URANIUM-ENRICHMENT IN SOILS AND PLANTS IN THE VICINITY OF A PITCHBLENDE VEIN AT LA-CREUSAZ-LES-MARECOTTES (W OF MARTIGNY, VALAIS, SWITZERLAND)

In the NE part of the Aiguilles Rouges Massif near Martigny, at the ea stern contact of the Variscan Vallorcine granite to adjacent gneisses, a series of pitchblende (UO2)-veins occur. This paper determines the level of enrichment and mobility of uranium in soils situated in the v icinity of such a UO2-vein 7 km west of Martigny. Within an area of 50 x 100 m, situated on a relatively steep slope and characterized by a strong gramma-ray anomaly, six soil profiles including their plant cov er and a reference soil profile outside the influence of the UO2-vein have been examined. The soil shows pH-values between 4 and 5 and is co lluvial. The applied analytical methods for the metal contents include extraction methods, common for soil studies, and bulk analysis perfor med with X-ray fluorescence and ICP-MS. Uranium contents found in the uppermost 20 cm of the soil profiles vary from 2,500 ppm close to the vein to 15 ppm at the lowermost point of the study area. The reference soil has around 3 ppm uranium. At greater depth (20 to 40 cm) the U-c ontent decreases to about half of the surface values, indicating a ver tical transport of uranium within the soil profile. No systematic depe ndance of uranium-contents to grain size (amount of clay) nor to the a mount of organic matter has been found. However, the good correlation between uranium and free iron oxide concentration suggests adsorption of uranium on iron oxy-hydroxides. The ashes of grass and mosses conta in up to 90 ppm U, the blueberry and redwood only up to 3 ppm. Our obs ervations suggest that at the surface the uranium is transported by do wnhill creep (solifluxion) of uranium-rich rock fragments. Liberated b y oxidation of the uppermost fragments in a given soil column, the ura nium migrates vertically until the conditions are favourable to adsorp tion onto Fe-oxy-hydroxides. However, as high U-contents of local surf ace water show, this adsorption does not lead to a significant retenti on of the uranium.