A study of brine flow through saturated-unsaturated potash tailings
Wong, Daniel Kam-hong
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The potash industry in Saskatchewan produces potash tailings as a waste material. These tailings are transported in a brine slurry to the tailings facility where they are spigotted onto the tailings pile. The current maximum height of the potash tailings piles in Saskatchewan ranges from 50 to 60 meters, in which only the bottom 3 to 5 meters of tailings is saturated. In the management of these waste piles, concerns regarding the environmental impact of the tailings pile on the existing groundwater, an tmderstand ing of a brine balance within the pile, and the stability of the pile arise. In order to address these concerns, an understanding of the flow of brine within the saturated-unsaturated potash tailings is critical. Analysis of the saturated-unsaturated flow syntem within the potash tailings requires that two basic relationships be established. These are the fluid retention and the permeability-suction curves. In order to examine the validity of the fluid retention curves evaluated in the laboratory and the calculated permeability-suction curves to the analysis of flow through the unsaturated potash tailings, a series of field infiltration tests were carried out on a tailings pile. These tests included single ring infiltrometer, shallow pit, open caisson, and trench type infiltration tests. The results of the single ring infiltrometer, shallow pit, and open caisson type infiltration tests Were used to establish the field saturated permeability of the tailings using analytical techniques. Monitoring of the trench infiltration test site for negative fluid pressure and fluid content within the tailings using tensiometers and neutron probes was used to establish the fluid retention curves for the tailings in the field. These field curves were then compared with laboratory evaluation of the fluid retention curve using the Tempe pressure cell method. The responses of the field instrumentation during the trench infiltration tests were also compared to the results of the computer simulation of infiltration which utilized the laboratory fluid retention curves and permeability functions as input data. In addition, sensitivity analyses of the various parameters characterizing the tailings material properties were performed also. In general, measurement of the field saturated permeability using the infiltration tests gives quite consistent results and indicates a significant degree of anisotropy in the permeability of the tailings. A good correlation is found between the fluid retention curve measured in the field using a gravimetric determination of the fluid content and the curves that were obtained in the laboratory. The field curves that were defined by using the neutron probe measurements indicate that fluid contents are too high in comparison to the laboratory curves. Numerical simulation of brine infiltration using laboratory data is in good agreement with field measurements both in terms of infiltration rate and pressure renponse. Sensitivity analyses illustrate that both the infiltration rate and the propagation of the wetting front are strongly dependent on the horizontal and vertical permeability, the degree of anisotropy and the fluid storage capacity of the tailings. In conclusion, the characterization of the potash tailings by a fluid retention curve determined in the laboratory and a calculated permeability-suction relationship is found to be adequate.