Past acidification and recovery of surface waters, soils and ecology in the United Kingdom: Prospects for the future under current deposition and land use protocols

The dynamic hydrogeochemical model MAGIC has been applied extensively throughout Europe and North America and has proven to be a valuable tool for predicting the chemical response of catchment soils and surface waters to future changes in atmospheric pollution and land use. Here we present an application of MAGIC to 22 sites in the UK Acid Waters Monitoring Network (UKAWMN) that incorporated uncertainty in model calibration (using 20 years of surface water observations) to evaluate past acidification and prospects for future recovery in surface waters, soils and key ecological indicators. Simulated ANC in 1860 indicated that prior to industrialisation, all UKAWMN surface waters were above the critical acid neutralising capacity (ANC) limit of 20 mu eq l(-1), with the exception of a naturally acid site, underlain by granite with large expanses of bare rock, scree and eroded peat in Northern Ireland (Blue Lough); this site had a baseline ANC value <20 mu eq l(-1). The significant increase in acidic deposition from 1860 to its peak during the 1970s resulted in surface water acidification at 14 of the study sites (ANC <20 mu eg l(-1)). The rate and magnitude of chemical recovery in soil is highly variable among sites owing to differences in factors such as soil mineral weathering and base saturation. Despite the significant reductions in sulphur and to a lesser extent nitrogen deposition since 1970, the simulated soil base saturation at all study sites either continued to decline or remained stable until the late 1980s, with marginal recovery detected at some sites in the past decade. Predictions were based on an emission reduction scenario (Gothenburg protocol) to 2020, and land use scenarios to 2050 based on approved Forestry Commission plans at the five afforested sites. Model predictions indicated that, in general, surface water acid status will continue to improve during the next decade and beyond under the Gothenburg protocol. Surface water recovery was primarily attributed to the significant decline in sulphate concentrations from a present day mean of 58.5 mu eq l(-1)-43.8 mu eq l(-1) in 2020. The contribution of nitrate (NO3-) leaching to the total acid status of surface water was small and predicted to decrease from 10.9 mu eg l(-1) in 2007 to 9.6 mu eq l(-1) in 2020. By 2100, NO3- concentrations increased slightly to 11.8 mu eg l(-1) having a small confounding influence on the rate of chemical recovery at most sites in the network. The future response of soil base saturation to reductions in acid deposition and land use change was mixed and the difference in behaviour between sites is unclear. Whilst recent studies (Malcolm et al., 2014a,b; Monteith et al., 2014) have demonstrated that forest practices have contributed to the acidification of surface waters, in this study, there was no evidence from the model predictions that forested sites will follow a different recovery trajectory to moorland sites. Planned reductions in coniferous forest cover amounting to approximately 13% across the five afforested sites resulted in a slight increase in ANC and pH. Ecological predictions highlighted the sensitivity of three indicator species (Baetis Rhodani, Achnanthes minutissima and salmonids) to changes in the acid status of surface water in the past and into the future. There was clear evidence of biological recovery by 2015 with some sites returning close to their pre-industrial biological status (Alit a'Mharcaidh) while at others (including Loch Grannoch and River Etherow) predicted recovery was incomplete for one or more of the studied indicator species. Crown Copyright (C) 2013 Published by Elsevier Ltd. All rights reserved.