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Acidification of freshwaters by red soil in a subtropical silicate rock area,Abstract Two samples of red soil, one from Gushikawa Recreation Center (GRC) and one from Okinawa Royal Golf Club (ORGC), were examined for particle size distribution, textures, minerals, and chemical compositions. The effects of particle size and grinding of clay minerals on pH, electrical conductivity (EC), and dissolved chemical species were studied in deionized water and river water. The results of red soil solutions were compared with those of acidic waters found in red soil dominated areas. The minimum pH values of soil solutions extracted by deionized water were 4.38-5.36 and 5.16-5.89 and the maximum values of EC were 4.91-16.98 mSm Ϫ1 and 3.54-11.23 mSm Ϫ1 for GRC and ORGC, respectively. In the river water samples equilibrated with red soils, the minimum pH values were 4.48-5.10 and 4.77-5.91 and the maximum EC values were 19.6-34.2 mSm Ϫ1 and 17.5-25.0 mSm Ϫ1 for GRC and ORGC, respectively. The values of pH and EC varied with the soil-solution ratio and the particle size. The chemical composition of river water without mixing with red soil shows Na ϩ ϾK ϩ and Ca 2ϩ ϾMg 2ϩ . After mixing with red soil, the trend of the concentrations changed to Na ϩ ϾK ϩ and Mg 2ϩ ϾCa 2ϩ , which is the same as that of soil solutions in deionized water as well as that of acidic waters found in the red soil area. The pH of the acidic waters was 4.95-5.81 and EC was 7.76-30.0 mSm Ϫ1 . Laboratory experimental results agreed well with those found in the field in terms of trend of concentrations of the chemical species and pH. Therefore, the results of this study suggest that the low pH and trend of the concentrations of chemical species of the acidic waters found in the red soil dominated areas were the result of the interaction of natural water and red soil.

The chemical characteristics of the Urauchi River water in Iriomote Island National park, Okinawa, Japan have been studied. Concentrations of PO 4 3− , NO 2 − , and NH 4 + were barely detectable. We compared the concentration ratios of Mg 2+ /Na + , HCO 3 − /Na + , and Ca 2+ /Na + in the Urauchi River to those of 60 large rivers in the world and indicated that the chemical composition of the river is most likely formed by the binary mixing of sea salt components and silicate rock weathering components. Although rock weathering in the catchments area is driven by both H 2 CO 3 and H 2 SO 4 , the role of H 2 CO 3 is dominant. The percentages of the concentration of each cation in the river water are almost the same as those of other rivers with drainage areas consisting of silicate rock and sandstone. Thus, the Urauchi River shows the typical chemical characteristics of a river in a silicate rock area that includes sandstone.

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