THE IMPACT OF CONIFER HARVESTING ON STREAM WATER PH, ALKALINITY AND ALUMINUM CONCENTRATIONS FOR THE BRITISH UPLANDS - AN EXAMPLE FOR AN ACIDIC AND ACID SENSITIVE CATCHMENT IN MID WALES

Results of a 5-year stream water quality monitoring programme, together with results from a complementary soil water chemistry study, of an upland afforested acidic and acid-sensitive environment, are presented. They show the impacts of tree harvesting on stream and soil water chemistry for the key components associated with acidification of the upland UK environment. Large temporal variations in alkalinity, pH and aluminium concentrations are reported. These are linked to variations in the contribution of acidic soil water and less acidic ground water to the stream. The effects of felling are shown to increase aluminium concentrations in the stream and the soils for the first 2 years: the major changes in the stream occur during the winter storm flow periods when concentrations are at their highest. Stream pH and alkalinity decline, for the first 2 years from the onset of felling, after which, the trends are reversed to give conditions similar to those pertaining prior to felling. These declines are associated with the summer baseflow periods when pH values and alkalinities are at their highest. The results are explained in terms of cation exchange reactions associated with release of nitrate following felling, changes in hydrological flow pathway and acidification of the ground water.