Below is a concept plan showing the area after rehabilitation. The open pit will become a lake, with an adjacent park, now the Surface Facilities Area. In order to achieve the final land use, the following activities will take place:
- Removal of buildings and plant
- Earthworks and recontouring
- Lake filling
Above: Planning for final rehabilitation is all part of initial planning for mining. When mining is complete, groundwater will return to a controlled level. The lake has been designed to be a major asset to the community. This is a conceptual plan for the future lake and recreation area.
In order to achieve the final land use, the following activities will take place:
Removal of Buildings and Plant
The crushers and conveyors will be demolished or removed as appropriate, as will other buildings, explosives magazines, fuel and oil tanks.
Earthworks and Recontouring
The present surface facilities area will be rehabilitated to form a park adjacent to the lake. The lake edge will be developed to create a shallow edge to the lake shore. This, in combination with the shallow area around the lake, will provide breeding and feeding areas for various species of fish such as eels and carp.
The crusher slot will be filled, and the surface facilities area covered with subsoil and topsoil as necessary and grassed. Walkways around the pit and the park, a boat ramp, an amenities block, access road and carpark will be constructed. Lookouts will be established around the perimeter of the pit, and a safety fence installed so that access to the pit lake and walls could only be gained through the park.
Lake Filling
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| Above: The present Surface Facilities Area will be rehabilitated to form a park adjacent to the lake. |
At closure, mine dewatering will cease, and due to inflows from both surface water and groundwater, the water level will begin to rise in the open pit. Left to fill naturally, it would take around twenty years to fill to the desired level. The actual time taken to fill the lake depends largely on rainfall. To accelerate the filling of the lake, water will be pumped from the Ohinemuri River during medium to high flows subject to a number of conditions. This will allow the lake to be filled within approximately five years, advancing the date at which the lake will become available as a recreational resource for the town. Stormwater runoff from the park area will be diverted to the lake.
Lake Level and Outlet
The lake level will be set at RL1104 (mine datum). Most of Waihi township within a 200 metre radius of the lake will be at least five metres above the lake level. A berm will be constructed at RL1103.5 to provide the following:
The lake level, and the variation in the lake level, will be controlled by constructing an engineered outlet from the lake to the Mangatoetoe Stream. The outlet will consist of a concrete intake structure, a tunnel, and a grassed open channel.
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2. Tunnel. 3. Open channel. 4. Mangatoetoe Stream. 5. Rehabilitated pit wall. 6. The Lake. 7. Moresby Avenue. 8. Savage Road. 9. Lookout Platform. |
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| Above: The present Surface Facilities Area will be rehabilitated to form a park adjacent to the lake. |
The concrete intake structure will be built on a pit bench. It will have a screen with a bar spacing of around 150mm to catch large debris and prevent human access. Provision will be made for slide gates to be put in place, to temporarily stop the flow when necessary for maintenance purposes.
Excess water will flow into the intake structure, and travel through a tunnel, approximately 135m long. The tunnel will end once construction is out of hard rock. The remaining 40 metres or so from the downstream end of the tunnel to the confluence with the Mangatoetoe Stream will be constructed as a grass-lined open channel.
The outlet will be constructed to allow migratory fauna such as eels to move between the lake and the stream. It will also be designed to minimise lake level fluctuations. Rises in the lake level have been modelled taking into account the probable maximum flood (PMF), together with extreme summer droughts. The modelling indicates that under PMF conditions, the lake level rise will be limited to one metre, and in an extreme summer drought, the lake surface would drop by about 0.25 metres. This is important for stability, and to maintain the littoral zone around the lake edge.
Lake Water Quality and Aquatic Biology
The lake will have an approximate area of 29 hectares and depth of 195 metres. The lake will be filled with the following types of water:
Because the pit slopes will contain areas of rock that are both potentially acid forming (PAF) and non acid forming (NAF), there is the potential for problems with the lake water quality. For this reason, modelling has been carried out to determine the lake water quality at filling, and into the long term.
