Reduction of iron(III) minerals by natural organic matter in groundwater

Construction of the entrance tunnel to the Aspo Hard Rock Laboratory, a prototype repository in Sweden for research into the geological disposal of spent nuclear fuel, has resulted in increased transport of organic carbon from the surface into the groundwater. This increased input of organic matter has induced accelerated oxidation of organic carbon associated with reduction of iron(III) minerals as the terminal electron acceptor in microbial respiration. Hydrochemical modeling of major solute ions at the site indicates an apparent first-order decay constant for organic carbon of 3.7 +/- 2.6/yr. This rapid turnover is not accompanied by an equivalent mobilization of ferrous iron. Thermodynamic calculation of iron mineral solubility suggests that ferrous clay minerals may form in hydraulically transmissive fractures. The conditional potentials for the oxidation-reduction of such phases coincide with measured redox potentials at the site. The calculated potential is sufficiently low so that such phases would provide reducing capacity against future intrusion of O-2 into the groundwater, thus buffering a repository against oxic corrosion of the engineered barriers. Copyright (C) 1999 Elsevier Science Ltd.