Trends in the hydrochemistry of acid-sensitive surface waters in the UK 1988–2008

AbstractWe conducted complementary linear and non-linear statistical modelling of the first 20 years of hydrochemical data from the 22 lakes and streams of the UK Acid Waters Monitoring Network (AWMN) in order to assess temporal patterns and rates of change in indicators of the key drivers of surface water acidification and acidity status. Over the period 1988–2008, concentrations of non-marine sulphate (i.e. of anthropogenic origin) fell in line with reductions in non-marine sulphur deposition, the principal driver of acidification. Most of the decline was confined to the latter half of the 1990s. Whilst these reductions were substantial, concentrations in recent samples from the most contaminated sites remained several times higher than those in the most remote, low-deposition regions. Nitrate (NO3−) concentrations also declined slightly at several sites in northern England and Wales, possibly reflecting a recent regional reduction in N deposition. Further north, NO3− concentrations increased slightly in some Scottish sites, reflecting a continued dominance of climatic variation on nitrogen cycling in this region. A combination of unusually high rainfall and seasalt inputs in the early years, gradual long-term reductions in hydrochloric acid deposition, and later and more substantial reductions in sulphur deposition, appeared to account for relatively linear increases in an alkalinity-based estimate of Acid Neutralising Capacity ([AB-ANC]) with time. However, variation between sites in the size of the long-term trend in [AB-ANC] was most clearly linked to the size of the long-term reduction in sulphate concentration.The chemical characteristics of changes in [AB-ANC] across the network were dependent on rates of change in acid deposition, the extent to which sites have previously acidified, and average dissolved organic carbon concentration ([DOC]). In the most acidified waters, the response in acidity to reductions in acid deposition was dominated initially by large reductions in inorganic aluminium concentrations ([Alinorg]. Responses in pH have become clearer at these sites as [Alinorg] has begun to stabilise. Over the wider network, pH has been rising more rapidly in sites with lower concentrations of organic acids. A substantial proportion of the deposition-driven increase in [AB-ANC] at several sites is accounted for by increases in [DOC]. For the non-acidified but acid-sensitive waters in the far north and west, changes in [DOC] represent the only clear response to the small changes in sulphur deposition.In the more acidified sites, a reduction in the size of occasional extreme concentrations of Alinorg, that occur after periods of high rainfall or seasalt deposition events, has been more rapid than the declines in average levels, implying that at most sites the persistence of these events should not be seen as an additional barrier to ecological recovery. In a comparison of geographically paired sites with forested and moorland catchments, consistently higher levels of inorganic aluminium concentration and lower AB-ANC provided clear evidence that the former group had acidified to a greater extent. There were few significant differences in the rate at which non-marine sulphate concentration declined or AB-ANC increased and the forested sites therefore remain in a more acidified condition