Hydrodynamics of a large oil sand tailings impoundment and related environmental implications

Abstract

Oil sand mine tailings are primarily contained by dykes constructed of permeable sand tailings. The environmental impacts of process waters released from these tailings facilities are controlled by groundwater flow within the impoundment structures. Transient groundwater flow characteristics were assessed within the Tar Island Dyke structure at the Suncor Inc. site near Fort McMurray, Alberta, to assist in quantifying potential long-term environmental impacts. A sequence of low-permeability fine tailings underlying the tailings water pond is supported by coarse sand tailings that are connected to and form the containment dyke structure. A clay unit separates the foundation from the underlying limestone that is hydraulically connected to the Athabasca River. The primary groundwater flow pathways are through the toe of the dyke and through the foundation. Field data and numerical simulations indicate that drainage of process water within the structure remains transient yet decreases over time. The fine tailings and pond become perched atop unsaturated coarse tailings, significantly restricting seepage and producing an environmental legacy with reclamation implications. The release of potentially contaminated process water continues to decrease, resulting in diminishing environmental risk. The lowering of pore-water pressures within the main dyke has increased its strength and long-term stability.