Calibration and Optimization of Initial Dilution Zone Theoretical Mixing Models Using Field Data

Resource projects may, at times, be required to discharge effluent or mine-affected runoff into the receiving environment. In such cases, effluent management guidelines permit chemical constituents to occur in elevated concentrations within a designated area of the receiving environment called the initial dilution zone, or IDZ. Consequently, the characterization of the IDZ has become an integral part of the permitting process for many mining operations. Determining the physical mixing behaviour of an effluent discharge into the receiving environment is a challenging but critical component of this work. Theoretical mixing models are commonly used to simulate mixing by integrating effluent and receiving environment flow and chemistry. The result is a spatial distribution of the concentration of chemical constituents, which is then compared against water quality guidelines at the boundary of the IDZ. While these theoretical mixing models are valuable tools for simulating effluent mixing at a desktop level, they are often limited by the accuracy of the input data and the underlying model assumptions and frequently lack real-world validation. Specific IDZ field programs have been developed to measure dilution and diffusion in the receiving environment. The procedure involves measuring conductivity within the IDZ by placing in-situ conductivity sensors at targeted locations in the receiving environment and injecting a continuous stream of brine to simulate the proposed effluent discharge. As the conductivity plume migrates, steady-state conditions are achieved. The sensors are then relocated farther upstream within the IDZ, and closer to the injection point, and the procedure is repeated. This is done across multiple sensor placements until conductivity has been measured at sufficient locations within a grid-like pattern throughout the receiving environment. This paper will expound on the methodology developed and used for calibration of theoretical IDZ mixing models, to support developing and operating mining projects throughout British Columbia. Canada.