Hydrogeochemical evolution and C isotope study of groundwaters from “Mina Fe” U deposit (Salamanca, Spain): implications for processes in radwaste disposal

Within the framework of the ENRESA natural analogue programme, the U ore deposit of “Mina Fe”, western Spain, has been studied as a natural analogue of radioactive spent fuel behaviour after burial, in spite of being an extremely perturbed geological environment due to mining activities. The main objectives of this project are to determine the hydrogeochemical evolution of the system and identify the main water/rock interaction processes that control the physicochemical variables (pH and Eh) of groundwaters, including the role-played by the organic matter. The hydrogeochemical results from 3 consecutive groundwater sampling campaigns, separated by a phase of restoration works, indicate that the groundwater of the site shows an evolutionary trend from image acid oxidising waters towards alkaline-reducing waters, though their evolution with respect to the alkaline and alkaline-earth elements is not clear. The image-acid waters are the result of the oxidation processes affecting the primary sulphide-rich U mineralisation, while the alkaline-reducing waters result from the buffer capacity of carbonates from fracture filling materials, as the δ13C values of DIC demonstrate. The reducing character of these waters mainly results from the microbiologically mediated partial oxidation of the abundant organic matter existing in the clayey walls of the major faults at the site, since other dissolved inorganic redox pairs are insufficient to explain the in situ measured redox potential. Thus, the high content in DOC of these waters is also explained, since the soil at the site is poorly developed. DOC, DIC or both can be responsible for the high U concentration measured in these groundwaters, in spite of its reducing character. The restoration works performed in the exploitation quarry have also restored the aforementioned evolutionary trend, which was the normal evolution of groundwater in the site before mining. Finally, the results are discussed in terms of the geochemical processes expected in a radioactive disposal site after closure.