Trace Element Mobility in Layered Oil Sands Mine Wastes

Abstract

Potential closure systems consisting of oil sands tailings and upgrading by-products were studied at the Syncrude Mildred Lake Mine in Northern Alberta, Canada. This research aimed to identify the geochemical implications of storing several different mine wastes and by-products together in integrated closure systems, with a specific focus on the behaviour of trace elements in associated porewater. To imitate such systems, layered assemblages of petroleum coke, centrifuged fine tailings (CFT), tailings sand, and peat-mineral mix soil (reclamation material) were placed in large, hydraulically contained lysimeters. Three layering schemes were chosen for the study: one with a reclamation material/petroleum coke cover overlying CFT; one with only petroleum coke overlying CFT; and one with uncovered CFT overlying a tailings sand underdrain layer–each having a duplicate to allow comparisons between fully- and partially-saturated systems. The geochemical behavior of each closure system was monitored with porewater samples obtained by continuous coring and by pumping multi-level piezometers installed throughout the depth of each lysimeter. Field measurements indicated that porewaters were sub-alkaline (7.7 ± 0.29 pH) with elevated specific conductivity (4.9 ± 1.9 mS cm 1). Oxidation-reduction potential typically decreased with depth, with fully-saturated systems exhibiting more reduced conditions than their partially-saturated duplicate. Porewater trace element compositions were dominated by Mo (220 ± 350 μg L 1), V (110 ± 200 μg L 1), As (59 ± 42 μg L 1), Ni (7.8 ± 13 μg L 1), and Se (4.5 ± 9.0 μg L 1). Additional sequential extraction studies showed that porewater elevated in exchangeable cations and inorganic anions produced during the dewatering of CFT could influence the release of trace elements bound to petroleum coke and associated mineral surfaces. The results of this study indicate that the dual-cover closure systems reduced trace element mobility at surface when compared with the single-cover systems, and that coke units in partially-saturated systems can accommodate dewatering of CFT within their pore space and reduce transport of trace elements to surface.