Levels of heavy metals (Co, Cu, Cr, Ni, Ph, and Zn) in agricultural soils of northwest Spain

Soil oxides, pseudototal (aqua regia extracted) and EDTA extractable conten ts of six potentially toxic metals, namely cobalt (Co), chromium (Cr), copp er (Cu), lead (Pb), nickel (Ni) and zinc (Zn) were determined in the topsoi l of domestic gardens and agricultural plots in Northwest Spain. The sample d 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 a nd 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 soluti on. Pseudototal and EDTA extractable heavy metal contents varied greatly de pending on the nature of their parent materials. Soil developed over ultram afic rocks have Cr and Ni contents that exceed current legislative safety l imits (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. I n soils close to urban centres, there was evidence of Pb accumulation due t o atmospheric deposition. The maximum EDTA extractable concentrations were 2.85 mg kg-l for Co, 14.7 mg kg-l for Cu, 96.9 mg kg-l for Ni, 23.2 mg kg-l for Pb and 23.5 mg kg-l for Zn. The soil contents in EDTA extractable heav y metals were closely correlated with the corresponding pseudototal content s.