REDOX POTENTIALS AND REDOX REACTIONS IN DEEP GROUNDWATER SYSTEMS

Both laboratory investigations and field studies of deep groundwater systems indicate that stable and reproducible redox potentials can be measured with a precision of +/- 25 mV. The E(h) data measured in the field are in good agreement with the half-cell reaction: "Fe(OH)3"(s) + 3H+ +e- half arrow right over half arrow left Fe2+ 3H2O involving hydrous ferric oxide and Fe2+ in solution, as indicated both by the slope of a plot of Eh vs. (3pH + log [Fe2+]), and the value of the solubility product for "Fe(OH)3"(s), calculated from these data. The solubility product: pK(s) = - log[Fe3+][OH-]3 = 40.9 +/- 1.1 falls in the range 37.3 < pK(s) < 44.1, as previously given by D. Langmuir and D.O. Whittemore for amorphous ferric hydroxide and crystalline goethite, respectively. The measurement and interpretation of field redox data from borehole investigations are complicated by the mixing of waters of different origin. Most groundwaters studied are not characteristic of the geochemistry of the particular section sampled; they are mixtures of water of different origin and may contain significant amounts of surface water (indicated by the tritium content) and drilling water used to cool the drill-bit. The initial oxygen content in these components is rapidly reduced by Fe(II) minerals with the formation of Fe(OH)3(s), resulting in mixed waters which are anoxic and containing dissolved Fe(II). Surface-mediated redox reactions seem to play an important role both in the reduction of oxygen and of uranium and other trace elements.