The labile fractions of metals and arsenic in mining-impacted soils are explained by soil properties and metal source characteristics.

Isotopically exchangeable metals in soil, also termed labile metals, are reversibly bound to soil surface and are a better index of the environmental risk of the metals than are their total concentrations. In this study, labile fractions of potentially toxic elements were surveyed in metal mining-impacted soils of Mexico to test the relative importance of soil properties (pH, effective cation exchange capacity, organic matter, etc.) or attributes of the mines (ore type and lithology, metal mineralogy, etc.) on the fractions of labile elements. Mining waste-impacted soils, corresponding uncontaminated soils and mining waste were collected around 11 metal mines in Mexico presenting contrasting ore types. Pseudo-total concentrations and labile fractions of Cd, Ni, Zn, Pb, Cu, and As were determined by aqua regia digestion and isotope dilution, respectively. Pseudo-total concentrations of these elements ranked: waste > contaminated soil > uncontaminated soils, and Zn and As dominated the concentrations of toxic elements. The labile fractions (% of total) in the soils ranked, with median values in brackets, Pb (22) > Cd (18) > Cu(15) > Ni∼Zn(13) > As(9). The labile fractions of waste samples were slightly higher than those of soil samples suggesting either a high weathering of mining wastes or the stabilization of heavy metals by soil. Stepwise multiple regression showed that soil properties rather than source attributes primarily explained the %E of most elements, except for Zn and As for which the ore lithology was the dominant factor. This study showed that earlier generic models explain metal lability adequately in mining waste-impacted soils.