Denis Tegg: How earthquake safe are the mine tailings dams at Waihi?

Written By: - Date published: 9:00 am, December 10th, 2016 - 18 comments
Categories: disaster, Environment, Mining - Tags: ,

Reprinted with permission from Denis Tegg’s blog Tegg Talk.

The two tailings dams near Waihi are some of the largest structures in the country.

Should either of these dams fail in an earthquake, we could be facing potentially New Zealand’s worst ever environmental disaster, and loss of life and livelihoods on a significant scale.

Therefore it is perfectly reasonable to ask the question – would these structures withstand a large earthquake? The question is even more pertinent because –

  • the Kaikoura quake unleashed thousands of huge landslides
  • a new GNS Science report on the nearby Kerepehi fault suggests a 7.4 magnitude earthquake is possible near Waihi, and also estimates that the interval between large quakes is 1000 years rather than 6000-8000 years as previously assumed.
  • the catastrophic failure of a tailings dam in Brazil in 2015 was due in part to a very small 2.6Mw quake acting as a triggering mechanism for the dam collapse

The huge number and the scale of the landslides triggered by the recent Kaikoura earthquake is astounding.

In 1996 a major landslide under the Golden Cross tailings dam at Waitekauri near Waihi threatened to cause the dam to fail and spill its contents into the valley below. A civil defence emergency was declared. The mining company – Coeur Gold – was forced to closed the mine permanently and to spend around $NZ30 million on stabilization work.

The conclusion to the 2016 GNS Science report on the Kerepehi fault states:

In the towns of the Hauraki Plains, in close proximity to segments of the Kerepehi Fault, the earthquake intensities are expected to reach MMI 9 (“destructive”) on some occasions and perhaps even MMI 10 (“very destructive”) … The risk is therefore at a level where buildings with high occupancy rates or which house critical facilities should undergo strength checks”.

A map presented to a Hauraki District Council civil defence workshop shows the proximity of Waihi and Waitekauri tailings dams to the nearest segments of the Kerepehi fault.

The design of the tailings dam at Waihi is based on a seismic hazard study undertaken in 2007, which was updated in 2010 resulting in a reduction of the perceived hazard for this site. However the 2016 GNS report suggests there is a significantly greater earthquake risk in the region than previously assumed. The fact that the Hauraki District Council held a workshop even before the GNS report was made public, is testament to that.

The past assessments of seismic safety for both tailings dam are outdated. They fail to take account of the latest science and events in Brazil and Kaikoura, and may significantly underestimate the earthquake hazard. (A 7.4 Mw quake is 8 times bigger and 22 times stronger (energy release) than a 6.5Mw quake)

The Fundao tailings dam failure is Brazil’s worst ever environmental disaster. The scale of the devastation is horrendous. 36 million cubic meters of toxic tailings (similar to the amount stored at the Waihi Martha mine) swept down 600 km of the Doce River and into the Atlantic Ocean. The rivers ecosystem has been poisoned, whole towns destroyed, 11 people were killed and thousands have lost their livelihood. Watch this one minute summary of the disaster or this 43 minute Australian Four Corners documentary.


Brazilian authorities have lodged a claim against Australian mining giant BHP et al for NZ $61 billion in damages – (note: billion not million).

[Compare this real life NZ $61 billion figure with the bonds posted for the Waihi mine – a paltry $26.2 million rehabilitation bond, and a $10.4 million capitalization bond. The mining company can uplift the rehabilitation bond upon “satisfactory mine closure”. Therefore any failure of the dam after “closure” would mean even that bond is not available. The bond for the Golden Cross mine is believed to be $12 million. The taxpayer will almost certainly have to pay most of the costs, should either of these tailings dam ever fail.]

The report on the Brazil dam collapse says three minor earthquakes with an intensity of just 2.2Mw, 2.6Mw and 1.8Mw which occurred 90 minutes before the dam breach were a “triggering mechanism” for a “liquefaction flow slide”.

Other causes of failure related to poor maintenance and management leading to the tailings becoming over-saturated.

In August 2016 I raised concerns with the Waikato Regional Council, and the Minister of Energy about the seismic safety of the Waihi dams. I suggested that an immediate independent peer reviewed investigation was justified. The Minister replied that this was a Regional Council matter and gave no substantive response. The WRC replied that it was satisfied with the two mining companies concerned carrying out their own in-house review, with no specific time-frame. The WRC did not consider there was any urgency.

I wrote to WRC again in October drawing attention to the conclusion in the report on the Fundao mine collapse that very small earthquakes were a triggering mechanism for failure, and pointing out that the 2016 GNS Science report had specifically recommended “the location of important facilities with respect to potential liquefaction-driven ground deformation and landslide risk should be mapped with urgency”.

