HYDROGEOCHEMICAL VARIATIONS IN HAFREN FOREST STREAM WATERS, MID-WALES

Results are presented for a study attempting to assess the effects of conifer planting/harvesting and acidic oxide deposition on streamwater quality in a 25 to 45-year-old sitka spruce plantation on acid moorland. This is undertaken to demonstrate the application of a multi-element survey in providing 'chemical fingerprints' for describing hydrological and hydrochemical controls within catchments. Hafren forest streamwater chemistry varies for different components: nitrate, bromide, total iodine and total organic carbon show seasonal oscillations varying in phase and amplitude; aluminium and hydrogen ion concentrations vary as a function of flow; manganese and cobalt remain approximately constant except under very dry conditions when concentrations reduce by up to 10 fold. No direct link exists between rain and streamwater chemistry: streamwater chemistry variations are determined primarily by hydrological and chemical reactions in the surface organicrich soils and the underlying inorganic soils/bedrock. Reactions in the organic-rich horizons involve the generation of acidic conditions and the mobilization/transport of transition metals that can be easily hydrolysed. Biologically mediated breakdown processes determine, in part, the hydrochemical behaviour of dissolved organic carbon, the nutrients, bromine and iodine. Reactions in the inorganic zones involve hydrogen ion consumption and the release of calcium and magnesium. Bicarbonate ions are generated by deprotonation of biogenically derived H2 CO3 and the decompostion of calcium carbonate in the bedrock. The initial effects of forest clearfelling are demonstrated; increases in nitrate and potassium occur. A simple mixing model is presented to show that either a large proportion of the storm water is derived from 'non-hillslope', 'groundwater', sources, or major modifications occur as soil water passes rapidly to the stream; whichever process is operative, it has not been identified directly within the catchment monitoring programme. © 1990.