River water quality formation on Iriomote Island of the Ryukyus was studied to understand the environment of an isolated subtropical island. The concentrations of the major ions in the water of five rivers and pH of each precipitation were measured for one year in 2005. The water quality was mainly classified into the alkali noncarbonate-type by mixing solutes of two different origins, i.e., sea salt and salt resulting from chemical weathering. In the five rivers, HCO3- concentrations ranged from 8 to 34 mg/l, and the Urauchi River, whose catchment area was the widest of the rivers on the island and was covered with siliceous sandstone, was characterized by a very low HCO3- concentration as a result of the chemical weathering of noncarbonate minerals. The average pH of the precipitation on this island was 4.7 except for the time of typhoons, and the river water chemistry reflected acid deposition. The total concentration of dissolved cations due to the chemical weathering of rocks (nss-cations) was equivalent to the sum of HCO3- and nss-SO42- concentrations, suggesting that acid deposition contributes to 10 - 20 % of the total chemical weathering. A further increase in the input level of acidic substances transported from long distances may negatively influence the quality of terrestrial water.
