Solution Exercise 4
Recharge on the eastern flank of the valley maintains a minimum water table elevation at the drainage divide of approximately 80 m. On the western flank of the valley, recharge maintains a water table divide at an approximate elevation of 92 m. Therefore, if the elevation of the tailings pond rises above an elevation of 80 m, there will be an outward hydraulic gradient driving groundwater flow from the pond to the valley to the east of the tailings impoundment. Groundwater recharge on the west side of the valley maintains a water table elevation significantly above the proposed pond elevation, so a hydraulic gradient toward the pond is maintained, as is hydrodynamic containment. On the eastern side of the facility, an outward hydraulic gradient is established as the water table comes into equilibrium with the rising pond level, hydrodynamic containment is overcome, with flow toward the valley to the east. If the pond elevation were limited to a height of say, 75 m, then both sides of the valley would provide hydrodynamic containment.
It is useful to note that an assessment of this nature needs to be based on water table elevations in the dry season, because they reflect the lowest elevations and so, represent the most vulnerable condition.