Rehabilitation: from open pit to recreational lake
Pictured here is a concept plan showing the open pit area after rehabilitation. The open pit will become a lake, with adjacent parklands. In order to achieve the final land use, the following activities will take place:
- removal of buildings and plant
- earthworks and recontouring
- lake filling
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
Areas surrounding the open pit will be rehabilitated to form parklands, where appropriate, adjacent to the lake. Some shallow areas 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 through some parkland have already been constructed and lookouts have been established around the perimeter of the pit.
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 more than 20 years to fill to the desired level. 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.
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:
- safety, so that anyone falling into the lake could swim to the edge and stand up
- a base for the concrete outlet structure
- a littoral zone around the edge of the lake which is important for plant establishment and aquatic biology.
The lake level, and the variation in the lake level, will be controlled by 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.
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 31 hectares and depth of 195 metres. The lake will be filled with the following sources of water:
- groundwater, which will begin to flood the pit once the dewatering pumps are decommissioned
- net precipitation (i.e. total rainfall minus total evaporation)
- the flow of water along the batters of the pit and the adjacent parklands
- water from the Ohinemuri River.
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 areas 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 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.
Lake water quality and aquatic biology
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.
We are continuing to trial ways of revegetating the PAF pit slopes, in conjunction with soil scientists in the Fertiliser and Lime Research Centre 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 it is possible that this method could be used to establish vegetation on the PAF areas.