Study on the Chemical Evolution of Groundwater in the Hydrological Cycle of a Limestone Area

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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 … More 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. Less