An isotopic assessment of oil sands mine site waters to improve water management practices
Current oil sands mining technology requires approximately 2 m3 in the production of 1 m3 of crude oil. This water demand has resulted in massive volumes of process-affected waters being stored on-site – a volume that is currently not well quantified, though estimated to be in in the order of billions of m3. A site wide water balance must be closed at each mine in order to effectively manage the on-site storage and reuse of process-affected waters, in addition to planning for future remediation and release of this water following site closure. Oil sands mining operators have identified the constantly evolving nature of both operational water demands and the tailings management infrastructure as key challenges preventing accurate closure of a site wide water balance. Stable isotopes of oxygen and hydrogen (18O and 2H) have been widely used as tracers to close water balances of natural reservoirs. To date, their application to mine water containment systems, such as tailings management facilities, has not yet been implemented. This study demonstrates the use of 18O and 2H as tools to track key components of the water balance associated with the Mildred Lake mine, operated by Syncrude Canada Ltd in Northern Alberta. There, process affected waters are stored in interconnected tailings ponds referred to as the recycle water (RCW) circuit, with approximately 200 million m3 of water accessible for reuse in the extraction and transport of bitumen. This thesis characterizes the primary mechanisms of each water balance component contributing to the seasonal and inter-annual evolution of isotopic signatures of the RCW circuit and an end pit lake located at Mildred Lake mine. The thesis uses isotopic “finger-printing” of contributing water sources to characterize each part of the system. Isotope mass balance techniques were implemented to estimate evaporative loss from these systems and are compared to traditional methods of estimating evaporation (e.g., Penman combination equation and eddy covariance towers). This study found that isotopic seasonality of both the RCW circuit and the end pit lake were muted compared to natural systems within the region due to the contribution of large volumes of highly enriched pore water to tailings pond water stores as a result of tailings settlement. Samples collected from tailings ponds showed a systematic shift towards greater isotopic depletion during the ice-on period. I hypothesized that this shift occurred as a result of fractionation during ice formation in addition to mixing with process water released from tailings. Such mechanisms appeared to contribute to the observable spring depletion of the RCW circuit in addition to depleted snowmelt during the spring freshet. The seasonal variations in the isotopic signatures of individual tailings ponds were consistent with differences in water management between ponds. I then used an integrated isotopic signature as a proxy for the entire RCW circuit in isotope mass balance modelling scenarios. The proportion of inflow lost to evaporation from the RCW circuit was calculated as a decimal ratio using isotope mass balance modelling. The evaporation/inflow ranged from 0.11 to 0.22 over a five-year period, consistent with a high through-flow system with low residence time. These ratios correspond to 67–133 mm yr-1 of inflow water lost to evaporation from the RCW circuit based on estimated volumes of the water balance inputs. A simplified isotope mass balance model of the RCW circuit was used to estimate evaporative losses based on observable temporal isotopic enrichment during the open water period and assuming all other outflows of the water balance were zero. Using this model evaporation rates were found to range from 418 to 931 mm yr-1, while evaporation rates measured on-site using eddy covariance ranged from 350-520 mm yr-1. The difference between the isotope mass balance and eddy covariance results suggest a contribution of highly enriched tailings pore water to the overall enrichment of the RCW circuit in addition to open water evaporation. The isotope mass balance model was also used to simulate the evolution of the daily isotopic signature of a highly monitored demonstration end pit lake. The simulated pattern of isotope evolution was used to obtain an optimized estimate of lake evaporation. This estimate was then compared to a measured water balance over a four-year period. The model showed good agreement when 18O was used as the tracer; however, when 2H was used as the tracer the model consistently under predicted open water enrichment - likely due to evaporative fractionation effects. This thesis represents the first study that we are aware of that applies isotope mass balance techniques to an engineered system within the Alberta oil sands region. Our results highlight the potential value of using stable isotope tracers to aid in on-site water management of tailings ponds as well as helping to improve our understanding of the transport and distribution of water moving through mine closure landscapes.
Isotope hydrology, deuterium, oxygen-18, isotope mass balance, oil sands, mine water management, tailings management
Master of Science (M.Sc.)
Civil and Geological Engineering