A SEARCH FOR THE ORIGIN OF CADMIUM IN THE SOIL OF THE SWISS-JURA

The top soil of a 14.5 km(2) region at la Chaux-de-Fonds in the Swiss Jura is exceptionally rich in cadmium. It contains an average of 1.3 m g per kg of soil. The spatial distribution of the metal has no simple pattern that could be explained by atmospheric deposition or agricultu ral practices. Thin soil contained most of its Cd at the surface; in t hicker soil Cd is mainly concentrated between 60 and 80 cm depth. No s pecific minerals or soil fractions could account for these accumulatio n, and the vertical distribution of Cd is best explained by leaching f rom the topsoil and further adsorption within layers of nearly neutral pH. The local Jurassic sedimentary rocks contained too little Cd to a ccount for the Cd concentrations in the soil. Alpine gravels from glac ial till were too sparse in soils to explain such a spreading of Cd. M oreover this origin is contradictory with the fact that Cd is concentr ated in the sand fraction of soils. The respective distributions of Fe and Cd in soils, and soil fractions, suggested that the spreading of iron nodules accumulated during the siderolithic period (Eocene) was n ot the main source of Cd. Atmospheric deposition, and spreading of fer tiliser or waste from septic tanks seem the only plausible explanation for the Cd concentrations, but at present few factors allow us to dif ferentiate between them.