Groundwater flow and salt transport in an undrained tailings sand dam is investigated at Syncrude Canada's Mildred Lake Oil Sands mine, in northeast Alberta. Two dimensional groundwater flow and salt transport are characterized using field data from two detailed piezometer transects. Calibrated steady-state groundwater flow and transient salt transport models simulate existing and future flow systems and flushing of process water. Dyke topography creates nested flow patterns, which are modified in some cases by variations in hydraulic conductivity. Greater relief of the backward-sloped bench design compared with forward-sloped benches results in larger local flow systems, a deeper water table, flushing of process water and focused discharge. Under the existing flow conditions captured by the model, salts will flush in decades at the local scale (bench) and centuries at the intermediate scale (perimeter dyke). The future flow regime will depend strongly on recharge rates across the reclaimed dam.