Characterization of redox conditions in the excavation disturbed zone of a drift in the Kamaishi Mine, Japan

Abstract

The excavation of drifts during construction of a geologic repository for high-level nuclear waste (HLW) could affect mechanical and hydraulic properties of the rock within a zone extending a short distance into the rock from drift walls. Related impacts on groundwater chemistry within such an Excavation Disturbed Zone (EDZ) are largely unknown, but the oxygen in air circulating through drifts could conceivably diffuse into groundwater within the EDZ and cause these solutions to become strongly oxidizing. A study was therefore undertaken of redox conditions within the EDZ of crystalline host rocks in the abandoned Kamaishi Fe–Cu mine in Japan, which is believed to be generally representative of conditions that could exist in the EDZ of a HLW repository. The chemical compositions of groundwaters flowing into three boreholes that were drilled various distances into the Kurihashi granodiorite were monitored continuously. The results indicated that dissolved oxygen concentrations tend to decrease with increasing distance into the rock. Oxygen penetrated, at most, a few meters into the rock from a drift (E.L. 250m drift) that intersected a specific fracture (Fracture No. 99), for example. A conceptual model was developed that takes into account both the reaction rate of dissolved oxygen with ferrous minerals and the diffusion rate of oxygen into the rock matrix. A quantitative evaluation of the model using a numerical solver indicated that the oxygen diffusion depth could vary from 3 to 30cm from the unsaturated/saturated zone boundary, depending on the reactive surface areas of ferrous minerals. These estimates may overestimate the migration distance of a redox front, however, because ferrous minerals other than biotite were not considered in the model and the partial pressure of O2(g) was fixed at the atmospheric value of 0.21bar throughout the unsaturated zone. More accurate assessments of redox-front behavior may be possible given better constraints on mineralogy and associated kinetic parameters (i.e., reactive surface areas and rate constants), and on microbial effects on oxygen consumption. Additionally, the use of more realistic boundary conditions on dissolved oxygen concentrations in the unsaturated zone would help reduce uncertainties in model results.