Chapter 2 Natural trace element input to the soil-sediment-water-plant system: examples of background and contaminated situations in Switzerland, Eastern France and Northern Italy

Abstract

Abstract Studies on environmental pollution in a given region often suffer from the lack of reliable information on the natural level of trace elements. The natural contribution to the trace element contents and fluxes of local and regional ecosystems strongly depends on the local composition of the lithosphere, i.e. the dominating rock type. The dispersion of trace elements in the different environmental compartments is mainly governed by weathering and mass transport. Based on examples from Central Europe, this paper gives an insight into the typical natural input of six major, geochemically defined rock types (“natural background”). It also presents two cases of regional “natural contamination” related to ore deposits. Each rock type contributes in a characteristic manner to the major and trace element budget of soils, sediments, plants, surface and ground waters. Whereas well soluble elements such as Ca, Sr, Mg, Na and K are enriched in the water phase, less soluble elements such as Al, Fe and in part Si and most trace elements are enriched in the soil compartment. In the latter, these elements are either part of, or are adsorbed on organic material, oxy-hydroxides or clay type minerals. From soils, trace elements are transported either to plants or to ground waters. The extent and manner to which chemical elements of the underlying rocks are dispersed in their surroundings depends first of all on the chemical composition of the parent material, varying greatly from one rock type to the other. Secondly, the typical pH created in the soil and water compartments strongly influences the sorption behavior and thus retention or release of specific elements. Soils formed on silicate rocks usually exhibit acid conditions (pH 4–6.5), related soil and ground waters typically are near neutral (pH 6.5–7.5). However, soils and waters related to carbonate rock environments are usually neutral to alkaline (pH 7–8.5). Soils on granitic silicate rocks have been found to be slightly enriched in Zr, Ti, Pb and Rb and often host ore deposits rich in U or As, which influence the water and sediment composition of whole catchment areas. Basaltic silicate rocks enrich the soils typically in Sr, Ti, Fe, V, Cr, Ni, Cu and Zn in concentrations up to three times those of granites. Mg-rich silicate rocks, such as peridotites and serpentinites, typically enrich soils, plants and waters with very high amounts of Mg, Fe, Cr, Co, Ni and V, but other trace elements remain very low. Al-rich silicate rocks, such as shales and micaschists, are often rich in sulfur and metal traces such as Zn, Pb, As and Cd. In contrast, carbonate dominated rocks, such as limestones or sulfate bearing dolomites, typically contribute to enrich their environment with Mn, F, S, Cl, Ba, Sr, As, Cd and radon. In conclusion, insufficient knowledge of this natural contribution can lead to misinterpretations of contaminated sites, especially for elements such as Ni, Cr, Zn, V, Cd and As.