Hydrological pathways and acid episodes in the Coalburn catchment
Hydrological pathways and episodic stream acidification were investigated by monitoring water quality in the Coalbum catchment during the later stage of canopy closure. Coalburn is a long-running experimental site, almost completely covered by a commercial coniferous plantation (part of the Kielder Forest in northern England). Regular sampling, event sampling and continuous monitoring were used to establish an extensive water quality data-base between 1993 and 1997. Cloud mist and precipitation at Coalburn are slightly acidic and the catchment can receive moderate levels of dry and wet acid deposition when the winds have tracked from an easterly direction. Coalburn streamwater chemistry displays well-buffered base-flows and acid episodes during high flows. Duration-frequency-magnitude analysis of the Coalburn acid episodes reveals they were less frequent than expected but had a longer duration. The acid events exceeded environment quality standards for freshwaters: pH and aluminium concentrations suggest a toxic impact on freshwater biota. However, the elevated calcium concentrations and presence of humic substances reduce the biological impact. The chemical signatures and variations in solutes were identified and used to postulate the causal mechanisms of the hydrochemical response and to produce the conceptual Coalburn runoff model. Mixing modelling was then applied to introduce a robust analysis into the process and produce the simplified Coalburn Model of acidification. Broadly, the Coalburn hydrochemical response can be explained by pre-event water sources and 'normal' catchment processes. However, during extreme events catchment conditioning and event-water can also influence the response. In improving knowledge of the likely water quality effects of coniferous plantation afforestation, the Coalburn study has contributed to calibrating the temporal and spatial variability of headwater acidification (and hence risks). This allows policy makers and environment managers to make informed decisions about land use/management and to apply the precautionary principle; the research findings were used to suggest a basic policy framework and protocols for catchment assessments to manage acidification issues through the cropping cycle within a headwater catchment.