Investigation on solute transport in heterogeneous fracture of crystalline rock by flow-through test and micro X-ray CT

Abstract:

Crystalline rocks such as granodiorites have been investigated as potential host rocks for the geological disposal of radioactive waste in many countries. Radionuclide transport in fractured crystalline rocks can be conceptualized by a dual-porosity model where radionuclides are transported by advective water flow through a fracture and are retarded by both diffusion and sorption into the surrounding rock matrix. Aperture variability within a fracture determines flow in a channel, whereas other areas of the fracture have almost stagnant water. The effect of different degrees of heterogeneity in fracture aperture distributions is critically important to quantify solute transport. In the present study, we identified flow channels and stagnant water zones in a fractured granodiorite by mean of a flow-through test, using imaging tracer coupled with micro X-ray CT measurements.

A cylindrical granodiorite sample (f5.0 cm × L10 cm) with a single natural fracture collected from the Grimsel Test Site (Switzerland) was used. A solution including 0.5 ~ 2 mol/L CsCl or NaI as an imaging tracer was flowed through the fracture saturated with distilled water, at flow rate 0.02 mL/min and the tracer concentration in the breakthrough solution was monitored. In addition, micro X-ray CT measurements (75 μm³ of the spatial resolution) were performed on the sample before and after the flow-through test.

The aperture distribution of a fracture of the sample was determined by the CT images, and showed the width of the fracture aperture varied between 0.1~10 mm. After the flow-through test, the tracer distribution in the fracture was detected in the CT images, and the tracer concentration map in the fracture was determined by the relationship between CT values and the tracer concentration between 0.1~2 mol/L. The concentration map corresponds to the aperture distribution, that is, the concentration was low in the narrow apertures with the width less than ~ 0.5 mm. This result suggests that the tracer selectively flows in the fractures with wider apertures and diffuses in the narrow apertures corresponding to the stagnant water zone. This finding is key information to evaluate the effect of the flow channel and stagnant water derived by fracture heterogeneity on radionuclide transport.