Potentiometrically measured Eh in groundwaters from the Scandinavian Shield

Measurement and interpretation of electrochemical Eh values in natural reducing groundwaters is a complex task. SKB, the company in charge of disposal of nuclear fuel wastes in Sweden, has developed a refined methodology for the determination of this parameter in packered sections in boreholes. The methodology consists of the simultaneous use of three different electrodes (Pt, Au and C) both at depth and at the surface, and maintaining continuous logging over a long period of time. Apart from Eh, the logging also includes other parameters such as pH, dissolved O2, conductivity and temperature. This methodology has been used since the 80s in the framework of the hydrogeochemical characterisation programs supported by SKB at different sites in the Scandinavian Shield. All the existing databases have been revisited in this work using a uniform set of criteria to select Eh values of the Swedish groundwaters as a function of depth.The selected Eh dataset ranges from −140 to −400 mV and corresponds to waters with pH values between 7 and 8.6 at depths between 110 and 1000 m. Eh shows no correlation with depth in any of the fractured aquifers of the studied sites. The Eh values, together with the results obtained from the geochemical modelling and the microbiological studies in the ongoing work at Laxemar and Forsmark have allowed the study of the relationship between the potentiometrically measured Eh and the redox pairs controlling it. The possible control of the redox conditions by the electroactive pair Fe2+/Fe(OH)3(s) in waters with a wide range of measured Eh values suggests either the involvement of different kinds of Fe oxyhydroxides or variations of crystallinity and/or particle size in them. The good fit found between the measured Eh and the results obtained with different S redox pairs, together with the occurrence of SRB and the equilibrium situations with respect to the amorphous Fe(II) monosulphides, suggest the participation of the S system in the measured Eh control.