QUANTIFICATION OF CATION EXCHANGE CAPACITY IN FRACTURED POROUS ROCK USING RADIOGRAPHIC DETECTION OF CESIUM

The measurement of porosity, diffusion coefficients and cation-exchange capacity (CEC) of low-permeability rocks is of interest to the nuclear power industry because of the relevance to solute-transport assessment and the integrity of deep geological repositories for management of radioactive waste. Cavé et al (2009; J. Contam. Hydrol., 103: issue 1-2, 1-12) describe a simple, non-destructive, radiography method for estimating 1D, spatially-resolved porosity profiles, and pore-diffusion coefficients (D_p) in geological materials. Here we present an extension of the radiography technique to provide estimates of the CEC for intact samples of Ordovician shale from southwest Ontario, Canada.

Measurements of CEC are commonly conducted on disaggregated samples using batch methods whereby ions on exchange sites are displaced by ions with higher selectivity, and the uptake of high-selectivity ions is equated to the CEC. One of the main limitations of this method is the potential for disaggregation to cause an increase in the mineral surface area available for ion exchange, leading to results that may overestimate the in-situ CEC. In addition, in cases where the natural pore water in these rocks is saline, ions on exchange sites may be a very small fraction of the total dissolved solids, leading to difficulty in distinguishing the CEC within analytical uncertainty.

The present technique utilizes measured changes in X-ray absorption properties of a sample as an X-ray attenuating CsCl tracer solution diffuses through the rock pores. A quantitative relationship is established between the measured X-ray absorption and the combined mass of Cs in the pore fluid and on exchange sites. This relationship allows for the measurement of 1D, time-series profiles for Cs mass versus distance during the tracer diffusion experiment. The profiles are interpreted using the reactive transport code MIN3P to estimate the CEC. Initial results for the Queenston Formation Shale from southwest Ontario (porosity 10.9%) indicate CEC = 14.6 ±0.4 meq/100g. This technique, combined with the method of Cavé et al (2009) for estimating porosity and diffusion coefficients, is capable of providing data for important parameters for assessing solute transport in low permeability rocks.