Tailings storage facilities
The tailings are stored in impoundments (tailings ponds) created by embankments constructed from waste rock. The embankments and impoundments are referred to as tailings storage facilities (TSF). Storage 2 commenced construction in 1987. Storage 1A commenced construction in 1999.
Placement of tailings to TSF 2 stopped in 2005 (The name may be somewhat confusing; although it is named number two, it was actually the first dam to be built and filled.) Only rainwater now enters the impoundment area at TSF 2.
For many years the dam has been a habitat for ducks, swans, geese and other birds. As expected, since TSF 2 was decommissioned, water quality has improved is now capable of supporting aquatic life. This improvement has been achieved in less than three years, the somewhat conservative prediction.
The water quality has improved to the extent that TSF 2 is able to discharge directly to the river, and has been doing so since November 2007 with the approval of the Waikato Regional Council.
The water is currently pumped to control the flow via a tributary into the Ohinemuri River. A weir allows the water to flow into the river once the pond reaches a set level.
The outflow is continuously monitored for turbidity, conductivity and pH, and is periodically tested for a wide range of parameters to ensure that its quality remains suitable for discharge.
Having TSF 2 reach this stage was an important milestone for the mining operation and in terms of closure. The lessons learned from water management here will be used in the continued operation and eventual closure of TSF 1.
The principal features of the tailings storage facilities:
- the embankment structures are engineered and constructed from mine waste rock
- waste rock is transported from the mine site to the waste disposal area by conveyor. From there it is transported by dump trucks and selectively placed to build the embankments
- the geometry of the embankments has been designed to accommodate all waste rock and tailings from the entire project, allowing some contingency
- the embankments abut rising ground to form large impoundments which contain the tailings pond. Tailings are pumped as a slurry into the tailings pond from the processing plant
- subsurface drainage intercepts tailings seepage, leachate from waste rock and groundwater. These drains include underdrains beneath the tailings, an upstream cutoff drain along the upstream toe of the embankment, an initial toe drain and downstream toe drain, and gully subsoil drains. In addition leachate collection drains are present within the embankment
- diversion drains above both tailings ponds intercept clean surface runoff from the adjacent hill and direct it to nearby streams to reduce the amount of water entering the tailings pond
- the tailings storage facilities are surrounded by perimeter roads, and a perimeter drain. This drain collects surface runoff from the embankment and directs it into collection/silt ponds. Depending on the water quality, this water is either discharged to natural water or pumped back to the water treatment plant
- in addition, decant water from the top of the tailings pond, and water from the underdrainage system is either reused for processing of ore, or treated and discharged along with water from mine dewatering.