The role of forest management in controlling diffuse pollution from the afforestation and clearfelling of two public water supply catchments in Argyll, West Scotland

摘要

Two catchment studies were undertaken in Argyll, West Scotland to test the efficacy of the Forestry Commission's Forests and Water Guidelines in controlling diffuse pollution from forestry. The first involved the peaty catchment of the Clachaig Water, which provides the potable supply for the villages of Muasdale and Glenbarr. Thirty-five percent of the area was ploughed and drained in spring 1990 and subsequently fertilised with rock phosphate. The water supply catchment of the Gleann Sabhail Burn at Tayvallich provided the second site for study. Following the construction of a 2.5 km length of forest road, around 40% of the catchment was harvested over a 4-year period. Forest management practices followed the working methods and measures described in the Guidelines. Water samples were collected at regular intervals from the water supply intakes to determine the impact of these practices on water quality, particularly on sediment release and water colour. The benthic macroinvertebrate fauna at both sites and the trout population at Tayvallich, were also monitored. Results showed that water quality was relatively unaffected by all of the forestry operations, with water clarity remaining high. No disruption was caused to the public water supply in either catchment. Impacts on stream biology were small and limited to a short-term decline in the number of macroinvertebrate taxa at one site at Tayvallich, due to the localised accumulation of brash and silt. There was no clear evidence of harvesting operations having an impact on the trout population. Overall, the Guidelines were effective in limiting site damage and minimising the impact of forest operations on the freshwater environment. The study demonstrates that good forest management can effectively control the threat of diffuse pollution within sensitive water catchments.