Guadiamar toxic flood: Factors that govern heavy metal distribution in soils.

Abstract

The remaining soil contamination after the removal of the sludge and the affected topsoil was studied in the northern part of the Guadiamar river valley affected by the Aznalcóllar toxic spill. The easily soluble (NH4NO3-extraction) and the oxidisable fraction (H2O2/HNO3-extraction) were analysed in samples of two calcareous and two non-calcareous soils. Correlations between soil properties and heavy metal concentrations were tested with special respect to depth distribution. The spatial distributions of the easily soluble and the oxidisable fraction were highly skewed in both examined depths (0–20 and 20–40 cm). Easily soluble heavy metal concentrations of a high percentage of samples exceeded thresholds that have been given in the German soil protection law particularly in the non-calcareous soils. Within the soil layers of the non-calcareous soils, the pH seems to control the easily soluble concentrations. However no relation between the pH and depth distribution of heavy metals within the profiles could be found. Physical properties, which determine partly penetration depth of the sludge and soil mixing caused by the removal may be the more important factors. Accordingly, high heavy metal concentrations are to be expected even in the subsoils of clayey sites. As heavy metal concentrations in the oxidisable fraction are still high, further oxidation accompanied by pH lowering has to be expected. Thus, the studied soils show a significant risk potential of available heavy metals even after the removal of the sludge.