3.2 Hydrodynamic Containment

When siting a tailings storage facility and in the assessment of potential seepage pathways beyond the proposed facility, the concept of hydrodynamic containment is important. If the elevation of the water table along the lateral flanks of the valley in which a tailings facility is located is maintained by local groundwater recharge at an elevation well above the ultimate pond level in the tailings facility, there will be an inward hydraulic gradient on the valley flank toward the pond that precludes supernatant water from flowing into the adjacent valleys by passing beneath the bounding ridge line. This situation is referred to as natural hydrodynamic containment. For example, Figure 9 shows a valley that at one time was considered as a potential site for a tailings storage facility. As long as the elevation of the tailings pond was well below the ridgelines in this valley, the only direction of seepage outflow would have been down the valley, at the location of the containment dam. Seepage control measures would need to be implemented only at the foundation and abutments of the proposed dam. During site investigations, water levels measured in piezometers installed along or near the ridgelines are key components in a field program to map the elevation of the water table and define its seasonal variation. If the ultimate pond level in the tailings storage facility is planned to be above the seasonal low elevation of the water table in the bounding ridges, as the pond levels rises to that elevation, conditions are established where natural hydrodynamic containment no longer exists and outward flow through the bounding lateral ridge becomes possible.

Figure 9 – Illustration of natural hydrodynamic containment that would be provided by an elevated water table beneath the lateral ridgelines of this valley.

In flat terrain, the absence of topographic relief precludes reliance on natural hydrodynamic containment for seepage control. The hydraulic gradient is everywhere outwards from the tailings facility. When slurry tailings are placed within a ring dyke, infiltration of water through the base of the facility contributes to the development of a water table mound superimposed on the regional water table profile. The requirements for and design of potential seepage control measures need to be evaluated along the full perimeter of the tailings storage facility. The control measures must be considered in conjunction with the interplay of the induced water table mound and the local and regional scale groundwater flow systems.