Acidification of freshwaters by red soil in a subtropical silicate rock area, Okinawa, Japan

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⁻¹ and 3.54–11.23 mSm⁻¹ 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⁻¹ and 17.5–25.0 mSm⁻¹ 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⁺≫K⁺ and Ca²⁺≫Mg²⁺. After mixing with red soil, the trend of the concentrations changed to Na⁺≫K⁺ and Mg²⁺≫Ca²⁺, 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⁻¹. 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.