Spatial distribution of trace elements in the soils of south-western France and identification of natural and anthropogenic sources

International audienceThe contamination of soils by trace elements is a major concern for soil quality. This study is based on the analysis of 356 samples from the RMQS soil monitoring network to establish the spatial distribution and origin of six trace elements (As, Cd, Cu, Cr, Ni, Pb) in soils of south-western region of France (area of 90,293 km2). An exploratory and multivariate statistical analysis, and geostatistics combined with a geographic information system (GIS) were used to identify and characterize any concentration anomalies in trace elements. For all the trace elements studied, the exploratory analysis shows that there are more anomalies in this region than in the rest of the country. Analysis of the semivariograms shows that the six elements are spatially auto correlated. The spatial structure of As highlights anisotropic behaviour with a direction that corresponds to the gold deposit and mining activities of the region. This indicates a dual origin anthropogenic and geogenic for As. The correlation between Cd and inherent features of calcareous soil (pH, CaCO3 and cation exchange capacity) suggest a mainly geogenic origin for this element; Cd origin is confirmed by its spatial distribution associated with the Jurassic limestone bedrock. The correlations between Cr, Ni and clays highlight a geogenic origin for these elements, as weathered parent material rich in clays is also rich in Cr and Ni. The high Cu concentrations are of anthropogenic origin, linked to viticulture and the spreading of Bordeaux mixture as a fungicide. Locally high Pb concentrations are associated with mining activities and automobile emissions in large cities in the region