Fractal Spatial Heterogeneity of Trace Element Abundance in Pristine Surface Soils: Implications for Pollution Assessment and Source Apportionment

The spatial heterogeneity of trace element geochemical backgrounds in surface soils poses a significant challenge for accuratly assessing the anthropogenic contributions to soil pollution. To evaluate the natural variation in trace element abundance, the composition and spatial distribution of 50 trace elements across 153 sites in the Hoh Xil Nature Reserve, a pristine region in western China, were determined. While the contents of the rare earth elements and heavy metal(loid)s closely mirror the composition of continental crust, moderate to extremely high spatial heterogeneity was found for these elements. The coefficients of variation (CV) indicate that 46 of the 50 trace elements display moderate to high spatial heterogeneity (10% < CV < 100%), while the remaining ones show extreme spatial heterogeneity (CV > 100%). Notably, the chalcophile elements exhibit particularly high spatial heterogeneity, and the spatial heterogeneity in trace element abundance increased with the geospatial scale of assessment. Normalization of the contents of trace elements with those of rock-forming element, Ti, under different geospatial scales reveals that the intrinsic crustal heterogenity in elemental abundance and the preferential enrichment or depletion of elements due to regional geological processes are the primary factors shaping the spatial viariation in their distribution in the pristine surface soils. The significant spatial heterogeneity in the distribution of trace elements and its fractal characteristics have important implications for evaluating the pollution of surface soils by trace elements and apportioning the contribution of anthropgenic activities based on subtraction of the regional background values.