POSSMs (ParsimOniouS Speciation of Metals in soils) is a simple model of the equilbrium speciation of nickel, copper, zinc, cadmium and lead in soils. In contrast to mechanistic speciation models, which predict concentrations and activities of multiple metal species, POSSMs predict concentrations only of the free metal ion, the metal complexed in the soil water, and the metal adsorbed to the soil solids. POSSMs also predicts the partition coefficient (Kd) of metal between adsorbed forms and the metal dissolved in the soil water.


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POSSMs is described in detail in an associated publication (Lofts, S., POSSMs: A parsimonious speciation model for metals in soils. Environmental Chemistry, accepted). The model is available at no cost, under the terms of the Open Government Licence, by emailing LAPollutionSupport@ceh.ac.uk.