The sources of excess water on site are summarised below:
Mine water is derived from the dewatering of the open pit (1 and 2) and represents one of the major sources of water on site. It comprises both groundwater (1) inflow into the open pit, and surface water (2) generated both in the pit and in the adjacent surface facilities area. Mine water is characterised by the presence of sulphates and metals, predominantly iron and manganese, in solution.
Consents allow up to a maximum of 15,000 cubic metres a day of water, or an annual average of 10,000 cubic metres per day, to be pumped from the open pit.
Decant water, water collected from the surface of the tailings pond, (3) consists of rain water combined with water from the tailings (4). This water source is the major flow requiring treatment for removal of cyanide and metals, particularly copper, complexed with cyanide derived from the tailings.
An extensive underdrainage system exists beneath the waste rock embankments (5). The underdrainage system collects both upward rising groundwater beneath the tailings storage facilities, and drainage from the tailings and the embankments. Initially, underdrainage tends to contain a high proportion of tailings seepage. Over time, as the tailings depth increases and the permeability decreases, the volume of tailings seepage decreases. There is an accompanying decrease in the cyanide concentration so that after a period of time, underdrainage water does not need to be treated for cyanide destruction at the water treatment plant.
Stormwater runoff is generated in various areas around the site, and depending upon its quality, the water is collected in collection/silt ponds (6). In Waihi, storms can occur that are of high intensity and short duration. In these circumstances the rivers and streams can flood rapidly, and it is not practical to contain all of the stormwater runoff on site. The objective is to provide sufficient containment systems to ensure that in the worst case, discharge arising from stormwater runoff from the site does not cause significant adverse effects on the receiving waters.
Collection ponds capture water that may require treatment. For example, collection ponds collect water from:
- around the processing plant (7) and water treatment plant areas, where chemicals are used and there may be a risk of spillage outside bunded areas
- around the base of the tailings storage facilities, where runoff from potentially acid-forming (PAF) material may be unacceptable for direct discharge if, for instance, liming of the PAF material was inadequate.
Water collected from the above sources is tested and if it meets the discharge criteria, it is released into the Ohinemuri River. Water which requires treatment is directed to the water treatment plant (8) to ensure it meets the required standards prior to release (9).
The collection ponds are sized to collect the runoff arising from a 10 year, 72 hour storm, and appropriate pumping systems are put in place to direct the water to the water treatment plant. This is to ensure that, in the worst case, discharges from the collection ponds will not elevate in-river metals concentrations to above the USEPA chronic guideline values, and therefore will not cause significant adverse environmental effects on the river or its aquatic biology.
In practice, the collection pond water quality is greatly improved by regular liming and sealing of PAF areas, and by carrying out progressive rehabilitation. Collection ponds that contain acceptable water quality can be reclassified as silt ponds, and the water can be discharged directly to the river. This, however, is subject to continuous water quality monitoring being carried out to prevent an out-of-specification discharge.
In contrast to the collection ponds, silt ponds tend to pick up runoff that may carry suspended solids, but that does not generally contain chemicals or metals. Often silt ponds exist in areas of general construction where no PAF material is present in the catchment, and the silt ponds are similar to those that can be seen in other construction areas, like housing subdivisions. The suspended solids can be removed by gravity settling within the silt ponds. Based on settling trials, the silt ponds are designed to provide a retention time of two hours for runoff generated during a two year return period storm event.
In storm events greater than the two year, two hour storm, the silt ponds will overflow. However, as described in the water treatment plant section, the sediment solids concentration in the Ohinemuri River increases as the river flow increases. The compliance limits placed on suspended solids concentrations from silt ponds take this into account.
Monitoring and inspections are carried out on a regular basis to ensure that the water contained in both the silt ponds and the collection ponds is being managed appropriately. Silt collects in both the silt ponds and the collection ponds and this needs to be cleaned out on a regular basis to ensure that the ponds continue to operate efficiently.
