Local background concentrations of trace elements in soils: a case study in the Grand Duchy of Luxembourg

Background concentration values of trace elements were determined for soils in the southern part of the Grand Duchy of Luxembourg, which are developed on the Bituminous shale (BS), the so-called “Bifrons” shale, and the “Minette” (M) Fe-oolitic sandstone substrates. Sampling sites with minimal anthropogenic influence were selected, and soil profile samples were analysed for major and trace element contents. A clear distinction is seen between the trace element content of the soils developed on the Minette sandstone on the one hand and those developed on shales on the other hand. For the Minette soil samples, most elements are present in concentrations exceeding those reported in literature for soils developed upon sandstones, with, for example, values of up to 278 mg kg⁻¹ As (median value 123 mg kg⁻¹), 287 mg kg⁻¹ Cr (median value 126 mg kg⁻¹), and 95 mg kg⁻¹ Co (median value 33 mg kg⁻¹). In the shale soil samples, concentrations are generally lower except for Cu and Mo. These elements reach maxima of up to 66 mg kg⁻¹ Cu (median value 41 mg kg⁻¹) and 59 mg kg⁻¹ Mo (median value 26 mg kg⁻¹). The results confirm the importance of determining background concentrations locally.Bivariate analysis shows a linear relationship between Fe content and elements such as As, Co, and Ni for the Minette soils. No relationship is found between trace element concentrations and clay or organic carbon (OC) content, but this could be due to the limited variation of these factors among the studied soils. A reconnaissance study with regard to the availability of the trace elements in the Minette soil samples shows that these elements are quite immobile and hence of minimal threat to the ecosystem.