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 concentrations in Swiss forest soils and to critically assess the measured values with respect to anthropogenic input vs. lithogenic background. Twenty-three sites were selected which represent a broad range of natural forest sites, bedrock material and soil types of Switzerland. At each site, samples were collected from all genetic soil horizons down to a C or B/C horizon. Total concentrations of As, Cr, Cu, Ni, Pb, and Zn in all samples were determined by X-ray fluorescence spectrometry. There were distinct differences in the geological background values estimated from the concentrations measured in the samples from the lowest soil horizon. Background concentrations for Cr and Ni were lowest in granite and gneiss, whereas Pb and Zn were highest in limestone and marl. Enrichment or depletion of the trace elements was assessed using Zr as reference element. Within the same profile, the six trace elements showed completely different enrichment/depletion patterns with depth. 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 anthropogenic 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 concentrations (MAC) for topsoil samples. The enrichment factors calculated as described here consider the effects of geological variation on metal abundances whereas the MAC does not. In order to obtain an estimate of soil solution concentrations, water extracts of the samples collected from a subset of 10 soil profiles were analyzed for the same trace elements. Solubility of all elements generally decreased with soil depth. An exception was Cr, Cu, and Ni solubility in the humus layer, which was lower than in the underlying mineral horizon. For all elements, solubility was higher for the collective of soil samples depleted in this element when compared to the samples, in which the element was enriched.