End pit lakes are common at open-pit metal mines around the world and are part of mine closure plans. Those at oil sand mines are no different except that the end pit lakes will be considerably larger, with the area averaging about 4 km2 and reaching up to about 15 km2. In a major study on end pit lakes for oil sand mines, the Cumulative Environmental Management Association defines an oil sand end pit lake as ‘an engineered water body, located below grade in an oil sands post-mining pit’. It may contain oil sand by-product material and will receive surface and groundwater from surrounding reclaimed and undisturbed landscapes. End pit lakes will be permanent features in the final reclaimed landscape, discharging water to the downstream environment. As a permanent feature, the long-term environmental effect of such an oil sand deposit must be carefully designed and monitored. If the end pit lake contains an appreciable thickness of tailings, the consolidation of tailings may continue for many decades. The effects of groundwater leakage are analysed and modelled to show the potentially large amounts of seepage into the underlying stratigraphic units.