ESTIMATING DIFFUSION RATES OF THE STABLE ISOTOPES OF WATER IN UNSATURATED MINE WASTE

The stable isotopes of water (δ¹⁸O and δ²H) have been extensively used as tracers to define water movement in hydrogeology; however, they have not been widely used to define water migration through deep, unsaturated, mine waste rock piles. One of the challenges in interpreting deep profiles of δ¹⁸O and δ²H though unsaturated mine waste is to determine the extent of diffusive transport that can occur in the piles. Defining the extent of diffusive transport is complicated by the multiphase (liquid and vapour) transport of the isotopes combined with the fractionation associated with equilibrium partitioning of the isotopes been phases. The combined diffusive transport of the δ¹⁸O and δ²H through the vapour and liquid phases of an unsaturated soils has been described in the literature, but has not been systematically compared to laboratory diffusion testing.

Double half-cell diffusion tests were used to measure the diffusive transport of δ²H through an unsaturated, fine grained sand. The cells were constructed from 200 mm long x 100 mm diameter PVC pipe with caps glued on one end. The diffusion cells were packed at a constant porosity of 0.4, but over a range of volumetric water contents (12%, 16%, 20%, 24%, and 28%). The water used to prepare the sample in one half of the diffusion cell was labelled with δ²H. The sand was then packed into the pipe in multiple lifts. Once all the sand was packed the cells were put together and sealed at the connection. After allowing the δ²H to diffuse for a pre-determined amount of time, the cells were cut into slices approximately 20 mm in length. The volumetric water content δ²H values within each slice were then measured.

The combined diffusive transport was termed the apparent diffusion coefficient. It was calculated assuming all transport was via the water phase. A least squares fit of the data to an Ogata and Banks diffusion equation was used to define the calculated apparent diffusion coefficient. The theoretical apparent diffusion coefficient as a function of water content was also found using estimation methods from literature. Plots of the theoretical and calculated diffusion coefficients versus volumetric water contents yielded similar values.