STUDIES OF SOILS, SOIL-WATER AND STREAM WATER AT A SMALL CATCHMENT NEAR GUIYANG, CHINA

Acid deposition is considered to be a major environmental problem in C hina, but information about effects on soils and waters is scarce. To contribute to increased knowledge about the problem a small catchment (about 7 ha) in the outskirts of Guiyang, the provincial capital of Gu izhou in south-western China, was instrumented for collection of preci pitation, throughfall, soil water and stream water. In addition soil s amples have been collected and analyzed for key properties. Median pH in the precipitation is 4.40 (quartiles: 4.19 and 4.77) and the median sulfate concentration 228 mu eq/L (quartiles: 147 and 334 mu eq/L). T he dry deposition of both SO2 and alkaline dust is considerable. The s um of wet deposition of sulfate and dry deposition of SO2 has been est imated to about 8.5 gSm(-2)yr(-1). The total S-deposition may be somew hat higher due to dry deposition of sulfate and occult deposition. In soil water, SO42- is the major anion, generally ranging from 300 to 25 00 mu eq/L in the different plots. Calcium is an important cation, but there is also a considerable contribution of aluminum from the soil. In some of the plots the concentrations of inorganic monomeric aluminu m (Ali) are typically between 200 and 400 mu m. Potential harmful leve ls of aluminum and/or high Ali/(Ca2+ + Mg2+) molar ratios occur in the catchment, but damages to vegetation have not yet been reported. In m ost cases exchangeable aluminum accounts for between 75 and 95% of the total effective cation exchange capacity (CECE) in the mineral soils. The aluminum chemistry cannot easily be explained by conventional mod els as the Gaines-Thomas ion-exchange equation or equilibrium with an Al(OH)(3) mineral phase. The stream water is generally less acidic and has considerably lower concentrations of aluminum than the soil water , even though quite acid events have been observed (pH < 4.4). The med ian pH values are 4.9 and 5.0 in the two first order streams and 6.3 i n the dam at the lower boarder of the catchment.