Forms of cadmium, lead, and zinc in polluted mining soils and uptake by plants (Soria province, Spain)

Soils and plants (grass and barley) were sampled in and around a lead (Pb)-zinc (Zn) mine in the province of Soria (Spain), from sites including the mine dumps and nearby cultivated areas along two transects: N-S and E-W. Total metal contents in soils ranged widely from background levels to gross pollution, which decreased with distance from the mine source in different dispersion patterns along the two transects, mainly due to topography and geological structure. The distribution of metals in a soil profile revealed a relative accumulation of the total contents in the surface layer, which apparently did not depend on the other major chemical properties of the soil. The distribution of cadmium (Cd) among the different forms or speciation revealed that the greatest amounts were in exchangeable form. By contrast the highest amounts of Pb and Zn were found in the residual and oxide fractions. This different pattern may be a consequence of a lower input of Cd in soils than Zn and Pb. The Cd contents in grass (Poa pratensis L.) were elevated, with highest relative accumulation found in the roots. The Cd contents in barley were high in relation to those in cereals from different countries, however, the degree of bioaccumulation was relatively low. Also, in barley, Cd contents decreased with distance from the mine dumps and were significantly correlated with total Cd in soils, apparently because the sum of exchangeable + carbonate forms (probably the amount bioavailable) is close to 50% of total Cd. The ratio of metal uptake to their amounts in the soils was Cd>Zn>Pb, in agreement with the most available forms of these elements in soil.