic deposition.
Ion-exchanger phase absorptiometry was applied to the flow analysis of trace amounts of copper. This method permitted a highly sensitive,selective determination of copper in karst groundwater samples without any preconcentration. By measuring the copper concentration response in the water after a storm,the infiltration rate of rain water from the soil zone to the underground river of this karst groundwater system could be evaluated to be about 10-20m day^<-1>. On the other hand,the tritium concentration response suggested that the faily long residence groundwater(about 40years)entered the groundwater system after a big storm.