Chemical Weathering Patterns of Diluted Bitumen Spilled into Freshwater Limnocorrals

Hydrocarbonsare rapidly depleted from diluted bitumen followingspills into freshwater, and diagnostic ratios help to elucidate majorweathering processes. Due to the sudden nature of oil spills, few controlledstudieshave documented how oil weathers immediately following accidentalrelease into a natural lake environment. Here, we evaluated the weatheringpatterns of Cold Lake Winter Blend, a diluted bitumen (dilbit) product,by performing a series of controlled spills into limnocorrals installedin a freshwater lake in Northern Ontario, Canada. Using a regression-baseddesign, we added seven different dilbit volumes, ranging from 1.5to 180 L, resulting in oil-to-water ratios between 1:71,000 (v/v)and 1:500 (v/v). We monitored changes in the composition of variouspetroleum hydrocarbons (PHCs), including n-alkanes,polycyclic aromatic hydrocarbons (PAHs), and oil biomarkers in dilbitover time, as it naturally weathered for 70 days. Depletion rate constants(k (D)) of n-alkanes andPAHs ranged from 0.0009 to 0.41 d(-1) and 0.0008 to0.38 d(-1), respectively. There was no significantrelationship between k (D) and spill volume,suggesting that spill size did not influence the depletion of petroleumhydrocarbons from the slick. Diagnostic ratios calculated from concentrationsof n-alkanes, isoprenoids, and PAHs indicated thatevaporation and photooxidation were major processes contributing todilbit weathering, whereas dissolution and biodegradation were lessimportant. These results demonstrate the usefulness of large scalefield studies carried out under realistic environmental conditionsto elucidate the role of different weathering processes followinga dilbit spill.

Automated summary

Diluted bitumen (dilbit) rapidly depletes hydrocarbons following spills into freshwater. This study evaluated the weathering patterns of Cold Lake Winter Blend dilbit by performing controlled spills in a freshwater lake. The results showed that evaporation and photooxidation were major processes contributing to dilbit weathering, while dissolution and biodegradation were less important.