Critical examination of trace element enrichments and depletions in soils:As, Cr, Cu, Ni, Pb, and Zn in Swiss forest soils

The aim of this study was to obtain an overview of trace element concentrat ions in Swiss forest soils and to critically assess the measured values wit h respect to anthropogenic input vs, lithogenic background. Twenty-three si tes were selected which represent a broad range of natural forest sites, be drock material and soil types of Switzerland. At each site, samples were co llected from all genetic soil horizons down to a C or B/C horizon. Total co ncentrations of As, Cr, Cu, Ni, Pb, and Zn in all samples were determined b y X-ray fluorescence spectrometry. There were distinct differences in the g eological background values estimated from the concentrations measured in t he samples from the lowest soil horizon. Background concentrations for Cr a nd Ni were lowest in granite and gneiss, whereas Pb and Zn were highest in limestone and mart. Enrichment or depletion of the trace elements was asses sed using Zr as reference element. Within the same profile, the six trace e lements showed completely different enrichment/depletion patterns with dept h. The various natural processes and anthropogenic inputs that can lead to these patterns are critically discussed. Based on this critical assessment, pollution of the investigated forest soils was found to be most severe for Pb and Zn and to a somewhat lesser extent for As and Cu, whereas anthropog enic input of Cr and Ni seems to be less important. The data suggest that a critical evaluation of enrichment factors is a better tool to assess soil pollution with trace elements than the use of maximum allowable concentrati ons (MAC) for topsoil samples. The enrichment factors calculated as describ ed here consider the effects of geological variation on metal abundances wh ereas the MAC does not. In order to obtain an estimate of soil solution con centrations, water extracts of the samples collected from a subset of 10 so il profiles were analyzed for the same trace elements. Solubility of all el ements generally decreased with soil depth. An exception was Cr, Cu, and Ni solubility in the humus layer, which was lower than in the underlying mine ral horizon. For all elements, solubility was higher for the collective of soil samples depleted in this element when compared to the samples, in whic h the element was enriched. (C) 2000 Elsevier Science B.V. All rights reser ved.