ABSTRACT
Windrowing is widely practised, across Europe and North America, in bole-only harvested coniferous forest plantations before replanting. Forest harvesting has been shown to significantly increase sediment and nutrient losses to watercourses in other studies but windrowing effects, which are as bad, have not been investigated in detail. To determine physico-chemical impacts on water quality and to help inform forest managers, the effects of windrowing were investigated in a headwater catchment. Water samples were collected from storm events pre- (PWR), during (DWR) and after windrowing (AWR). Total suspended solids (TSS), total phosphorus (TP), soluble reactive phosphorus (SRP), total ammonia, nitrate, stream discharge, water level and velocity were measured. Results showed that peak and flow weighted mean concentrations (FWMC) of TSS concentrations increased significantly during windrowing when compared to pre-windrowing concentrations. Peak TSS increased from 88 mg/l (PWR) to 502 mg/l (DWR) and decreased to 163 mg/l (one year AWR) and 225 mg/l (two years AWR). Peak and FWMC of TP also increased during windrowing when compared to pre-windrowing concentrations. Peak TP concentrations increased from 0.1 mg/l (PWR) to 0.4 mg/l (DWR) and decreased to 0.1 mg/l (AWR). SRP and nitrate concentrations increased during windrowing when compared to pre-windrowing but remained low overall. TSS and TP concentrations were highest when flows greater than 0.3 m(3)/s (exceeded 6.3% of time) were recorded in the channel. It was highlighted that high-resolution sampling of storm events is important, where precise measurements of windrowing-sourced outputs are required. Windrowing was shown to generate very high concentrations of TSS and TP, comparable to those recorded during harvesting. This research helps to identify potential impacts on physico-chemical water quality that arise during windrowing and demonstrates the need for measures to minimize impacts on surface waters as required by the EU Water Framework Directive and similar legislation elsewhere.
ABSTRACT
Episodic surface water acidification is common in many regions worldwide; the driving processes are dependent on a variety of physicochemical and climatic characteristics, and acid deposition pressures, which have changed significantly over the last two decades. This study provided a unique opportunity to re-examine the drivers of acidity in an environment of low anthropogenic input. In three geologically distinct acid-sensitive regions of Ireland during 2009-2010, 34 headwater streams were evaluated in peat-dominated catchments draining moorlands without forest, 20-50% (low) forest cover and >50% (high) forest cover. Results indicated episodic acidity/alkalinity loss in headwater streams, despite significant reductions in acid deposition. Both the differences in pH between base and storm-flow (∆pH) and the number of pH events≤5.5 were higher in forested streams. Dissolved organic carbon and inorganic aluminium concentrations were also higher in forested catchments. The primary driver of acidity was strong organic anions, which generally increased with increasing forest cover. Base-cation dilution was also prominent in west and southern regions, while surprisingly chlorine anion acidity from sea-salts had little or no influence on stream acidity. The contributions of excess non-marine sulphate (xSO(4)) and nitrate (NO(3)) to storm-water were low, with no observed increases in xSO(4) with increasing forest cover, although contributions of NO(3) were higher in forested catchments in the east. The results suggest that episodic acidification in Ireland is primarily driven by organic acids. However in peat dominant catchments, plantation forest, climate change and/or reductions in xSO(4) appear to also be having an effect on stream pH from increased DOC, with some forested streams previously unaffected by deposition now showing low pH (<5.5) during storm-flow. As quantified from this study, observed changes in stream acidification in Ireland may provide a better understanding of future chemical responses to declining acid deposition and climate change elsewhere.
