Rehabilitation: from tailings to pasture, plantings and ponds
Below is a concept plan showing what the waste rock embankments may 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. As required, 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 from 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.
Native trees and shrubs have been planted on various 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 continue.
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.
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 metre subsoil layer and a 0.1 metre topsoil layer, similar to the embankments. This will be sown in pasture, and a littoral zone will be planted between the pasture and the pond edge.
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. Tailings Storage 2 was decommissioned for tailings deposition in 2005. Water quality in the Tailings Storage 2 pond has reached a level suitable for direct discharge to the river without the need for treatment. 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 1A, and the results are considered to be similar to those for Storage 2.
Four types of water are expected to mix in the tailings ponds at closure:
- an initial pond of decant water
- annual precipitation (total rainfall minus total evaporation)
- overland flow of surface water into the pond
- upward directed flow of pore water which occurs as the tailings consolidate.
At present, the upstream diversion drain catches any runoff from the adjacent hill and diverts it from the tailings ponds.
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, as experienced at Tailings Storage 2. Modelling indicates that the water in Storage 1A will be suitable for discharge to the Ohinemuri River approximately three years after tailings deposition ceases. Experience indicates that this may occur in a shorter period of time.
Aquatic biology of the tailings ponds
When water quality of the tailings ponds improves this water is discharged without having a significant effect on the water quality, or the aquatic biology of the receiving water.
In the longer term the pond outlet structures will 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 and wetlands. Fish that are good climbers will be able to access the ponds from the Ohinemuri River. The 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.
A variety of birdlife inhabits the tailings ponds and waste rock embankments. Ducks, a dabchick and dotterels are pictured below.