The water management system
The water management system operates to:
- reduce the volumes of water that require treatment
- to reuse and recycle water where possible
- to provide adequate containment systems and alarming to minimise the risk of an out-of-specification discharge
- provide buffer storage and pumping facilities to ensure that the site can cope with large rainfall events
- manage the flows to the water treatment plant to ensure that the above objectives are met
- treat water at the water treatment plant and the reverse osmosis plant, and discharge the treated water to the Ohinemuri River, subject to a number of conditions as stipulated in the resource consents
- carry out monitoring and inspections to ensure that the objectives of the water management system are being met, and that compliance with the relevant consent conditions is being achieved.
Reduction, reuse and recycling of water
To minimise the volumes of water requiring on-site management and treatment, best practice dictates the diversion of natural water away from active working areas where practical. For example, upstream diversion drains exist above the tailings storage facilities. These drains capture clean runoff that would otherwise enter the tailings pond, increasing the quantity of water that requires management.
Water is re-used on site where possible, often for dust control and irrigation. For example, mine water is reticulated around the open pit where it can be used to dampen down working areas. Mine water is also used in the water carts for dampening haul roads. Water is reused in the vehicle washing facilities on site, for conveyor belt washing and for general wash down in areas where this is needed. Clean water from springs and diversions is captured and used for drinking water for stock.
Where possible, water is recycled on site. The most obvious example of this is the decant water from the tailings pond, which is recycled through the processing plant. Underdrainage water can also be used for this purpose.
The end result is that minimal amounts of water need to be derived from sources outside the the mine. External water sources include:
- water drawn from the Ohinemuri River for use in the elution circuit
- water drawn from the town water supply, for use in amenities blocks and tea rooms.
Containment systems and alarms
- tailings from the processing plant to the tailings pond
- out-of-specification water from the water treatment plant to the tailings pond
- stormwater runoff from around the processing plant and water treatment plant to the tailings pond
- decant water from the tailings pond to the processing plant and the water treatment plant
- underdrainage water from the tailings storage facilities to the water treatment plant, and the processing plant.
These pipelines cross the Ohinemuri River adjacent to the Mill Bridge. Measures are in place to ensure that, in the event of leakage or breakage from pipelines, the material cannot spill into the river. These measures include:
- the use of trenches and contingency ponds, and pumping arrangements. These are sized to contain spillage from the entire pipeline, assuming that such a spillage may occur during a 10 year, 72 hour storm event
- the use of float level alarms on the contingency ponds. These provide warning that the ponds are filling, and that action needs to be taken to prevent an out-of-specification discharge
- the use of flow differential alarms on pipelines, to indicate when the flows along the lines are significantly different and that a spill from the pipeline may be occurring
- regular inspections and audits to ensure that the systems described above are performing appropriately.
Buffer storage and flow management
An important part of the water management system is to ensure that sufficient buffer storage exists to:
- prevent overtopping of the tailings pond
- prevent out-of-specification discharges from the collection ponds
- prevent flooding of the open pit.
In order for sufficient buffer storage to be maintained, some planning needs to be incorporated in the water management system. Such planning needs to take into account the mine schedule, which will dictate the predicted depth of mining within the open pit and consequently the required dewatering rate. Similarly, the water management system needs to take into account the construction schedule for the waste rock embankments, and the expected rate of tailings production. This will ensure that adequate freeboard is maintained on the tailings pond. The planning also needs to take into account the river flow. In summer, when the river flows are lower, the amount of water that can be discharged from the water treatment plant also reduces.
The weather in Waihi can be unpredictable, and site water management needs to be conservative and flexible to allow changes to be made that are appropriate to the weather and operational requirements. Similarly, sufficient capacity needs to be provided, particularly in the collection ponds, to prevent out-of-specification discharges. For this reason, pumping facilities must be sized adequately to allow pond levels to be reduced quickly, in response to rainfall and river flow.