Influence of geological setting on geochemical baselines of trace elements in soils. Application to soils of South–West Spain

A collection of 235 samples were taken from 115 sites (representing a density of 1 sampling site ca. 130 km) on rural soils derived from the major rock types in the southern Iberian Massif. The geochemical baselines of selected trace elements (As, Co, Cr, Cu, Ni, Pb and Zn) were determined on the b2 mm soil fraction. The sampling sites were not directly influenced by external pollution. Soil geochemical baseline and threshold values were calculated for each element in two geologically different zones: the Ossa-Morena Zone (OMZ) and the South-Portuguese Zone (SPZ). All the trace elements showed significantly high median concentrations when compared to reference values for soils of Andalusia, the European Union and the world indicative of regional scale enrichment of topsoils, particularly with respect to As (24.7 mg kg), Cu (32 mg kg) and Pb (37.9 mg kg) in the SPZ, and Zn (78.5 mg kg) in the OMZ. The distribution patterns of element concentrations are primarily influenced by the lithology and geochemistry nature of bedrock and the occurrence of metallogenic belts in the survey area, notably the Iberian Pyrite Belt. In the SPZ the highest median values of As (34.4 mg kg ), Pb (56 mg kg) and Cu (57.4 mg kg ) were found in soils derived from acid igneous rocks of the Iberian Pyrite Belt. By contrast, the soils developed on carbonate rocks of the OMZ recorded the largest median values for As (27.8 mg kg), Pb (44 mg kg) and Zn (83.1 mg kg), probably regarded to base-metal (SEDEX) deposits. These results indicate that regional geology is an important determinant of soil geochemical baselines for soil pollution assessment. © 2008 Elsevier B.V. All rights reserved.