The Impact of Conifer Harvesting on Stream Water Quality: A Case Study in Mid-Wales

A 12-year record of water quality data for runoff from a spruce forested hillslope with podzolic soils shows the impacts of conifer harvesting and replanting in relation to nitrate generation and its influence on surface water acidification. With felling, nitrate increases from a background of 18 Eq/l to about 50 Eq/l after 1 to 2 years and then declines to background levels over the next 1 to 2 years and to lower concentrations thereafter. This change is mirrored by an acidification process as manifest by a change in Gran alkalinity, acid neutralization capacity (ANC) and aluminium concentrations as well as pH. For example, Gran alkalinity and ANC, which start at negative concentrations prior to felling (about –20 and –50 Eq/l, respectively), become more negative (–30 and –100 Eq/l, respectively) at high nitrate concentrations. Correspondingly, pH decreases from about 4.7 to 4.5 and aluminium concentrations increase from about 14 to 16 M. Subsequently, the acidification is reversed as nitrate concentrations decline and after five years post-felling the system has higher pH, Gran alkalinity and ANC together with lower aluminium concentrations than even before the felling took place (the post-felling values are about 4.9, -15 Eq/l, –20 Eq/l and 7 M/l, respectively).Other determinands show clear changes over time. For example, there is a marked increase in sodium and chloride prior to and around the time of felling (200 to 300 and 230 to 400 Eq/l, respectively), with a subsequent decline in concentration to pre-felling and to lower values of around 160 and 170 Eq/l, respectively, thereafter. This change is probably associated with abnormally high inputs of sea-salts from the atmosphere during the first quarter of the year of felling, and dilution thereafter, rather than a direct consequence of the felling activity itself: this change in sea salt loading has had an impact on stream acidity. Dissolved organic carbon and iron also change with concentrations increasing over time (60 to 200 and 1.0 to 1.5 M/l, respectively) and this mirrors a general pattern observed across the Plynlimon catchments irrespective of whether or not there has been felling activity.The implications of the findings are discussed in relations to environmental management and hydrochemical processes.