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| Above: A conceptual
plan for the rehabilitated waste disposal area . |
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| Above: Rehabilitation
in progress on the waste rock embankment. Waste rock, topsoil, pasture, and a native species area can all be clearly seen. |
On the right is a concept plan showing what the waste disposal
area will look like once rehabilitation is complete.
Waste Rock Embankments
As the embankments are constructed, they are progressively rehabilitated to pasture. This involves applying a 0.5 metre layer of subsoil material followed by a layer of topsoil, approximately 0.1 metre thick. The upper 0.15 metres of subsoil is modified with lime and superphosphate. The topsoil is fertilised with potassic superphosphate at conventional rates required to ensure successful pasture establishment on these volcanic soils. If necessary lime is applied. A standard agricultural seed mix is used. Low ground pressure agricultural machinery is used for soil conditioning and seeding, as opposed to mining machinery that would over compact the soil. The land is grazed by young dry cattle.
Earthworms tend to move out of topsoil stockpiles and for this reason, earthworms must be re-introduced to the area. Earthworm seeding is carried out on all rehabilitated areas 18 to 24 months after pasture establishment.
The success of the pasture establishment is monitored by regular soil testing, and by comparing the pasture dry matter production on the rehabilitated areas with an adjacent area undisturbed by mining. These measurements confirm that the rehabilitated pasture is performing well. The first rehabilitation took place in 1991, and there is now a substantial amount of data to measure performance.
Native trees and shrubs have been planted on some areas of the embankments. These plantings provide a food source and nesting sites for birds as well as preventing 'tracking by cattle' on the steeper slopes. While most of the plantings are not above PAF rock areas, investigations have been carried out to determine the rooting characteristics of the species used. Species have been chosen that do not have tap roots, and it was found that the roots were concentrated within the subsoil and topsoil layers, and they did not penetrate the zone G cap. Further investigations are planned.Collection Ponds
On completion of rehabilitation of the waste rock embankments, the collection ponds will be collecting only clean runoff and therefore can be filled in. Outlets will be reconstructed to allow water to flow from the perimeter drain to adjacent water courses without causing erosion.
Tailings Capping
When tailings are deposited as a slurry, they immediately begin to consolidate. Because the grains will be finer, consolidation will increase further away from the embankment deposition points. For this reason, the tailings rehabilitation will involve a combination of a dry cover and a permanent water body. The dry cover will be placed adjacent to the embankments. Acid drainage control will be achieved by adding additional limestone to the tailings surface, and by achieving high levels of saturation within the tailings.
The capping layer will consist of a running surface with a minimum thickness of 0.6 metres, covered by a layer of growing medium. This consists of a 0.5 metres thick subsoil layer and a 0.1 metres thick topsoil layer, similar to the waste rock embankments. This will be sown in pasture, and a littoral zone will be planted between the pasture and the pond edge.
Above: A conceptual cross-section
of the littoral zone planting at the tailings storage facilities.
1. Grassed waste rock embankment crest. 2. Capping. 3. Planting
of species such as cabbage trees, raupo and flaxes to provide
nesting cover and food for birds. 4. Maximum pond level. 5.
Consolidated tailings. 6. Waste rock embankment.
Tailings ponds
When tailings deposition ceases, there will be an initial period in which the water quality within the pond will require monitoring and treatment. During this time, the tailings pond will be pumped to a low level to assist the tailings surface to gain strength. At Storage 2, while the tailings pond is being pumped low, a discharge channel and variable height weir will be constructed to allow the pond to overflow to a tributary of the Ohinemuri River. It is possible that the tailings pond of Storage 1A may be directed to the Storage 2 tailings pond. Once dischargeable water quality is achieved, the decant system will be removed and the pond allowed to rise, with its surface level controlled by the discharge channel and weir. The tailings will continue to consolidate for a number of years, deepening the pond.
Modelling of the pond water quality was carried out for Storage 2, and the results are considered to be similar to those for Storage 1A.
Four types of water are expected to mix in the tailings ponds at closure:
At present, the upstream diversion drain catches any runoff from the adjacent hill and diverts it from the tailings ponds. This situation is expected to continue until the water quality in the tailings ponds is satisfactory for direct discharge into the Ohinemuri River.
When tailings deposition ceases there will be some upward directed flow of pore water as the tailings consolidate. Consolidation upflow will reduce quickly with time and it is expected that the water quality will also improve quickly. Modelling indicates that the water will be suitable for discharge to the Ohinemuri River approximately three years after tailings deposition ceases. Experience at the nearby Golden Cross Mine indicates that this may occur in a shorter period of time.
Aquatic Biology of the Tailings Ponds
After approximately three years, the water quality of the tailings ponds will improve and this water will be able to be discharged without having a significant effect on the water quality, or the aquatic biology of the receiving water.
The pond outlet structures will be constructed to allow fish passage between the ponds and the Ohinemuri River. The ponds will be able to support a range of aquatic organisms, including submerged and emergent plants, insect larvae, eels and waterfowl, typically found in such pond-like environments. Fish that are good climbers will be able to access the ponds from the Ohinemuri River. The proposed riparian planting adjacent to the pond edge will trap sediment and nutrients in the runoff waters and assist in maintaining the quality of the ponds at a level suitable for a range of aquatic organisms.
