Dr. Tom Al : Studies of diffusion and reaction... (24-11-2016)

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Dr. Tom Al (U. Ottawa)

Studies of diffusion and reaction processes in porous rocks: examples from nuclear waste management and contaminant hydrogeology

Jeudi 24 novembre 2016 à 13h30/ Thursday, November 24, 2016, 1:30pm

Local PK-7605, 201 ave. Président-Kennedy, UQAM

Résumé / Abstract:

This talk will focus on rock transport and reaction properties as they relate to nuclear waste management and contaminant hydrogeology (hexavalent Cr). In characterizing the geologic properties of candidate sites for a deep geologic repository (DGR), the diffusive properties of rocks are of fundamental importance. We have developed new magnetic-resonance-imaging (MRI) and radiation-imaging (RI) methods for measurement of diffusion and reaction properties including pore- and effective- diffusion coefficients (Dp and De), tracer-accessible porosity (φtr), cation-exchange capacity (CEC) and ion selectivity coefficients. Site investigations for the DGR proposed by Ontario Power Generation for low- and intermediate-level waste at the Bruce nuclear site in Ontario provided opportunity to take the work from lab to field and study the effect of diffusion on the distribution of natural tracers (18O, Cl and Br) across 700 m of Paleozoic stratigraphy in southwest Ontario. Ongoing research is focussed on measurement of diffusion coefficients for dissolved gases (e.g. CH4, CO2 and He) and refinement of the RI methods to improve tracer detection so that the method can be applied to rocks with very low porosity, and to expand the range of tracers for both conservative and reactive transport.

We have also been working to understand controls on the transport and fate of CrVI in fractured porous bedrock. We developed a method for determination of CrVI concentrations in porewater from rock cores allowing for collection of high-resolution data and improvements in the assessment of the subsurface Cr(VI) distribution. The data demonstrate that, as a result of diffusion, most of the contaminant mass resides in the porous matrix and is therefore not accounted for by standard groundwater sampling methods. Most recently we have developed an extraction method for determining the amount of labile CrIII precipitate present in the rock matrix. This allows for closing the mass balance to provide a quantitative assessment of the extent to which CrVI is immobilized by reduction to CrIII. Our first complete dataset is from a CrVI plume in redbed sedimentary rocks in New Jersey. These are oxidized hematite-rich rocks and we did not expect to find significant evidence for natural CrVI reduction. Surprisingly, the results indicate that approximately 50% of the total Cr mass is present as CrIII. This work is an example of how scientific methods can assist regulators faced with the decision to permit monitored natural attenuation or to push for more aggressive remediation options. Ongoing research focusses on identifying the principle reaction processes, reaction rates, and the long-term stability of the CrIII solids.