Lessons from Tui mine
Whenever anyone wants to point to the toxic legacy of the mining industry, Tui mine on the slopes of Mount Te Aroha is always at the top of the list. Even mining companies agree, Tui mine is a significant environmental issue. The abandoned underground workings and exposed tailings dam are leaching base metal sulphides into surrounding streams, resulting in localised and downstream contamination. Until recently the site was regarded as the most contaminated in New Zealand. A programme of remediation is now complete.
How did it come to this, and could modern mines such as the Martha open pit and Favona or Trio underground mines at Waihi end up in the same condition?
The Tui Mine site is located on the edge of the Kaimai range in the Waikato. The site is within the catchments of the Tui and Tunakohoia streams, both of which flow into the Waihou River at the base of Mount Te Aroha. The mine opened in 1884. Miners worked the base metal sulphide deposit that contained copper, iron, nickel, lead, zinc and other heavy metals. The lead was used in gold refining in Waihi. Contrary to popular belief, Tui mine was never a gold mine in these early days, although some ore was mined. The mine first closed around the end of the nineteenth century
Tui reopened in 1967. Norpac Mining Ltd extracted copper, lead and zinc. The mine also produced some gold and silver. Unacceptable levels of mercury were found and the Japanese company buying the ore pulled out in 1973. Norpac Mining went broke and the site was abandoned. By this time 160,000 tonnes of ore had been mined from underground, producing 20,000 tonnes of copper, lead and zinc concentrate. 2.5 tonnes of gold and silver concentrate were also produced.
Over the past 30 years discharges of heavy metals and acid rock drainage from the abandoned site have left surface water toxic to aquatic life and unsafe for recreational activities. The source of these contaminants is the sulphide minerals in the rock that oxidise over time and are then leached by rainfall forming acidic runoff containing elevated concentrations of copper, lead, zinc and other trace elements.
How could the mining company just walk away? Forty years ago there was no Resource Management Act. There were no bonds required. No closure plans had to be developed prior to mining starting or implemented after closure. The industrys and general communitys view of the environment was very different to what it is today.
In contrast, the Martha, Favona and Trio mines at Waihi typically contain low concentrations of sulphides, particularly of copper, lead, zinc and other trace elements. Newmonts mines must also comply with a range of legislation including the Resource Management Act and bonding requirements to ensure there is never another Tui.
What happens when a modern mine is closed? Consent conditions require that the site is left in a safe, stable, self-sustaining rehabilitated state is At Martha Mine these conditions will have been met when:
- all structures (such as pit walls and tailings impoundments) are shown to be stable.
- any water discharging from the site, and any groundwater under the site, will be of a quality that will not affect aquatic life, or other users of the water resource.
- all revegetation is complete, and monitoring demonstrates that the site is sustainable.
Once closure of the site is complete, the post closure period commences. At this time, a charitable trust will manage areas such as the Tailings Storage Facilities and Water Treatment Plant and take title to the land upon which they stand. The Trust will also take title to the proposed park adjacent to the lake, and maintain it for general use by the public. The Trust will also be responsible for the monitoring and maintenance of the lake. A sum of money will be handed over by the Company, and the interest generated from that money would allow the Trust to manage, monitor and maintain the land, and provide the necessary insurance cover.
The company is required to have in place a rehabilitation bond to cover the costs of closure whenever that may occur. The amount of the rehabilitation bond is reviewed annually, and this bond is only released once closure of the site is completed. The rehabilitation bond provides certainty. In the unlikely event that mining finishes early, and the mining company walks away from the site, funds will be available to ensure that rehabilitation is completed. A capitalization bond is also required. This bond ensures that a sum of money will always be available to allow the Trust to carry out its activities.
Tailings impoundment construction
There is also a significant difference in the way the Tui and Martha tailings storage areas have been constructed. At Tui, the tailings are stored behind a simple bund at the head of a steep valley. At Martha the tailings storage impoundment walls are carefully engineered structures constructed in zones and feature a comprehensive drainage system to capture any seepage.