I noted that a high-intensity tropical storm could cause the tailings to be saturated and if followed quickly by an earthquake could make the tailings more prone to a liquefaction flow slide – a “perfect storm”. (Wellington suffered major flooding the day after the recent earthquake).

The people living in Waihi, the Karangahake Gorge, Thames, Miranda and in around the Hauraki Gulf – in fact all New Zealanders – are entitled to the assurance that these two tailings dams are seismically safe. Given the potential catastrophic environmental and human losses which would arise from a tailings dam failure, it is reasonable to insist that the Regional Council and/or central government should carry out their own independent peer-reviewed study. Office buildings and houses are routinely re-assessed by independent engineers for earthquake safety, even although the probability of a large quake is low. Why should tailings dams be treated differently? The stakes are far too high to let the companies self-regulate or to simply assume that all is well.

Update 7 December : Waikato Regional Council have advised that “GNS is currently undertaking updated estimates of seismic hazard, which once completed, will be taken into account as part of the annual Tailings Storage Facility Monitoring Report provided each year.” Also, the latest reports on the tailings dams at Waihi and Golden Cross had been received by WRC, and I have been promised copies. It will be interesting to see whether the seismic assessment is based on outdated 2010 data, or whether the latest GNS Kerepehi Fault report is taken into account?

 

18 comments on “Denis Tegg: How earthquake safe are the mine tailings dams at Waihi? ”

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  1. John up North 1
    10 December 2016 at 1:26 pm

    So what is the final plan for these poisonous mining by products? Or is that it, put it in a hole/dam and walk/slink away leaving nothing but mega tons of toxic sludge and no financial commitment when (not if) it turns to custard.

    Sounds like farming and the waterways but a much more concentrated toxic mixture……….. just spray and walk away!

    • rob 1.1
      10 December 2016 at 7:52 pm

      Probably glossed over with trees?transplanted! not germination or growth but a false look, until it’s damage becomes aware and then yrs of bullshit until nothing is done!
      Does that sound about right?
      They do as they always have done and most people will no what’s next. SADLY.

  2. Draco T Bastard 2
    10 December 2016 at 1:52 pm

    Therefore it is perfectly reasonable to ask the question – would these structures withstand a large earthquake?

    Although a good question in context the real question that needs to be asked is:

    Why are we still allowing tailings dams?

    Having huge, man-made, poisonous lakes that will cause untold damage to the environment is obviously not the best solution. These tailings need to be treated and not just left.

    • dukeofurl 2.1
      10 December 2016 at 3:26 pm

      They say the tailings dams now support aquatic life such as ducks etc.
      Cynanide is of course found in the stones of pip fruit and in such things as almonds along side cigarette smoke.
      I think the gold mining process recovers the cyanide but the mine rocks become acid forming when exposed to air and water due to the sulphur content.

      • Draco T Bastard 2.1.1
        10 December 2016 at 3:51 pm

        [citation needed]

        Really, ‘I think’ isn’t good enough for such a critical discussion.

        • dukeofurl 2.1.1.1
          11 December 2016 at 7:24 am

          http://www.waihigold.co.nz/sustainability/environment/rehabilitation/rehabilitation-from-tailings-to-pasture-plantings-and-ponds/

          On the surface it seems the long term plan is Ok, but I dont think we have enough independent information to see if it really is that good.

          Of course the area was a virtual wasteland at the height of the mining in the 1920s, but the the regeneration since then has been mostly natural and not man made.

          There is a high level of knowledge put into these regeneration processes now, but of course that costs serious money.
          eg
          “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.”

          and
          “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.”
          However there could be a strong chance some of these outcomes are not going to occur as hoped for. Its just the way of the world and engineering design doesnt have all the answers.

      • RedLogix 2.1.2
        10 December 2016 at 5:31 pm

        Old tailings dams are indeed the worst legacy of the mining industry. Especially ones that pre-date current standards which mandate very low cyanide levels. But even then, rock which has been in contact with cyanide … even when the ‘weakly available’ cyanide levels are very low, also has other heavy metals such as arsenic, mercury, lead and zinc liberated into environmentally unstable forms.

        It is now entirely possible to design the process so that ONLY clean tailings (waste rock) that has never been in contact with cyanide is stored on the surface; and for very small amounts that are potentially toxic to be stored safely back underground. The industry should be required to move quickly to this standard.

        Even so … vast amounts of liquefiable rock that has been crushed down to a micron level (chemically clean or not) is a long-term hazard. More work is needed to find effective ways to stabilise these dams. Left as they are, in the long-term disaster of some kind is almost inevitable.

  3. Lloyd 3
    10 December 2016 at 4:11 pm

    Drowning in acidic mud is so much less fatal than cyanide poisoning.
    (Irony).

