To check the performance of the subsurface drainage systems at the tailings storage facilities, groundwater is monitored in both shallow and deep wells constructed downstream of the tailings storage facilities. Pneumatic and electric piezometers and standpipe piezometers, installed in the embankments also assist in assessing the performance of the subsurface drainage.
Right: A schematic diagram showing typical shallow and deep groundwater monitoring wells.
1. Shallow well
2. Deep well
3. Well cap
4. Shallow aquifer (ash/alluvium)
5. Deep aquifer (bedrock)
6. Shallow water level
7. Deeper water level
8. Seals (cement grout/ bentonite pellets at base)
9. Filter (fine gravel with sand at top)
10. Screen (slotted PVC pipe)
The shallow wells allow the groundwater to be monitored in the alluvium and ash materials that overlie the bedrock. The deep wells allow the groundwater in the underlying, lower permeability bedrock to be monitored.
In the past, several of the monitoring wells have shown intermittent and minor effects, principally in the shallow groundwater system. These effects were generally derived from ponded areas, particularly collection ponds, which previously were not lined.
As a consequence of the intermittent and minor effects seen in the shallow groundwater system, the collection ponds and perimeter drain are now lined. Monitoring of the groundwater continues on a regular basis.