From determinism to fractal processing, structural uncertainty, and the need for continued long-term monitoring of the environment: the case of acidification

✍ Scribed by Colin Neal

• Publisher: John Wiley and Sons
• Year: 2002
• Tongue: English
• Weight: 77 KB
• Volume: 16
• Category: Article
• ISSN: 0885-6087
• DOI: 10.1002/hyp.5055

✦ Synopsis

Over the last 30 or more years a dominant environmental issue has been the impact of acidic deposition on acidic and acid-sensitive systems. The view of acidification is broadly expressed in terms of the deposition of sulphur and nitrogen oxides from industry, nitrogen oxides from car exhausts, and ammonium from industry and agriculture, which acidify sensitive soils, leading to acidic runoff and to ecological damage such as the loss of fish populations. This view is broadcast in the press and taught in schools and universities. Few, including myself, would not subscribe to it, and the view has hardly changed much over the past 15 years. Intuitive and mathematical models were developed, particularly in the 1980s, to explain the acidification process. From both a conceptual and a modelling standpoint it was assumed that nature operates in a relatively simple way that seems eminently clear and sensible. Thus, acid deposition falls on a landscape to acidify the soils and the streams. In the process of transfer of hydrogen ions and other acidifying agents from the atmosphere to the stream, chemical equilibrium or kinetic reactions are invoked whereby the acidity is modified in the soil by a variety of reactions, including cation exchange, sulphate adsorption, and weathering. The primary measure of acidity used is the acid neutralization capacity (ANC), the difference between the sum of the strong base cations and the strong acid anions. This corresponds to the difference between the sum of the weak acid anions (the acidity buffers, mainly bicarbonate and organic anions) and the weak base cations (hydrogen ions and aluminium). ANC values of zero or less correspond to acidic waters and situations where ecological damage can occur.

The understanding and modelling of the acidification process has not been smooth, and the demise of one of the most influential dynamic models of acidification developed in the early 1980s, the Birkenes model, is of particular relevance, for it implied something about the processes operative and the modelling process itself (Christophersen and Neal, 1990), the implications of which still have not been resolved (Neal, 1997). The Birkenes model considers catchments in terms of two reactive homogeneous zones where rain passes on its way to the stream, a soil zone where cation exchange and sorption reactions occur together with solubilization of aluminium, and a groundwater zone where weathering reactions partially neutralize