STREAM CHEMISTRY IMPACTS OF CONIFER HARVESTING IN WELSH CATCHMENTS

Hydrochemical data have been collected for between 6 and 9 years from forest harvesting experiments in small catchments (>10 ha) at Plynlimo n and Beddgelert, Wales, UK. Felling resulted in rapid increases in NO 3- and K+ concentrations at both sites. A maximum of 3.2 mg N L(-1) wa s observed at Plynlimon about one year after the start of felling. Con centrations declined to control stream values (0.5 mg N L(-1)) after 5 years. At Beddgelert, NO3- concentrations in the manipulated catchmen ts remained above those in the unfelled control catchment for three ye ars, before declining below control values. The NO3- pulse was related to increased rates of mineralization acid nitrification in the soil a fter felling. The initial increase in K+ concentration after felling a t Plynlimon was followed by a slow decline, but concentrations were st ill above those in the control stream after 5 years. From 4 to 8 years after felling at Beddgelert, K+ concentrations fell below and then ge nerally remained lower than control values. The NO3- pulse after felli ng at Plynlimon sustained inorganic anion concentrations above those i n the control stream for the first 18 months after felling. As the NO3 - pulse declined, inorganic anion concentrations decreased to below th ose in the control stream about 4 years after felling. At Beddgelert, the smaller increase in NO3- concentrations had less of an effect on i norganic anion concentrations which decreased after felling relative t o values in the control stream. The increase in NO3- was associated wi th temporary streamwater acidification in the felled catchments due to the increased rates of nitrification and nitrate leaching. At Plynlim on, streamwater filterable Al concentrations declined after felling, b ut controls on Al behaviour are complex and not explained by simple eq uilibrium relationships with Al(OH)(3) or by variations in inorganic a nion concentrations. At Beddgelert, felling had no effect on stream wa ter filterable Al concentrations. Felling at Plynlimon led to a large reduction in streamwater Cl-, Na+ and SO42- concentrations. At Beddgel ert reductions in SO42- and 'sea salt' ion concentrations were less cl ear, reflecting the smaller proportions of the catchments which were h arvested. Felling had no deleterious effects on water quality, apart f rom a temporary slight further decline in stream pH at Beddgelert. Inc reases in NO3- concentrations were short-lived and concentrations were well below drinking water standards. Filterable Al concentrations wer e already higher than statutory standards, but were not increased or d ecreased through felling.