Solubility of major cations and Cu, Zn and Cd in soil extracts of some contaminated agricultural soils near a zinc smelter in Norway: modelling with a multisurface extension of WHAM

Mechanistic modelling offers a means of simulating the speciation and solubility of trace metals in soils. The WHAM/Model VI model has previously been used to simulate pH buffering and Al solubility in acid soils, and metal partitioning in highly organic soils, but has not previously been applied to agricultural soils. Here we have extended the WHAM/Model VI framework to include surface complexation to oxides and cation exchange, and applied it to batch titrations of limed agricultural soils contaminated by emissions of Cd and Zn from a metal smelter. In contrast to previous studies on forest soils, model predictions were most sensitive to the size of the geochemically ‘active’ soil Ca pool. Following optimisation of this pool the model reproduced trends in pH and major cations well. Blind predictions of soil metal (Cu, Zn, and Cd), using estimates of the active soil metal obtained by extraction with 0.22 M HNO3, were mostly very reasonable. Where predicted metal solubility was biased the model could be fitted to the data by optimising the size of the active metal pool. In some cases the optimised metal pool was unrealistically large, indicating a possible deficiency in the way the model considers binding activity and competition. Organic matter was the dominant binding phase in these soils. These results support the contention that speciation modelling has great promise in providing a holistic description of ionic chemistry in soils for both major and trace elements.