REDOX CHARACTERISATION OF THE SWEDISH CANDIDATE SITES FOR DEEP GEOLOGICAL DISPOSAL OF SPENT NUCLEAR FUEL

Two sites in Sweden are being characterised as possible candidates for hosting a deep geological repository for the storage of spent nuclear fuel: Forsmark and Laxemar-Simpevarp, in the eastern coast of Sweden. The main geologic, hydrologic and hydrogeochemical features of both sites are being studied and integrated, with especial emphasis on their redox characteristics, which are key to understand the past, present and future geochemical evolution in and around the repository area. As a part of the geochemical characterisation, an innovative methodology for the measurement of representative potentiometric Eh values has been developed (Auque et al., 2008) and its results have been completed and evaluated with the assistance of geochemical modelling calculations (Laaksoharju et al., 2008) and integrated with the outcomes from microbial and mineralogical studies.

With regard to the redox parameters and indicators, Eh values are clearly reducing (from -143 to -310 mV) in both sites, which indicates the good redox-buffering ability of microbial or water-rock interaction processes. This is consistent with the detected presence of Fe(II)-bearing minerals (mainly chlorite and pyrite) in the fracture fillings at all depths in Laxemar-Simpevarp and Forsmark area. These mineral occurrences prove that oxidising episodes have not exhausted the reducing capacity of fracture filling minerals even in the shallowest part of the studied systems. Another typical redox feature of both studied systems is that neither Eh values nor concentrations of redox-sensitive elements (iron, sulphur, manganese and carbon) nor calculated potentials from redox couples nor microbiological populations show any clear trend with depth. This could be a consequence of their complex hydrological setting and palaeohydrogeological evolution.

References

Auqué L.F.; Gimeno M.J.; Gómez J.B. and Nilsson A.C. (2008). Potentiometrically measured Eh in groundwaters from the Scandinavian Shield. Applied Geochemistry 23: 1820-1833.

Laaksoharju, M.; Smellie, J.; Tullborg, E.-L.; Gimeno, M.J.; Molinero J.; Gurban I. and Hallbeck L. (2008). Hydrogeochemical evaluation and modelling performed within the Swedish site investigation programme. Applied Geochemistry 23:1761-1795.