Effect of acid precipitation on the mobilisation and leaching of heavy metals from contaminated soils

A leaching experiment with reconstructed soil columns was conducted in order to assess the effect of irrigation with water at different pH values on three soils affected by different sources of heavy metals (mining works, industry and vehicle emissions). The leachates obtained were characterised by their pH, conductivity, alkalinity, oxidizable matter and heavy metal concentrations. The relationship among these parameters was studied by factor analysis. Metal concentrations were low and decreased quickly in successive leachates. Therefore, not very hazardous leachates are expected from these soils, unless continuous metal input causes greater concentrations in the leachates produced in the field compared with the laboratory experiment. No significant differences were observed in the leachates produced by the different treatments. The experiment caused a drop in the pH of one of the soils and a decrease in the base saturation and an increase in the contents of CaCl2-extractable Cd, Mn and Zn in the three soils. These changes observed in the soils were not related with the pH of the irrigation water, but must be linked to an internal acidity source in the soils.