Rehabilitation studies on tailings storage facilities in an arid hypersaline region

A field study was established in 200 1 primarily to evaluate a range of cover designs for the rehabilitation of tailings storage facilities (TSFs) in arid hypersaline regions. The success of a cover system in an arid hypersaline area is dependent on the extent to which the cover limits the upward evaporation-driven migration of salts from the tailings into the cover (depending on whether vegetation of the TSF forms part of its rehabilitation plan). The trial covers therefore comprised a capillary break layer overlain by a growth medium layer. This study presents the results of field testing of trial covers on hypersaline tailings, establishing the extent of the upward migration of salt. The results are compared with that of similar testing on a nearby natural salt pan. The results of field erosion studies conducted on TSFs and a laboratory column test conducted to simulate tailings deposition, desiccation and flooding are also presented. A discontinuous cover is one particular system that was trialled. Aside from being a cost-effective alternative to placing a conventional continuous cover, the concept of a discontinuous cover system was built on the idea the limited lateral dimensions of the cover elements (or mounds), would also limit the height to which the salt mound would rise. The sides and tops of the mounds would then in theory be suitable for the growth of vegetation, while the uncovered tailings between mounds would remain salinas where vegetation could not grow, mimicking a natural salt pan. However, the research showed higher salt concentrations within the discontinuous covers than the continuous covers. Two-dimensional solute transport modelling was conducted to assist in identifying the key mechanisms causing the difference in behaviour between the continuous and discontinuous cover systems. It was found that the cause was primarily a combination of the lateral movement of salts from beneath the salt-crusted tailings surrounding the small cover elements as they undergo evaporation; and the presence of a thin outer layer of fines covering the capillary break material (providing an access route for advective salt transport). This mechanism was found to be coupled with the low intermittent rainfall of the region that dampens the surface of the mound and subsequently increases the unsaturated hydraulic conductivity of the outer layer of fines. The interpretation of the observed behaviour and numerical modelling outcomes demonstrate the need for the growth medium to be fully encapsulated by the capillary break material. Conventional cover systems are generally designed to shed water in order to minimise infiltration into potentially contaminating mine wastes and seepage into the foundation. This research demonstrates that in an arid climate, where the evaporative flux dominates, a cover system should be designed to promote evaporation, and water storage for vegetation, rather than water shedding. This approach vastly differs to current mine site practice and regulatory guidelines, and would aid in minimising rainfall runoff ponding against low points along the outer wall leading to overtopping of the wall and subsequent erosion gullying. A discontinuous cover system in the form of crescent-shaped dams, which are much cheaper than a continuous cover, may be constructed to spread ponded water over as large a proportion of the surface as possible to maximise evaporation and prevent water shedding. There is a potential to reduce erosion loss, caused by incident rainfall on the outer TSP walls, by half to twothirds, by constructing concave-shaped TSP outer slopes that mimic natural slopes (rather than the conventional uniform, flatter slopes). These can be as steep as the angle of repose over the upper section, if durable, inert rock is used. This is supported by natural analogues.