Abstract
In order to investigate the influence of watershed geology, a water sampling survey was conducted on Ishigaki and Iriomote Islands, southwest of Japan, whose basement rocks are carbonate and siliceous sandstone, respectively. Samples from Miyara River in Ishigaki showed generally high total alkalinities and DIC concentrations reflecting the effect of carbonate erosion and those concentrations drastically increased downriver. In contrast, samples from Urauchi River in Iriomote showed lower DIC concentrations, reflecting slow chemical erosion of silicate rocks and less groundwater contribution. The stable carbon isotope (d13C) values of high-DIC samples suggest that dissolution of carbonate rocks are enhanced by continental and/or other acidic matters.
