Abstract
Abstract. Long-term simulations of the water composition in acid forest soils require that accurate descriptions of aluminium and base cation chemistry are used. Both weathering rates and soil nutrient availability depend on the concentrations of Al3+, of H+, and of base cations (Ca2+, Mg2+, Na+, and K+) . Assessments of the acidification status and base cation availability will depend on the model being used. Here we review in what ways different dynamic soil chemistry models describe the processes governing aluminium and base cation concentrations in the soil water. Furthermore, scenario simulations with the HD-MINTEQ model are used to illustrate the difference between model approaches. The results show that all investigated models provide the same type of response to changes in input water chemistry. Still, for base cations we show that the differences in the magnitude of the response may be considerable depending on whether a cation-exchange equation (Gaines–Thomas, Gapon) or an organic complexation model is used. The former approach, which is used in many currently used models (e.g. MAGIC, ForSAFE), causes stronger pH buffering over a relatively narrow pH range, as compared to state-of-the-art models relying on more advanced descriptions in which organic complexation is important (CHUM, HD-MINTEQ). As for aluminium, a “fixed” gibbsite constant, as used in MAGIC, SMART/VSD, and ForSAFE, leads to slightly more pH buffering than in the more advanced models that consider both organic complexation and Al(OH)3(s) precipitation, but in this case the effect is small. We conclude that the descriptions of acid–base chemistry and base cation binding in models such as MAGIC, SMART/VSD, and ForSAFE are only likely to work satisfactorily in a narrow pH range. If the pH varies greatly over time, the use of modern organic complexation models is preferred over cation-exchange equations.
Highlights
Acid rain has been an environmental issue of concern ever since Svante Odén presented his famous newspaper article in 1967, in which the acidification of water systems in the Northern Hemisphere was first described (Odén, 1967)
Modern organic complexation models such as WHAM, NICA-Donnan (Kinniburgh et al, 1999), and the Stockholm Humic Model (SHM; Gustafsson, 2001) have been applied to numerous soil systems, and the results showed the correctness of the above conclusions (e.g. Lofts et al, 2001; Weng et al, 2002; Gustafsson and Kleja, 2005)
The results suggest that the difference in model performance is rather small as long as the pH remains within a confined pH range; the most significant differences are seen for proton buffering and for base cation dynamics
Summary
Acid rain has been an environmental issue of concern ever since Svante Odén presented his famous newspaper article in 1967, in which the acidification of water systems in the Northern Hemisphere was first described (Odén, 1967). Most of the atmospheric acid deposition was caused by sulfur emissions from fossil fuel combustion. In Europe and North America successful efforts were made to reduce the emissions, and in 2014 the atmospheric sulfur emissions in western Europe were less than 20 % of what they were around 1970, when the emissions peaked (Engardt et al, 2017). Despite the drastic cuts of emissions, there are still acidified soils and waters for which the critical loads are exceeded at the current atmospheric S deposition level. In 1980, the critical loads were exceeded for 58 % of all Swedish lakes.
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