    • joe90 3.1
      11 December 2016 at 3:12 pm

      Landing in acidic water ain’t too flash, either.

      Several thousand snow geese may have died after landing in a former open-pit copper mine in Montana that is now filled with highly acidic water.

      A Nov. 28 snowstorm forced the migrating geese to take refuge in the Berkeley Pit, a 900-foot-deep pit lake of heavy metals and other dangerous chemicals that is now a federal cleanup site in Butte, Mont.

      The site has seen a horror like this before: In 1995, 342 geese landed on the pit, drank its water, and suffered fatal burns to their tracheae and other internal organs.

      http://www.latimes.com/nation/la-na-montana-toxic-geese-20161207-story.html

  4. Robert Glennie 4
    10 December 2016 at 9:43 pm

    Coromandel is fortunately not that seismically active. It is not to say there is no seismic activity because nowhere is entirely immune to seismic activity, BUT, if there is a large quake that causes damage to the tailings dams it will probably come from a fault in the Taupo Volcanic Zone where you can expect up to a magnitude 7.0 on some of the larger faults.

    Once you get into Waikato, the seismic hazard diminishes significantly as you are well away from the tectonic belt parallel to the tectonic plate boundary off the Hawkes Bay coast.

    Here are two maps from G.N.S. One shows deep quakes and the other shows shallow quakes. The big gap in deep quakes in the South Island is the Alpine Fault.

    http://info.geonet.org.nz/display/quake/Earthquake?preview=/950275/1278078/Deep_Seismicity.png

    http://info.geonet.org.nz/display/quake/Earthquake?preview=/950275/1278077/Shallow_Seismicity.png

    • Robert Glennie 4.1
      10 December 2016 at 10:08 pm

      If people are really concerned, they should contact Environment Bay of Plenty (assume Waihi falls in their boundaries)and ask if there are significant fault lines nearby and what they are capable of doing. The Regional Council will have that information on file.

      • Denis Tegg 4.1.1
        11 December 2016 at 10:44 am

        Robert
        You clearly haven’t read the article or the latest report from GNS Science on the Kerepehi Fault which is less than 20 km away from these tailings dams. This 2016 report says the rupture interval is 1000 years, rather than 6 to 8000 years, as previously thought, and that a magnitude 7.4 earthquake is possible if multiple segments of the fault ruptured together. The report says that towns close to the fault could experience MM8 MM9 or even MM10 (very destructive) intensity damage.

        Which is the whole point as to why the seismic safety of the dams needs to be re-examined.

      • Macro 4.1.2
        11 December 2016 at 1:15 pm

        Waihi does not form part of Environment Bay of Plenty – it is part of Waikato Regional Council. See the article above!
        The Kerepehi Fault is an active fault line – it is unusual in being a rift valley – the Coromandel Penisula is moving away from Auckland (who wants to be part of them anyway/ sarc) at around 10cm /century. The Firth of Thames forms part of that rift valley. And yes the geological activity in the Taupo area will have and has had significant impact on the Hauraki plains – The Waikato river for instance used to flow into the Firth until the massive eruption from Taupo closed that and forced the river to flow out to the Tasman rather than the Pacific.
        As Dennis outlines in the article above, the current GNS report of 2016 raises considerably the risk of significant earthquake on the Kerepehi fault. Previous assesments were not so alarming.
        It should also be noted that while those of us in Thames did not feel the Kaikoura quake (and my home is situated directly on top of the Thames fault line), the people on the plains around the Kerepehi fault certainly did.

    • Jenny Kirk 4.2
      11 December 2016 at 8:45 am

      Many people once thought Christchurch wasn’t seismically active, as well. How wrong they turned out to be !
      And as the Kaikoura earthquakes indicate, seismic activity goes a long way into areas other than the nearby centre.
      What is more, seemingly long dead volcanoes can also stir awake .
      I don’t think anyone can say there is no place in NZ that is immune from earthquake or volcano activity ……. and the hazard of tailings dams has not been diminished in any way by the fact that birds might visit them, nor by what geotech experts suggest might happen in the future.

      • dukeofurl 4.2.1
        11 December 2016 at 3:03 pm

        Christchurch has all ways been seen as close to the highest siesmically active zones, obviously they have now moved the boundary to include the city area. But it was only say from about the Waimakariri River moved to south to about lake Ellesmere/Te waihora.
        I imagine the extreme zone will be adjusted south a bit too to now include Kaikoura

  5. Jenny Kirk 5
    10 December 2016 at 9:50 pm

    This is useful information. Thanks for posting it.

  6. saveNZ 6
    11 December 2016 at 6:45 am

    Shocking!