Multiscale analysis of heavy metal contents in soils: Spatial variability in the Duero river basin (Spain)

Abstract

Source identification of heavy metals in soil is not straightforward since several inputs of either an anthropogenic or natural origin contribute to their total content. Here we explore the spatial variation and covariation of seven heavy metals (Cd, Cr, Ni, Pb, Zn, Cu and Hg) in the agricultural soils of the Duero river basin (one of the largest in Spain) where both anthropogenic activities (mainly agriculture and industry) and natural factors may be responsible for their total concentration. Factorial kriging and principal components analysis were used on a data set that comprised 721 soil samples. We found that the concentrations of heavy metals in the analysed samples do not exceed the limits set by Spanish legislation excepting mercury that presents high values in a limited number of samples (maximum 1041μg/kg). The linear model of coregionalization–the basic model for factorial kriging analysis–was composed of two structures (representing two scales of variation) with ranges of 20km (local scale) and 130km (regional scale). Six of seven elements (Cd, Cr, Ni, Pb, Zn and Cu) were found to be strongly correlated regardless of the spatial scale considered. In contrast, correlations of Hg with other elements were small at the local spatial scale but augmented substantially at the regional scale. We conclude that agricultural practices in the Duero basin have not altered yet the natural content for Cd, Cr, Ni, Pb, Zn and Cu. On the other hand, Hg inputs from human origin, most probably related to airborne emission and deposition from industrial plants, are observable at the local spatial scale. Finally, no human-induced correlations among heavy metals were detected at the regional spatial scale. Based on the results of this study and in accordance with the results obtained in the nearby Ebro river basin (Rodríguez et al., 2008) we conclude that anthropogenic heavy metals in soil are visible only at local spatial scales. In contrast, natural factors maximize their influence on the distribution of heavy metals when considering larger spatial scales.