In many mines throughout the world, waste rock and tailings are disposed of separately to form waste dumps and tailings dams. At Waihi the situation is unusual, in that waste rock is used to form engineered embankments that are progressively raised in order to ‘keep up’ with tailings production. This allows for sufficient storage capacity to ensure that the water within the tailings pond will be contained, even in extreme rainfall events. Storage capacity is provided for a 1200 mm rainstorm, (the Probable Maximum Precipitation), plus an additional 1.0 metre minimum freeboard.
There are two tailings storage facilities, known as Storage 2 and Storage 1A. Construction of Storage 2 commenced in 1987, and has reached its final crest height. It was decommissioned from tailings deposition in 2005. A substantial database spanning more than twenty years demonstrates that the performance of Storage 2 has been very good. The design of Storage 1A is based largely upon the design of Storage 2. While there are some technical differences, the description of Storage 2 can also be applied to Storage 1A.
A number of environmental considerations have been taken into account in planning and designing the waste rock embankments and tailings storage facilities:
- identification of the best location for the tailings storage facilities, taking into account the status of the land with respect to ownership, geology, archaeological features, flora and fauna
- the design and construction of the embankments, considering short and long term structural stability
- ways that potentially significant adverse effects such as noise, dust, visual effects, acid drainage, and cyanide can be avoided, remedied and mitigated
- the surface and subsurface drainage systems required to ensure that potentially contaminated water can be collected and managed
- the water management and water treatment facilities required to ensure that there are no significant adverse effects on the surrounding rivers and streams
- the requirements for rehabilitation and closure of the site.
To address these issues, advice has been sought from a team of experts including archaeologists, geologists, hydrogeologists, engineers, geochemists, rehabilitation consultants and landscape architects.
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 and 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.
A typical cross section through a tailing storage facility.
- Natural ground
- Ponded water
- Tailings beach
- Embankment crest
- Zoned Waste Rock Embankment structure
- Consolidated tailings.
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.