Major, minor and trace element mobility in the acidic upland forested catchment of the upper River Severn, Mid Wales

Variations in the major, minor and trace element concentrations in two streams within a coniferous forest are described and are related to hydrological and hydrogeochemical factors. The rainfall chemistry reflects a maritime influence although there are occasional pollutant inputs (pH 4–5). Streamwater chemistry variations are strongly dependent upon flow. Baseflow chemistry, determined by reactions in the inorganic soils and bedrock, is of a Mg-Ca-HCO 3 type (pH 6–7.5); mineralization yields up to a four-fold increase in Ca, HCO 3 , Zn and SO 4 concentrations (pH ~ 0.5 units higher). Stormflow chemistry reflects hydrogeochemical processes in the organic soil horizons; the stormflow waters are acidic (pH ~ 4.5) and are enriched, by an order of magnitude, in H + , AI, Y, Co and Mn. Mineral equilibrium calculations have been applied to aluminium hydroxides, crystalline and amorphous aluminosilicates and hydroxy aluminium sulphates to determine the degrees of mineral saturation in the streamwaters. Ion exchange reactions have been evaluated to describe the consequences of natural variations in atmospheric salt loadings and evapotranspiration on the stream water chemistry and to explain low Na/Cl ratios in storm waters.