Levels of Heavy metals (Co, Cu, Cr, Ni, Pb, and Zn) in agricultural soils of Northwest Spain

Abstract

Soil oxides, pseudototal (aqua regia extracted) and EDTA extractable contents of six potentially toxic metals, namely cobalt (Co), chromium (Cr), copper (Cu), lead (Pb), nickel (Ni) and zinc (Zn) were determined in the topsoil of domestic gardens and agricultural plots in Northwest Spain. The sampled soils were developed over a wide distinct range of geological materials, ranging from felsic to mafic materials, i.e. granite, schist, limestone and mafic and ultramafic rocks. Soil texture varied between sandy‐loam, loam and silty‐loam. Soil oxides were characterised semi‐quantitatively by X‐ray fluorescence, pseudototal contents were determined after digestion in aqua regia, and extractable contents were estimated by extraction in EDTA solution. Pseudototal and EDTA extractable heavy metal contents varied greatly depending on the nature of their parent materials. Soil developed over ultramafic rocks have Cr and Ni contents that exceed current legislative safety limits (NMHPPE, 1991), as a result of high natural concentrations of parent material. There was evidence of anthropogenic contamination by Cu and Zn in some soils, due to the traditional use of animal manures as fertilizers. In soils close to urban centres, there was evidence of Pb accumulation due to atmospheric deposition. The maximum EDTA extractable concentrations were 2.85 mg kg‐1 for Co, 14.7 mg kg‐1 for Cu, 96.9 mg kg‐1 for Ni, 23.2 mg kg‐1 for Pb and 23.5 mg kg‐1 for Zn. The soil contents in EDTA extractable heavy metals were closely correlated with the corresponding pseudototal contents.