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

摘要

This study assesses the major chemical processes leading to acid extremes ina small, moorland stream in mid-Wales, UK, which has been monitored since1979. Results suggest that base cation (mainly calcium) dilution, the"sea-salt effect", and elevated nitrate pulses, are the major causes ofseasonal/episodic minima in acid neutralising capacity (ANC), and that therelative importance of these drivers has remained approximately constantduring 25 years of decreasing acid deposition and associated long-termchemical recovery. Many of the chemical variations causing short-termreductions in stream acidity, particularly base cation dilution and organicacid increases, are closely related to changes in water-flowpath andtherefore to stream discharge. Changes in the observed pH-dischargerelationship over time indicate that high-flow pH has increased more rapidlythan mean-flow pH, and therefore that episodes have decreased in magnitudesince 1980. However a two-box application of the dynamic model MAGIC, whilstreproducing 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 uppersoil (the source of relatively acid water during high-flow episodes)stabilising close to zero beyond 2030. With climate change predicted to leadto an increase in maximum flows in the latter half of the century, high-flowrelated acid episodes may actually become more rather than less severe inthe long term, although the model suggests that this effect may be small.Two other predicted climatic changes could also detrimentally impact on acidepisodes: increased severity of winter "sea-salt" episodes due to higherwind speeds during winter storms; and larger sulphate pulses due tooxidation of reduced sulphur held in organic soils, during more extremesummer droughts. At the Gwy, the near-coastal location and relatively smallextent of peat soils suggest that sea-salt episodes may have the greatestinfluence.