Effects of decreasing acid deposition and climate change on acid extremes in an upland stream

Abstract

Abstract. This study assesses the major chemical processes leading to acid extremes in a small, moorland stream in midWales, UK, which has been monitored since 1979. Results suggest that base cation (mainly calcium) dilution, the "seasalt effect", and elevated nitrate pulses, are the major causes of seasonal/episodic minima in acid neutralising capacity (ANC), and that the relative importance of these drivers has remained approximately constant during 25 years of decreasing acid deposition and associated long-term chemical recovery. Many of the chemical variations causing short-term reductions in stream acidity, particularly base cation dilution and organic acid increases, are closely related to changes in water-flowpath and therefore to stream discharge. Changes in the observed pH-discharge relationship over time indicate that high-flow pH has increased more rapidly than meanflow pH, and therefore that episodes have decreased in magnitude since 1980. However a two-box application of the dynamic model MAGIC, whilst reproducing this trend, suggests that it will not persist in the long term, with mean ANC continuing to increase until 2100, but the ANC of the upper soil (the source of relatively acid water during high-flow episodes) stabilising close to zero beyond 2030. With climate change predicted to lead to an increase in maximum flows in the latter half of the century, high-flow related acid episodes may actually become more rather than less severe in the long term, although the model suggests that this effect may be small. Two other predicted climatic changes could also detrimentally impact on acid episodes: increased severity of winter "sea-salt" episodes due to higher wind speeds during winter storms; and larger sulphate pulses due to oxidation of reduced sulphur held in organic soils, during more extreme summer droughts. At the Gwy, the near-coastal location and relatively small extent of peat soils suggest that sea-salt episodes may have the greatest influence.

Stream geochemical response to reductions in acid deposition in headwater streams: Chronic versus episodic acidification recovery

“…As acid anion concentrations decrease in streamwater, previously paired base cations would also be projected to decrease, with the relative rate indicating if base cations have been depleted (greater rate of decrease), are stable (similar rate of decrease), or regenerating (lower rate of decrease or increasing). For baseflow conditions, decreasing SAA trends (Figure S4c and Table S3) were counterbalanced by SBC trends (Figure b and Table S3) at sites with more moderate base cation concentrations (Paine Run and Staunton River) similar to findings in the central Appalachian river basin (Kline et al, ) and a small mooreland stream in mid‐Wales, United Kingdom (Evans et al, ). In contrast, Piney River had an increasing base cation trend during low‐flow periods, demonstrating that base cation regeneration has occurred in the deeper soils.…”

Modelling the impacts of climate change on upland catchments in southwest Scotland using MIKE SHE and the UKCP09 probabilistic projections

“…These flow paths are associated with water that is relatively acidic, base-cation dilute and enriched in organic acids. The impact of acidic episodes is more pronounced in catchments, such as those modelled in this study, where rapid changes in discharge result in the shift between highly acidic storm flow and buffered baseflow (Evans et al 2008). Surface water acidity may also be impacted by the lower summer flows which would lead to increases in solute concentrations (Evans 2005;Evans et al 2005).…”

“…Chiverrell et al 2007). It has been hypothesised that the recovery of acidified catchments following reductions in sulphur and nitrogen emissions from fossil fuel combustion may be delayed as a result changing hydrological pathways brought about by higher winter rainfall (Evans et al 2008) whilst it is possible that the increases in dissolved organic carbon (DOC) resulting in the discoloration of water from upland catchments is partly linked to climate change (e.g. Evans et al 2006).…”

Restoration and recovery from acidification in upland Welsh streams over 25 years

“…While climate‐change effects on precipitation are still difficult to discern from background variability (Wilby 2006), future projections for Western European precipitation are for an overall increase in winter by 10–30% and a decline in summer by 10–20% over the next 20–40 years (United Kingdom Climate Impacts Programme http://www.ukcip.org.uk/). In turn, projections from hydrochemical models and available data suggest strong climatic interactions with recovery from acidification in Wales and other locations (Evans 2005; Evans et al . 2008).…”