Chemical properties of surface waters in the limestone regions of western Japan: Evaluation of chemical conditions for the deposition of tufas

In order to understand the chemical conditions for the depositionof tufas, chemical properties of were investigated for the 201 surface waters collected from the limestone regions including newly discovered localities of tufas. The collected waters were categorized, according to the association with tufas, mainly into; category N (waters without tufa deposition), category T (waters depositing tufas), and category S (spring waters of tufa-depositing streams). These three categories clearly differ in Ca contents, equilibrium CO 2 partial pressure (PCO2), and saturation index for calcite (SIC); all of which are the most important chemical properties for the deposition of tufas. The waters of the category N are characterized by small Ca contents (mainly 15～35 ppm) and low PCO2(350～1,000 µatm) which indicate that they are basically flowing on a limestone substrate without an efficient CO2 uptake from soils. Their SIC never exceeds +0.5. The waters depositing tufas (category T) normally exhibitCa contents more than 45 ppm and PCO2 ranging 500～2,000 µatm. The waters of category S show comparable or slightly larger Ca contents than category T, however their PCO2 is considerably higher (than 2,000 µatm). Their raised values of Ca contents and PCO2 are ascribed to efficient CO2 uptake in a soil layer and subsequent dissolution of CaCO3. Furthermore, their SIC around 0.0 indicates that the waters dissolve CaCO3 until they reach the saturation in underground water systems. The large difference in PCO2 between categories T and S results from degassing of CO2 during flowing on the streams. The degassing increases pH and SIC of the waters. The SIC of category T mostly exceeds +0.5, that is probably the most important chemical condition for an efficient deposition of tufas. This study indicates that tufas are not very rare, but also that their distribution tends to be concentrated in a certain area, such as northwestern Okayama Prefecture. Local geological and hydrological conditions can be also important controls for deposition of tufas.