ENVIRONMENTAL RISK ASSOCIATED WITH HEAVY METAL POLLUTION IN SOILS BASED ON GEOCHEMICAL FRACTIONATION

In order to estimate the environmental risk associated with heavy metal pollution in the Ervedosa mine area (North Portugal), the soil and tailings samples were classified according to a risk assessment code (RAC) based on the strength of the bond between metals and the different geochemical fractions in soils and the ability of the metals to be released and enter into the food chain. The RAC, expressed as a percentage, is defined by taking the ratio of the water soluble (FI) and exchangeable (FII) fractions to the total concentration of elements in the soil. According to the used RAC, there is no risk when FI+FII fractions represent lower than 1% of the total concentrations, low risk for 1-10% RAC, medium risk for 11-30% RAC, a high risk for 31-50% RAC, and a very high risk for higher percentages of RAC. In general, tailings samples showed a higher risk than the soil samples probably due to the deficient physico-chemical properties of tailings (i.e., very low clay and organic matter contents, sandy texture) that influence processes of interaction with heavy metals. The soil and tailings samples collected from the area showed high total concentrations of As, Sn, U, W, Bi and Cu. However, the risk assessment based on those elements suggested that potential dangerous situation in the area can be derived mainly from U pollution because most of the analyzed samples suggest medium to high risk based on U concentrations. The soil samples suggest low risk for As, due to a consequence of the strong adsorption of As on the secondary ferric phases at the acidic pH of the samples.