Examining saturated and unsaturated hydraulic parameter changes as a result of geochemical reactions in tailings

Freshly deposited tailings define finely crushed rock rarely altered by weathering processes. In modelling water flow and solute transport for saturated and unsaturated conditions in deposited tailings, the hydraulic parameters are essential required parameters. It has been proposed that these parameters vary as the materials evolve through weathering processes which transform minerals and hence chemical and physical properties. Several methods, destructive and non-destructive, have been used to determine the change of these parameters. The evaporation method as a non-destructive method, for deriving hydraulic parameters of porous media can be employed to monitor the overall changes in the pores as the properties of the porous matrix changes. In order to test this hypothesis, 25% of pyrite, 25% of dolomite, 35% of quartz and 15% of chlorite, reflecting typical acid producing, acid neutralising and inert minerals were mixed to form artificial tailings. The mineral admixture was mixed at a water content of 10% and allowed to equilibrate for two weeks before the start of the experiment. The mineral admixture was packed to a bulk density of 1.4 g/cm³. The hydraulic parameters for the near saturation state were measured using the simultaneous measurement of water content and water potential as a consequence of evaporation. The mineral admixture was subjected to 9 drying and wetting cycles over a period of 9 months. The hydraulic parameters were estimated using RETC and HYDRUS 1D. After 9 months of drying and wetting, there was a slight change in the bulk density of the material. This change had an overall effect on the subsequent hydraulic parameters. This study implies that feedback effects should be considered in modelling ageing of mine tailings.