Definition of regional background values of trace element in soils of Castilla-La Mancha region, Spain

The present study defines geochemical threshold values (GTV) and background values (GBV), in addition to analysing of the spatial distribution of major and trace elements in uncontaminated soils in the Castilla-La Mancha (CLM) region (Spain). A total of 869 samples were analysed from the entire region by X-Ray Fluorescence to determine the presence of Ti, Cr, Mn, Fe, Ni, Cu, Zn, As, Rb, Sr, Zr, and Pb. A principal components analysis (PCA) was performed in conjunction with the observation of the regional lithology. This process resulted in the delineation of seven geochemical zones. The calculation of GTVs was undertaken using different approaches for the complete dataset and by geochemical zones (percentile-based techniques (p95 and p98); Tukey's inner fences (TIF); median+2xmedian absolute deviation (mMAD); and cumulative probability plot (CP)). The results obtained from the CP plot are considered the most robust for defining the GTVs of CLM, because of the capacity to observe subpopulations and conduct a lithological comparison of potential geochemical anomalies. In order to calculate the GBVs, outliers above the GTVs were eliminated to create geochemical interpolated maps for CLM. These maps and values were then used to make comparisons with regional, national and European references levels. The results obtained indicate significant enrichment of Ti, Mn, Fe, Zn, Zr and Pb, suggesting an association with metallogenetic districts. Elevated Rb values were identified within the granitic units, whereas Cr and Ni are strongly associated with the Campo de Calatrava volcanic field. In contrast, the Cenozoic gypsum-rich evaporitic deposits are characterised by Sr enrichment. This research enhances the existing GTV and GBV forms for CLM, providing a robust foundation for future studies in the domain of soil investigation, environmental management and land use.