Masaru Yamanaka

Geochemical Characteristics of Acidic Stream Water on Yakushima Island, a World Natural Heritage Site

We determined the chemical compositions of acidic stream waters on the rural, mountainous island of Yakushima (YK), and compared them with those of near-neutral stream waters in other mountainous watersheds in Japan. The pH and concentrations of the predominant sea-salt components (i.e., Cl−, Na+, Mg2+) in YK water, particularly in streams with small watersheds, decrease with elevation, while the concentration of Al increases. In contrast, water in northwestern YK contains higher concentrations of non-sea-salt SO42− and NO3− and of rock-derived cations such as Si and Ca2+ than water in eastern to southern YK. This clearly shows that the anthropogenic pollutants on YK are dominantly derived from the northwestern to western Asian continent rather than from Japans main islands, which lie to the northeast. These pollutants are responsible for stream acidification on YK and for accelerating chemical weathering, mainly of plagioclase, in northwestern YK. Large amounts of acid rain on YK may enhance the formation of secondary minerals that strongly adsorb heavy metals, which can partly account for the low concentrations of heavy metals in YK waters.