The Tui tails impoundment would be endangered by a moderate seismic event or an extreme weather event. The tailings impoundments at Waihi are designed to withstand what engineers describe as a Maximum Credible Earthquake (MCE) based on a seismic hazard study of the site in accordance with international guidelines. The MCE has been conservatively assessed to be a magnitude MW 7 earthquake at a distance of nine kilometres from the site. The embankments are designed as earth/rockfill water-retaining structures. This is conservative, as with time the tailings will consolidate and essentially become soils with inherent shear strength, unlike water, which is only kept in place by the dam walls.
The embankment crest level is designed to provide a safe height above the tailings plus stored water level. Storage capacity is provided for a 1200 mm rainstorm, (the Probable Maximum Precipitation), plus an additional 1.0 metre minimum freeboard. The downstream shoulder of the embankment is considerably flatter than would normally be the case for water retaining embankments because of the need to dispose of large quantities of waste rock. The slope of the downstream shoulder of Storage 1A is typically 3.2:1. This compares to 2.2:1 for the Benmore Dam, which is a large water storage dam used for hydro-electric power generation, and the highest hydro dam in New Zealand
What about the risk of a really big earthquake in Waihi? Risk can be defined as the potential for an activity to have an undesirable outcome. Risk assessment procedures quantify the hazards presented by a situation and identify any mitigating actions required to lessen the risk. For example, recent reports suggest that there is the likelihood of a very large earthquake happening off the coast of Chile every 50 to 100 years. It is predicted that such an earthquake would create tsunami waves travelling thousands of kilometres across the Pacific Ocean, hitting Auckland's east coast and inundating land up to 3.5m above sea level. Low lying areas immediately adjacent to the Pacific Ocean, such as the east cost of the Coromandel Peninsula would most likely get higher water levels. Do we decide to evacuate all inhabited areas on the east coast of New Zealand because of this risk? No, but we do make preparations such as installing tsunami warning sirens, having Civil Defence practices or knowing the best route to take to higher ground. We mitigate the risk by taking appropriate action. We do this every day in everything we do. Will that car have time to stop if I step out onto the pedestrian crossing now? The speed limit is 100 km/h, but its raining really hard so I will slow down. Ive got a frying pan on the stove so I need to stay in the kitchen.
The risk of an earthquake breaching the Waihi tailings impoundment is considered to be very low. In designing the project, the risks have been identified, and where possible, the risks have been minimised by allowing for them in the project design. The risks have been engineered out. For example the open pit slopes and the tailings storage facilities have been designed to achieve long-term stability, and the risk that remains is minimal.
This 'residual risk' is covered by the monies and insurances held by the Trust. Risk assessments have been carried out on how the tailings impoundment area will be managed after mine closure. A residual risk assessment has been undertaken to determine what risks remain once closure is complete, and to determine the costs of remediation and/or clean up in the unlikely event of a problem. Some risks are insurable, and sufficient money has been put aside for insurance purposes. Some risks are not insurable, and the costs of dealing with the consequences of those risks are taken into account in determining the amount of money that the trust needs to carry out its tasks. Unlike Tui mine, the cost of any maintenance or remediation work at Martha and Favona will not be passed on to the public.
There is no doubt, however, that Tui mine remediation will be costly. In one way or another we all pay. Its a legacy from a bygone era of different laws, rules and views that needs our attention now.
Can it happen again?
It is understandable that the general public look at Tui mine and ask if it could happen again. A simple analogy would be that to compare the Tui mine and tailing area with the mines in Waihi is much the same as comparing a car from forty years ago with one produced today. To the uninitiated they might look similar, they both have four wheels and doors and a steering wheel, but their design, construction, environmental considerations and safety features are worlds apart.
For more information on earthquakes and tailngs dams click here