URANIUM MOBILIZATION IN GROUNDWATERS: GEOCHEMICAL INVESTIGATIONS AND WATER-ROCK INTERACTION MODELLING IN A COASTAL AREA OF EASTERN SARDINIA (ITALY)

Uranium is concentrated in late-stage magmas and therefore granitoids are usually enriched in U compared to many other rocks. The extent of U mobilization in ground water depends on a wide number of hydrogeological (water-rock ratio, water flow and residence time), geochemical (main dissolved components, pH, redox conditions) and mineralogical factors, such as phases containing U (Wanty and Nordstrom, 1993, in: Regional Ground-Water Quality, Alley, ed., VanNostrand Reinhold). Ground waters in granitoids may have high U concentrations, although they rarely exceed 20 µg/L (Gascoyne, 1989, Appl. Geoch., v. 4, p. 577). Dissolved U concentrations up to 100 µg/L have been recognized in ground waters of eastern Sardinia (Lorrai and Mereu, 1999, Rend. Sem. Fac. Sc. Univ. Cagliari, 69). Rocks in this area include a low-grade metamorphic basement of Cambrian to lower Carboniferous age, intruded at the end of Hercynian orogeny by calcalkaline magmas of granodioritic composition (Secchi et al, 2002, Per. Mineral., 70). Post Hercynian (Permian) rhyolitic volcanics are also present in the northern part of the study area. Water samples collected from shallow wells, drilled wells and springs have been analysed by IC (major anions), ICP-OES (major and some trace cations) and ICP-MS (U and many other trace elements). Waters circulating in the Permian volcanics have low TDS (220 to 460 mg/L) and low U concentrations (0.2 to 1.8 µg/L). Waters of metamorphic aquifers have TDS ranging from 1500 to 3800 mg/L and dissolved U ranging from 0.7 to 11 µg/L (mean 3.7 µg/L). Waters circulating in granodiorites and their superficial weathering zones have variable TDS, ranging from 130 to 2400 mg/L and U concentrations ranging from 0.2 to 100 µg/L (mean 13 µg/L). A field radiometric survey has located a swarm of mineralized fractures in granodiorites, with pyrite, magnetite and epidote in the area with the higher dissolved U concentrations. The high variability in salinity may be related to water-rock interactions or to the influence of seawater on coastal aquifers. However, U appears to be more closely related to alkalinity than to salinity, in agreement with other studies. Numerical modelling of the geochemical data will be used to help in understanding this variability and to draw a model of water-rock interaction and U mobilization in this area.