Experimental and theoretical studies on leach pad hydraulics and transport behavior during rinsing

Rinsing is currently a common method for leach pad closure. The objectives of this research are: (1) to conduct large field scale solute transport experiments under natural rinsing conditions; and (2) to determine numerically the representative hydraulic and solute transport characteristics, applicable to the spent leach pad.; Bromide tracer tests were conducted at the AA leach pad of Barrick Goldstrike Mines Inc. The breakthrough curves (BTCs) obtained from chromatograph analysis were simulated using the dual-porosity, CDE and dual-permeability models, and analyzed with the temporal moment method.; The strong variability of solute movement between different lysimeters was found in the field tests. However, the simulated parameters derived from the area-averaged BTCs and full data set yielded almost identical results. The simulated results demonstrated that dispersivity of the AA pad was about 100 ft., mobile water contents were high, and mass transport between mobile and immobile water content were significant. Although these general hydrologic parameter set can not exactly match individual BTCs, it could follow the basic trend of the full data set and the mean simulated errors are small.; Individual BTCs simulations demonstrated that solute transport in the leach pad was complicated. Some lysimeters could be simulated with CDE models or large dispersion dual-porosity models. To these lysimeters, the simulated dispersivities, which were scale dependent, and partition coefficients were similar to the laboratory column test results. This finding suggests that even though the column tests cannot completely reflect field situations, but they can serve as primary prediction prototypes for the field objects.; More than half of the lysimeters could only be fitted by small dispersion dual-porosity models. For these lysimeters, dispersions in the mobile phases were limited to the molecular diffusions, similar to what were commonly found in fractured rocks. Both leach pad and fractured rocks have highly permeable portions which can conduct water and solute rapidly and both include low permeable matrix serving as a water and solute reservoir. Meanwhile, the mass balance calculations using temporal moment analysis and field dye infiltration tests demonstrated that preferential flow were significant in the pad.