Speciation and multivariable analyses of geogenic cadmium in soils at Le Gurnigel, Swiss Jura Mountains

Mineralogical and geochemical analyses were performed on six closely spaced (within a perimeter of 300 m) soil profiles, which are naturally enriched in cadmium (Cd) at Le Gurnigel, Swiss Jura Mountains. The soils consist of Cambisols and Cambic Luvisols, the latter including an important allochthonous, aeolian fraction (loess). All soils are associated with bedrock composed of middle Jurassic (Bajocian) oolithic carbonate — a lithology, which has been identified as a source of geogenic Cd. Cd concentrations generally increase down the analyzed soil profiles, showing maxima (up to 16.3 mg·kg− 1) near the soil–bedrock interface. A sequential extraction analysis suggests that most Cd resides in the carbonate and organic fractions of topsoils, whereas the amorphous iron and manganese oxyhydroxide fraction become the most important Cd-bearing phase in the middle and deeper parts of the soils. A principal component analysis shows that Cd, Zn and Cr are positively correlated with comparable distributions in the soil profiles suggesting a common bearing phase such as Fe oxyhydroxides for these three elements. A complex transfer pattern of Cd is proposed for the analyzed soil profiles, which starts with the release of Cd from the underlying bedrock, and its transfer into oxide, hydroxide, carbonate and organic phases. Additionally, the lateral advection of Cd-rich soils on top of these soils adds Cd to them, which is transferred from the topsoil towards the deeper horizons by biological and pedological processes. The amount of readily exchangeable and therefore potentially bioavailable Cd is low in these soils (on average 0.2 mg·kg− 1), provided that the pH remains above 5. Under stronger acidic conditions the concentration of bioavailable Cd may increase in the topsoils of thin Cambisols (3.0 mg·kg− 1) and Cambic Luvisols (1.2 mg·kg− 1). Under stronger acidic (pH < 4.5) and oxidizing conditions, Cd bound to organic matter may be mobilized in addition, and the bioavailability of Cd would range between 3.3 and 5.4 mg·kg− 1 in Cambisols and reach up to 1.7 mg·kg− 1 in Cambic Luvisols.