Oil sands, also known as tar sands or bituminous sands, are sand deposits impregnated with dense, viscous petroleum called bitumen. The two major oil sand mining companies currently producing synthetic crude oil in the province of Alberta are Syncrude Canada Ltd. and Suncor Energy Inc. Coke is a waste product formed during the heavy oil upgrading processes. The amount of coke produced currently at the Syncrude and Suncor operations is significantly more than what can be combusted and/or sold. Therefore, it must be stored on site making coke a necessary component of future reclamation landscapes. It is as yet unclear whether or not coke poses a significant toxicological risk (i.e. through leaching of constituent chemicals and elements) to the aquatic or terrestrial environments if used in a long-term reclamation capacity. The primary objective of this research was to evaluate whether stockpiled Syncrude Canada Ltd. and Suncor Energy Inc. coke can safely and effectively be used as a substrate amendment in an aquatic reclamation option.
Syncrude and Suncor coke were exposed to three different sets of water quality conditions: high and low dissolved oxygen, pH (5, 7.5 and 10) and freeze-thaw cycles. These conditions have the potential to affect the leaching of constituents from coke once placed in an aquatic environment in northern Alberta. Metals were observed to leach from both coke types under all treatments. Some of these metals (cobalt, copper, manganese, molybdenum, nickel, vanadium and zinc) reached levels of toxicological interest by exceeding published LC50 values for the aquatic macroinvertebrate Chironomus sp. and/or established Canadian Water Quality Guidelines for the protection of aquatic life.
To determine the source of metals observed to leach in the leaching experiments, an accelerated weathering experiment was conducted. Coke from both Syncrude and Suncor were pre-treated by rinsing with water to remove any natural soil or sediment particles that become associated with the coke during storage, and with peroxide to remove any metals bound to the surface of the coke. It was concluded that the metals leaching into the overlying water and pore water of the coke were from either the natural soil/sediment particles, and/or were weakly bound to the surface of the coke. Rinsing coke with water before adding it as a substrate amendment in an aquatic reclamation strategy may decrease its potential toxicity to aquatic organisms by removing the soil/sediment debris and metals loosely bound to the surface of the coke, reducing initial amounts of metals available to the organisms in the pore and overlying waters.
As part of these leaching experiments, coke from both Syncrude and Suncor were further characterized with elemental analysis and qualitative analysis with polarized light microscopy and scanning electron microscopy. These analyses showed distinct differences in the physical properties (i.e. particle size and structure) of Syncrude and Suncor coke. However there appeared to be little variation in the elemental and structural composition among the different weathering treatments within each coke type.
Following leaching, coke from Syncrude and Suncor was used as a substrate in 10-d toxicity tests with Chironomus tentans. These experiments evaluated whether the leachate generated from coke, or the coke itself, caused of an adverse effect on the survival and/or growth of this aquatic invertebrate. Decreased survival and growth of C. tentans was observed when Suncor coke was used as a substrate. However, further experimentation showed that these effects were due to the physical properties of Suncor coke, rather than any constituents that leached from the coke.
In a separate experiment, combinations of fresh oil sands waste materials were added to experimental enclosures constructed at Syncrude Canada Ltd in May 2002. Core samples of these waste materials were collected after aging in situ for periods of 2 and 14 months. The aged samples along with fresh (0 month) samples, were used as substrate material in 10-d toxicity tests with C. tentans to assess the potential toxicity of both fresh and field-aged combinations of waste materials. These experiments found that there is a potential for greater initial stress or toxicity to aquatic organisms when the substrates are freshly added to the reclaimed wetlands. However, this initial potential toxicity of oil sands waste materials can decrease with aging in an aquatic environment.