As Ohinemuri River water will be pumped to the lake, the initial quality of the lake water will be determined by the quality of the river water. As the pit lake rises, much of the PAF rock on the pit slopes will be covered by the lake water, and therefore not exposed to atmospheric oxygen. This is an effective way of controlling the onset of acid producing conditions.
When the lake is full, there will be some PAF material exposed above the lake level. Taking into account the runoff from the park area PAF runoff will comprise only around 14% of the total surface runoff. While it can be expected that this runoff will consume some of the alkalinity within the lake water, groundwater, with its high alkalinity, will continue to flow into the lake. For this reason the final pH of the pit lake overflow will remain near 7.5, and the metals concentrations will remain low.
While the chemical composition of the lake is important, consideration has to be given to the interaction between the physical, chemical and biological processes within the lake. This determines the type of lake that develops and its possible uses.
An important feature of lakes is whether the waters fully mix, or whether they stratify (form layers). Modelling of the lake dynamics suggests that the entire lake will become fully mixed about every five years. In the intervening time, only the uppermost portion of the lake is expected to be mixed due to thermal and mechanical inputs. It is reasonable to assume that there would be annual mixing in at least the uppermost 20 metres of the lake, with the depth of mixing likely to reach 50 or 60 metres in most years.
The lake is predicted to be a productive (eutrophic) lake. This is primarily due to the quality of the dominant inflow, being the Ohinemuri River water. This water contains concentrations of nutrients adequate to stimulate the productivity of the lake water. Frequent mixing of the lake will result in nutrient laden waters being made available to phytoplankton (planktonic algae) in the upper waters. The long term scenario is that the quality of the lake water will improve as a result of a significant decrease in nutrient input to the lake following initial filling from the Ohinemuri River.
The lake will support populations of coarse fish, such as carp and eels, and wildfowl like ducks and shags. With a water clarity of 2.0 to 2.5 metres, the inshore lake water will appear quite clear while the deeper area of the lake will appear green, due to of the depth of the lake. It is likely that the lake will be used for non-motorised boating activities like rowing, canoeing and sailing.
The Pit Lake Formation and Discharge consents require a programme of monitoring pit lake water quality during filling and for a minimum period of five years after the lake first overflows. The monitoring programme is to include an assessment of the quality of the runoff from the pit walls, and an assessment of the aquatic life found in the lake.
Revegetation of the Pit Slopes
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| Above: Rehabilitation trials on batters around the open pit. |
Since September 1990, the pit slopes have been progressively hydroseeded. While hydroseeding has been successful in revegetating NAF areas, including oxidised andesitic rock and volcanic ash on the pit slopes, the low pH of some material has prevented successful revegetation to date. Experience shows that once grasses and legumes become established on the pit slopes, other species such as trees and shrubs will establish naturally in the area. Native trees and shrubs will be planted around the pit edge to provide seed sources to assist this natural process.
Around 25% of the pit slopes above lake level will be PAF, and as described above, the water quality modelling suggests that this will not affect the lake water quality. It is also important to consider what the pit slopes will look like in years to come.
If revegetation of the PAF areas does not occur, the pit
walls will consist of a mixture of bare rock and vegetation,
as shown in the concept plans for the lake. Road cuttings
in the area are similar in appearance.
The Company is continuing to trial ways of revegetating the
PAF pit slopes, in conjunction with soil scientists in the
Fertiliser and Lime Research Centre (FLRC) at Massey University.
Trials have been carried out using tyres, filled with soil,
to establish grasses and shrubs on these areas. The trials
to date have been successful, and accordingly a trial on a
larger scale is being planned. If these trials are successful,
it is possible that this method could be used to establish
vegetation on the PAF areas.
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| Above: Tree planting is an integral part of rehabilitation, over 200,000 native trees and shrubs have been planted on the site. Pasture growth trials determine the most productive species for post-mining activities. | |
