At Storage 2 an extensive network of subsurface drainage was constructed beneath the tailings pond and embankment to intercept and control seepage from the tailings and leachate from the embankment, as well as naturally occurring groundwater flows. A similar underdrainage system has been constructed for Storage 1A. The drains include:
- tailings underdrains, located beneath the tailings, that intercept downward seepage from the tailings and shallow groundwater flow
- upstream cutoff drain, incorporated in the upstream side of zone B at foundation level that functions to intercept tailings seepage and shallow groundwater flow
- initial toe drain, located along the downstream toe of the initial (first stage) of the embankments that intercepts seepage through the surficial soils or bedrock beneath the initial embankment
- toe drain, located along the downstream toe of the completed embankments: this drain is in contact with bedrock and functions to intercept shallow seepage through the surficial soils and upward rising groundwater flow through the bedrock
- gully drains located along the centreline of gullies that underlie the embankments, that intercept any seepage from the embankments as well as groundwater seepage; they also initially assist in draining and consolidating any alluvium left in place in the foundations
- leachate drains that collect seepage from the PAF rockfill zones within the embankments during construction, and after closure, from the bulk fill zone.
All drains connect to a series of sumps located around the perimeter of the embankments, where flows can be measured and samples taken for chemical testing. The combined underdrainage water is either sent to the water treatment plant, or recycled through the processing plant.
The subsurface drainage system for Storage 2 is shown below.
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