In the NE part of the Aiguilles Rouges Massif near Martigny, at the
eastern 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
vicinity 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 cover
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
colluvial. The applied analytical methods for the metal contents include
extraction methods, common for soil studies, and bulk analysis performed
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-content decreases to about half of the surface values, indicating a
vertical transport of uranium within the soil profile. No systematic
dependance of uranium-contents to grain size (amount of clay) nor to the
amount 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 contain up
to 90 ppm U, the blueberry and redwood only up to 3 ppm. Our
observations suggest that at the surface the uranium is transported by
downhill creep (solifluxion) of uranium-rich rock fragments. Liberated
by oxidation of the uppermost fragments in a given soil column, the
uranium migrates vertically until the conditions are favourable to
adsorption onto Fe-oxy-hydroxides. However, as high U-contents of local
surface water show, this adsorption does not lead to a significant
retention of the uranium.