Evaluation of uncertainty of redox potential in deep groundwater

A case study in the Horonobe area, Hokkaido

Abstract

Redox potential of deep groundwater in the Horonobe area, Hokkaido, was measured, and uncertainty of the measurement and thermodynamic interpretation was evaluated. The redox potential of groundwater obtained using monitoring units in the Underground Research Laboratory ranged between －250 and －100 mV, and the effect of the excavation of drifts on the redox potential is considered to be small in spite of its temporal variation. The redox potential is controlled by the reaction of chemical pairs of SO₄^2－/FeS₂, SO₄^2－/HS^－ and CO₂(aq)/CH₄(aq). The comparison between the equilibrium potential for these reactions and the measured redox potentials suggests that ±50 mV of uncertainty for the measurement of the redox potential is appropriate.