Petroleum hydrocarbon content, leaching and degradation from surficial bitumens in the Athabasca oil sands region

Mine reclamation has become a topic of considerable research in the Athabasca Oil Sands Region of Northeastern Alberta, Canada. In this area some of the largest open pit mines in the world extract bitumen, a type of heavy oil, from the oil bearing McMurray Formation. At the close of mine operations, lease holders are required to return the land to equivalent capability. To achieve this, several existing waste piles will need to be capped and a functioning ecosystem re-established. Proposed borrow pits for capping material were recently found to contain reworked bitumen materials of various shapes and sizes. This reworked bitumen has been named tarballs by local industry. The use of that name is maintained throughout this study. The tarball accumulations are very abundant in some areas but also occur infrequently in apparently clean areas. In this study, the petroleum hydrocarbon signature and leaching characteristics of the tarballs were determined. The hydrocarbon content and composition of the tarballs were assessed using the Canadian Council of Ministers of the Environment (CCME) methods. The data were presented in terms of the four CCME fractions. Leaching characteristics were determined using a custom designed, unsaturated soil column experiment carried out at the University of Saskatchewan. The soil column was monitored for petroleum hydrocarbons, total carbon and total organic carbon in leachate water and soil respiration as a surrogate for biodegradation. The bitumen was found to consist primarily of heavy hydrocarbons, CCME Fractions 3 and 4, at levels of thousands and tens of thousands of mg/kg respectively. Gravimetric analyses indicated that a significant proportion of the tarball materials are very heavy hydrocarbons beyond the range of high temperature gas chromatography. These very heavy hydrocarbons are greater than carbon number C90. Lighter and more mobile hydrocarbons were occasionally identified at the core of larger accumulations. Where identified, the lighter fractions were typically accompanied by F3 and F4 hydrocarbons at one to two orders of magnitude greater than typical tarball material. Leachate was found to contain F2 hydrocarbons at less than 0.2 mg/L, a small fraction of the CCME clean water guideline of 1.1 mg/L. F3 hydrocarbons were identified at levels up to 0.6 mg/L. Soil respiration indicated a very low activity system, suggesting limited potential for biodegradation. Tarball materials are concluded to be of little concern for potential impacts to groundwater based upon the hydrocarbon fingerprint and the observed leaching characteristics of the tarball materials