Engineering & Mining Journal

OCT 2018

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Page 82 of 91

OCTOBER 2018 • E&MJ 81 PROCESSING SOLUTIONS These included brine evaporation in surface ponds or in a brine concentrator and discharging brine into surface wa- ter. Surface evaporation was considered unfeasible owing to the climatic water surplus of the region, while a brine con- centrator would incur the excessive cost of a necessary lengthy high-voltage power transmission line. In any event, the sub- sequent disposal of concentrated brine would still need to be managed. Discharging to surface water, while practiced in other jurisdictions, was not favorable for this project despite the high natural attenuation potential in the re- gion's rivers. "Brine injection to surface infi ltra- tion basins was also considered but did not appear feasible due to an increased potential for affecting shallow ground- water resources — and potentially surface water — in the vicinity of the infi ltration ponds," she said. The option deemed most practical — and evaluated further through reviewing available information, fi eld testing and numerical groundwater modeling — was the injection of brine to a deep aquifer using injection wells. The solids discharged from the hori- zontal belt fi lter will be conveyed to the tailings management area (TMA) de- veloped over the life of the mine and through the post-closure period. Some of the tailings are sent underground and used as backfi ll in mined-out panel rooms while the rest of the tailings are stored in two tailings facilities on the surface. Upon settling of the solid material, the resultant brine needs to be removed and disposed from the tailings dam. Situated on plastic and clay com- posite liners, the tailings would consist largely of waste sodium-chloride salt, with some impurities such as other salts, sand, silt and clay. Rainfall would pro- gressively dissolve the salt, leaving a small residue of insoluble material. Brine generated through this process and from the recycling of brine from the process plant would collect in lined settling ponds at the base of each TMA. "The potential options for managing waste brines depend very much on having the right geology and hydrogeology," Im- rie said. "In this project area, these par- ticular aquifer conditions — where there is suffi ciently low permeability near the surface and higher permeability with depth — allowed us to identify an aquifer about 400 meters deep that has the po- tential to be a good injection site." While unusual, these conditions pre- sented a good opportunity for further study, to ensure that vertical movement of the brine would be limited. This would allow the brine to attenuate horizontal- ly over time without fi nding its way to groundwater receptors. A preliminary evaluation of brine in- jection feasibility — in line with the ear- ly-stage evaluation of the project as a whole — was undertaken through a com- bination of numerical groundwater and geochemical modeling. While the ground- water modeling evaluated the likely pres- sure increases and brine plume migration associated with injection, the geochemi- cal modeling focused on the potential for chemical precipitation resulting from the interaction of oxygenated brine with anoxic brackish groundwater, as this could poten- tially clog the aquifer and injection wells. "I was appointed to assist with the pre- feasibility numerical groundwater model- ing scenarios to simulate potential impact of multiple injection wellfi eld designs," said Imrie. "We developed a 2D, axisym- metric radial groundwater fl ow model to evaluate pressure effects and plume mi- gration — as the hydrostratigraphy was relatively horizontal and homogeneous — and to model density-driven groundwater fl ow. We evaluated numerous brine injec- tion scenarios, and a range of sensitivity analyses were undertaken to help address inherent uncertainty in the knowledge of hydrogeological conditions." The next stage of investigation, she said, will include follow-up drilling, in- stallation of test wells, piezometric mon- itoring, and hydraulic testing to confi rm the injection horizon continuity and hy- draulic characteristics. The installation of deep, nested vi- brating-wire piezometers to provide reli- able piezometric head profi les will also be necessary, as will the optimization of the injection fi eld design, including monitor- ing points and program details based on outcomes of the investigations. "This will allow the remodeling of ex- pected pressure response and brine migra- tion on the basis of the updated informa- tion," she said. "Numerical groundwater models are essential tools for investigating the behavior of aquifer systems in time and space. By ensuring that each phase of model development is appropriately de- signed to meet current project objectives, we provide our clients with a cost-effective and defensible means to evaluate system responses and potential impacts of cur- rent and future groundwater-related man- agement options under consideration." Model results showing estimated brine TDS concentrations at 400 m below ground level, as well as alterations in the level of the local groundwater table, under the assumptions of brine injection at 16 wells at a rate of 1,295 m 3 /h and concentration of 300,000 mg/L, at the end of the life of the mine (23 years).

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