The impacts of conifer harvesting on runoff water quality: a regional survey for Wales

Major, minor and trace element chemistry of runoff at stormflow and baseflo w from 67 catchments (2 to 5 ha in area) has been determined to investigate the effects of clear felling and replanting of conifers on stream water qu ality across Wales. Samples, collected by local forestry workers (Forest En terprise staff) on a campaign basis on up to eight occasions, were for 16 m ature first rotation standing forest: the remainder represented areas compl etely clear felled from less than one to up to forty years previously. As t he waters drain acidic and acid sensitive soils, acidic runoff is often enc ountered. However, higher pH values with associated positive alkalinities a nd base cation enrichments are observed due to the influence of weathering reactions within the bedrock. There is little systematic variation in water quality between baseflow and stormflow for each site indicating a complex and erratic contribution of waters from the soil and underlying parent mate rial. 80% or more of the data points show hardly any changes with felling t ime, but there are a few outlier points with much higher concentrations tha t provide important information on the processes operative. The dearest out lier felling response is for nitrate at five of the more recently felled si tes on brown earth, gley and podzolic soil types. ANC, the prime indicator of stream acidity, shows a diverse response from both high to low outlier v alues (>+400 to -300 mu Eq/1). In parallel to nitrate, aluminium, potassium and barium concentrations are higher in waters sampled up to 4 years post felling, but the time series response is even less clear than that for nitr ate. Cadmium, zinc and lead and lanthanides/actinides show large variations from site to site due to localized vein ore-mineralization in the underlyi ng bedrock. The survey provides a strong indication that forest harvesting can have mar ked local effects on some chemical components of runoff for the first four years after felling but that this is confined to a small number of sites wh ere nitrate production and aluminium leaching are high. In general, defores tation leads to a reversal of acidification when the nitrate pulse is low. The variability in water quality from catchment to catchment is too high fo r generalized conclusions to be made over the extent of the potential chang es from site to site. The value of an organised campaign of opportunistic s ampling using an infrastructure of enthusiastic staff from regionally dispe rsed organisations associated with environmental matters (in this case the forestry industry) is highlighted.