Zn in the soil solution of unpolluted and polluted soils as affected by soil characteristics

Current legislative frameworks for soil pollution focus predominantly on total metal content. However, environmental risks posed by heavy metals are not only function of their overall presence in the soil, but also of their chemical speciation. Soil solution levels of Zn are indicative of its availability to plants and soil (micro)biota. In addition, levels present in the soil solution can tell us something of the potential risks for leaching and subsequently elevating ground water levels. In this research, a total of 29 polluted and unpolluted soils were characterised and assessed for the presence of Zn in the soil solution using Rhizon soil moisture samplers. Interactions between the soil properties and observed solubility of Zn was described and put into empirical multivariate formulations. All obtained equations without exception contained pH as a key factor in determining Zn solubility. Finally, soil solution speciation of Zn was assessed using 2 independent software packages (Visual Minteq 2.23 and WHAM VI), based on pH and the presence of other metals, exchangeable bases, anions, and dissolved organic matter. Both models were largely in disagreement in the fractionation between organically bound and inorganic Zn in the solution. Further investigation into the parametric background of both models will therefore be required to ascertain the cause and nature of the observed discrepancy.