Fate, behaviour and microbial response of diluted bitumen and conventional crude spills in a simulated warm freshwater environment.

Diluted bitumen (DB), one of the most transported unconventional crude oils in Canada's pipelines, raises public concerns due to its potential spillage into freshwater environments. This study aimed to compare the fate and behaviour of DB versus conventional crude (CC) in a simulated warm freshwater environment. An equivalent of 10 L of either DB or CC was spilled into 1200 L of North Saskatchewan River (NSR) water containing natural NSR sediment (2.4 kg) in a mesoscale spill tank and its fate and behaviour at air/water temperatures of 18 °C/24 °C were monitored for 56 days. Oil mass distribution analysis showed that 42.3 wt % of CC and 63.6 wt% of DB resided in the oil slicks at the end of 56-day tests, consisting mainly high molecular weight (HMW) compounds (i.e., resins and asphaltenes). The lost oil contained mainly low molecular weight (LMW) compounds (i.e., light saturates and some aromatics) into the atmosphere, water column, and sediment through collective weathering processes. Notably, weathered CC emulsified with water and remained floating until the end, while the weathered DB mat started to lose its buoyancy after 24 days under quiescent conditions and resurfaced once waves were applied. Analysis of the microbial communities of water pre- and post-spills revealed the replacement of indigenous microbial communities with hydrocarbon-degrading species. Exposure to CC reduced the microbial diversity by 12%, while exposure to DB increased the diversity by 10%. During the early stages of the spill (up to Day 21), most dominant species were positively correlated with the benzene, toluene, ethylbenzene, and xylenes (BTEX) content or polycyclic aromatic hydrocarbon (PAH) content of the water column, while the dominant species at the later stages (Days 21-56) of the spill were negatively correlated with BTEX or PAH content and positively correlated with the total organic carbon (TOC) content in waters.

Response of microbial communities in North Saskatchewan River to diluted bitumen and conventional crude under freeze-thaw-refreeze cycle.

Microorganisms are the first responder to oil spills and their response provides insight into the ecological effects of oils on aquatic ecosystems. Limited information is available about the impact of oil spills on freshwater ecosystems under seasonal river-ice regimes. This study aimed to investigate the microbial response of North Saskatchewan River water to diluted bitumen (DB) and conventional crude (CC) during the freeze-thaw-refreeze cycle. In two separate experiments, equivalent to 2 L of fresh DB and CC were spilled on the ice-covered river water within a mesoscale spill tank. The microbial response (changes in abundance and diversity) to oils under the freeze, thaw, and refreeze cycles were assessed for 10 days using 16S rRNA gene sequencing. The results showed that microbial communities exhibited different responses to the DB and CC oils. The effect of oils was more pronounced than that of the freeze or thaw cycles. The river microbial community rapidly responded to both spills, which coincided with a steady increase in the organic content of water throughout the freeze-thaw-refreeze cycle. Microbial diversity increased after the DB spill, but remain unchanged after the CC spill, regardless of the cycles. A higher number of new taxa emerged during the ice-covered period, while more microbial enrichment (increase in abundance) was observed during the thaw cycle. Flavobacterium (37 ± 5%) and Pseudomonas (36 ± 4%) remained the most predominant genera post-DB and CC spill, respectively. The results of this study suggest that ice coverage of 5 cm did not prevent the microbial communities from the effects of oils. Thus, a quick clean-up response to an oil spill on ice-covered water is equally critical to avoid the effects of oils on the underlying freshwater ecosystems.

The effect of temperature on hydrocarbon profiles and the microbial community composition in North Saskatchewan River water during mesoscale tank tests of diluted bitumen spills.

Despite many studies of diluted bitumen (DB) behavior during spills in saltwater, limited information is available on DB behavior in fresh water. This study examined the collective weathering processes on changes of fresh DB spilled in the North Saskatchewan River water and sediment mixture in a mesoscale spill tank under average air/water temperatures of 14 °C/15 °C and 6 °C/2 °C. Temporal changes of the hydrocarbon and microbial community compositions in the water column were assessed during the two 35-day tests under intermittent wave action. The contents of total organic carbon (TOC), benzene/toluene/ethylbenzene/xylenes (BTEX) and polycyclic aromatic hydrocarbons (PAHs) in water decreased with time during both tests. The final contents remained at higher values in warm water (15 °C) than in cold water (2 °C) after the collective weathering processes. A quick response of the main phyla, Proteobacteria and Actinobacteria, was observed, where the members of Proteobacteria enriched during both DB spills. In contrast, the members of Actinobacteria reduced with time. The microbial shifts coincided with the changes of PAHs in the waters at both temperatures. A comparison of the physical properties and chemical compositions of fresh and weathered DBs at both temperatures showed that the oil had undergone weathering that increased oil density and viscosity due to losing the light oil fraction with boiling points < 204 °C and emulsifying with water. This corresponded to losses of 19.0 wt% and 17.2 wt% of the fresh DB at 15 °C and 2 °C tests, respectively. For organic compounds in the DB with boiling points > 204 °C, there were small losses of saturates and 2- & 3-ring PAH aromatics (more during the 15 °C test than the 2 °C test), and negligible losses in the subfractions of resins and asphaltenes by the ends of the tests. <1.0 wt% of the DB was recovered from the bottom sediment, regardless of the temperature.