Analysis of the impacts of major anion variations on surface water acidityparticularly with regard to conifer harvesting: Case studies from Wales and Northern England

Data on the water quality of streams draining a range of acidic and acid se nsitive, mainly afforested, upland catchments in mid- and north-Wales and n orthern-England are described to investigate the acidification effects of c onifer harvesting in relation to natural variability. Most sites show a lar ge range in pH and major cation and major anion concentrations. The waters draining from the smaller catchments are more acidic and aluminium bearing reflecting a higher proportion of runoff from the acidic soils in each area . However, there is often a less acidic component of runoff under base-flow conditions due to ground-water contributions particularly within the large r streams. Higher concentrations of nitrate occur for sites which have been felled although declines in concentration occur several years after fellin g. Multiple regression analysis reveals the importance of cation exchange a nd within catchment acidification associated with sulphate and nitrate gene ration. Sulphate also has a component associated with weathering but the pa tterns vary from catchment to catchment. Analysis of the influence of chang ing anion concentrations associated with tree harvesting reveals that the a cidification induced by increases in nitrate can be offset or reversed by t he lowering of chloride and sulphate concentrations due to decreased atmosp heric scavenging by the vegetation, reduced evapotranspiration and increase d surface runoff diluting the acidity generated. It is concluded that conte mporary UK forestry guidelines with an emphasis on phased harvesting of cat chments over several years and careful harvesting methodologies can allevia te most problems of stream acidification associated with felling activities and in some cases can reverse the acidification pattern.