Assessing the environmental availability of heavy metals in geogenically contaminated soils of the Sierra de Aracena Natural Park (SW Spain). Is there a health risk?

Soil developed on mineralised bedrock areas of the Sierra de Aracena Natural Park (SW Spain) is geochemically characterised by naturally high levels of heavy metals and metalloids (notably Zn, Pb, Ag and Cd, but also As, Sb, Cu and Tl). To assess environmental availability, geochemical speciation and potential health risk of such geogenic trace elements, 24 randomly selected soil samples were subjected to one-step extraction procedures (0.01M CaCl2 and 0.05M EDTA), aqueous speciation modelling, and site-specific risk analysis. Metal fraction available for plant uptake or leaching to groundwater was found to be negligible (<1%) due to the low activity of dissolved and exchangeable ions in soil solution, as predicted from the CaCl2 extracts. Based on modelling calculations, free metal ions, primarily Cd(2+) and Zn(2+), were the dominant species in solution over the soil pH range of 5.8 to 7.8. For most metals, the EDTA-extractable fraction generally accounted for <5% suggesting that a limited reservoir of trace elements, mainly bound to Fe oxy-hydroxides, could be potentially available. The results of the health risk assessment for ingestion exposure to groundwater affected by soil leaching revealed that the hazard quotients of heavy metals are within the acceptable risk level. The cumulative hazard index (HI=0.55) fell below the regulatory threshold value of 1.0, even in the worst-case scenario being evaluated, leading to the conclusion that no toxic effects are expected to humans under the conditions and assumptions of the assessment.