Larissa Mikkelsen's MSc thesis abstract

Pit lakes are often a planned part of an open pit mine closure where the excavations are expected to flood and water quality is not an issue. Common environmental issues regarding pit lakes include their rebound rate, hydrodynamic behaviour and water quality. The water quality of pit lakes can be influenced by their hydrodynamics, for example overturn in a holomictic lake can transport dissolved oxygen down to submerged tailing resulting in the production of acid mine waters if sulphide minerals are present, or the unexpected overturn of a meromictic pit lake can bring stagnant, dissolved metal laden waters to surface that may be toxic to aquatic life. Where water quality is of concern and pit lakes outflow into adjacent watersheds their behaviour can determine if noxious material will be brought to the surface and released. At the former Steep Rock Iron Mines property near Atikokan, Ontario, three pit lakes are currently flooding and will eventually join to form a super pit lake before they outflow into the West Arm and subsequently Seine River system. Previous studies on two of the pit lakes, Caland and Hogarth, have shown that the pit lakes are meromictic and holomictic, respectively, and that both have elevated sulphate concentrations. The aim of this research was to: i) evaluate existing rebound models by modeling rebound and assessing which parameters exert the greatest influence on the rebound rate; and, ii) develop hydrodynamic models of Caland and Hogarth pit lakes to assess if their current limnology will change as rebound continues and they outflow into the West Arm.

Rebound models are constructed using two approaches and compared to the Ontario Ministry of Natural Resources Regional Engineering model that accurately predicted water levels to 2011. The first rebound modeling approach uses two curves to model the stage-volume relationships, a hypsometric curve and a surface area versus elevation curve. The second approach fits an exponential curve to measured water elevations and then future water elevations are forecasted by extrapolation. Rebound Model 2B constructed following the first approach matched measured water elevations best for the two pit lakes and predicts 2010 measured water elevations better than the Regional Engineering model. Model 2B predicts that Caland will flow into Hogarth in 2070 and that the new Steep Rock pit lake will outflow into the West Arm in 2087, 18 years longer than predictions made by the Regional Engineering model. Based on the water balance parameter sensitivity analysis, the difference between this study's predictions and those of the Regional Engineering model is the result of different pit volume calculation methods. In this study's rebound models the stage-volume relationships for Hogarth are more accurate than for Caland, suggesting that in future work, at minimum, linear interpolation should be used to define the volume in Caland pit lake.

This study is the first to model the hydrodynamics of Caland and Hogarth pit lakes. The Dynamic Reservoir Simulation Model (DYRESM) was used to: i) assess if it can accurately model the current pit lake conditions; and, ii) model the future conditions in Caland and Hogarth for when the pit lakes join and when they outflow to the West Arm. The model salinities are discussed to assess the future toxicity of the pit lakes. DYRESM simulations of current conditions accurately portray the observed limnological characteristics of Caland and Hogarth pit lakes, including: i) that Caland is meromictic and has a lower salinity relative to Hogarth; and, ii) that Hogarth develops a temporary meromix. Simulations of when the two pits join indicate that the freshwater lens in Caland will be maintained, but is thinner, and that Hogarth develops a meromix, which is maintained throughout the simulations. Simulations of when the pit lakes outflow into the West Arm indicate that Caland will maintained its upper freshwater lens and that a fresh water lens is only briefly present in Hogarth. In most cases, variations of the simulations for current and future pit lake conditions, including additional inflows, alteration of the inflow salinities, and the use of a slower rebound rate to define the DYRESM water balance, only produced minor changes in the simulation.

A linear trend between sulphate concentrations and salinity exists for water samples from Caland and Hogarth. Based on this trend, the DYRESM salinity profiles suggest that the waters that outflow from Caland into Hogarth will have sulphate concentrations ranging from 0 mg/L to 100 mg/L and that waters that outflow from Hogarth will have sulphate concentrations ranging from 1700 mg/L to 1900 mg/L. In general, the sulphate concentrations in Caland are below maximum acceptable limit of all water quality standards while those in Hogarth exceed all water quality standards. These results suggest that the waters that outflow from the pit lakes will be toxic.

DYRESM can be used to simulate the future hydrodynamics of Caland and Hogarth pit lakes, however, future studies and field investigations should address some of the areas of uncertainty in the DYRESM simulations for Caland and Hogarth pit lakes, including constraining seep and groundwater volumes and chemistry, on site meteorological monitoring and measurement of the light extinction coefficient.

A copy of the thesis can be downloaded here