Turbidity Mitigation in an Oil Sands Pit Lake through pH Reduction and Fresh Water Addition

Currently, tailings ponds are used to contain oil sands process-affected water (OSPW) and fluid fine tailings (FFT) generated from the Alberta oil sands mining operations. In 2016, the government of Alberta introduced a tailings management directive providing guidance for reclamation efforts in Alberta's oil sands mining projects. Pit lakes, one of the proposed reclamation strategies, are designed to support a healthy aquatic ecosystem and to provide a storage location for FFT and OSPW. In 2013, Syncrude Canada Ltd. began testing a full-scale demonstration pit lake project, named Base Mine Lake (BML). However, high turbidity [approximately 71-231 nephelometric turbidity units (NTU)] has been observed in the BML cap water due to the resuspension of FFT; this turbidity inhibits the development of a healthy aquatic ecosystem. The goal of this water column study, which tested two different water dilutions (60% OSPW and 20% OSPW), was to improve BML cap water clarity by reducing pH through CO2 addition. Of these mixtures, only the one containing 60% OSPW with added CO2 showed significant water clarity improvement (P <= 0.001), and an approximately 60-day lag period preceded a rapid increase in water clarity (96.8% turbidity reduction). By contrast, only minor improvement in water clarity was observed for other mixtures: 20% OSPW with CO2 added (77.9% turbidity reduction) and 60% OSPW with no CO2 added (81.6% turbidity reduction). Taken together, these results suggest that water clarity can be improved by CO2-induced pH reduction and that the ionic strength of the solution is an important factor in turbidity removal through CO2 addition. (C) 2018 American Society of Civil Engineers.