'Acidic episodes' in surface waters in Europe

Abstract

Pronounced and short-term changes in freshwater chemistry ('acidic episodes') can have a significant biological impact. More attention has been paid to chronic acidification, and there has been no previous attempt to produce a regional summary of occurrence of episodes in Europe, to describe their chemical characteristics, or to synthesise what is known of their controlling processes. These are the objectives of this review. The basic characteristics of episodes are explained, along with the problems of establishing an adequate description. Because of the relative paucity of other information on water composition, the review is based largely on measurements of short-term pH depressions, although other data are considered where available. Most of the studies reported here were conducted in Norway, Sweden, Scotland and Wales. The differing physical characteristics of the various study catchments are assessed; these influence the hydrochemical responses to hydrological events (snowmelt or rainfall). Antecedent conditions are also important. Nevertheless, it is possible to arrive at some generalising conclusions about episodes. Snowmelt appears to instigate the most pronounced episodes, particularly in lakes. However, rainfall-induced episodes are generally much more frequent. Direct links between the chemistry of the snowmelt or rainfall which induced the episode and surface water chemistry are relatively rare, except in the case of a large input of sea-salt. Sulphate is the strong acid anion most strongly related to pH depression during episodes, although nitrate is important in some locations at snowmelt. Organic acids can play an important role in some peaty catchments. Dilution of base cations during episodes was also observed in most of the studies reviewed. Some depression of pH in surface waters seems ubiquitous during hydrological events, but in areas which have experienced large depositions of pollution, long-term acidification has conditioned catchments for the episodic release of acid. Changing flowpaths through hydrological events are of overwhelming importance in controlling the chemical character of episodes. © 1992.