Categories
Remediation

Briefing Note on Hamilton et al 2020 paper

The following provides some comments from Mount Polley Mining Corporation (MPMC) on the Hamilton paper (Hamilton, et al. 2020)[1] regarding Quesnel Lake in relation to the TSF Breach at Mount Polley. The note is divided into general comments, specific comments, and then provides an update on Quesnel Lake water quality, and some key observations from recent sediment and aquatic life monitoring, which support the MPMC comments on the paper. This is not a comprehensive review of the paper.

General

The Hamilton paper provides a summary of a considerable amount of monitoring data collected in Quesnel Lake, including  from automated moorings. (Note: MPMC contributed to this research through the purchase of a number of new instruments for the moorings in the fall of 2014.)

The paper focusses on seasonal observations of a slight increase in turbidity deep in the West Basin, and on physical lake dynamics. It also introduces some hypotheses regarding new mechanisms of lake water movement. MPMC is pleased to have contributed to this enhanced understanding of water movements in large lakes.

However, we are concerned that important monitoring data, available on our web site or directly from MPMC or our consultants, was not referred to or incorporated into interpretations made in the Hamilton paper. The use of information that is readily available from MPMC’s web site or directly from MPMC or its consultants would have helped address some of the authors’ concerns, particularly about future impacts to aquatic life and contamination.

Unfortunately, the paper does not include data from the mine’s monitoring nor any other data on these topics. The paper contains a number of interesting scientific observations, but these do not necessarily indicate an environmentally consequential measurement.  

Specific Comments on the Hamilton et al (2020) paper:

  1. Mount Polley’s monitoring data indicates that contaminant levels in Quesnel Lake are not elevated. The paper identifies a small turbidity signal at depth, but turbidity does not necessarily indicate contamination.  (See below for a description of “what is turbidity”.)
  2. Hamilton et al’s data from 2015 to 2017 indicate a significant decline in the seasonal turbidity signal they measured since the spill in 2014. This observation agrees with MPMC’s monitoring data.
  3. The turbidity values measured by both MPMC and Hamilton et al are below BC water quality guidelines, which are based on a 30-day average. (The BC Guidelines allow for increases to 10 NTU for short durations.)
  4. There are no data presented in the paper from 2018, 2019 or 2020. This is a significant shortcoming of the paper being able to speak to the current situation, or to future impacts. MPMC has monitoring data for 2018 to 2020 for a number of sites in the lake that the researchers could have used to assess trends after 2017 for both water quality and aquatic ecosystem health.
  5. The levels of turbidity measured by Hamilton et al deep in the West Basin are quite low. (Between winter 2015 and winter 2017 they range from highs of approximately 2.3 FTU, to less than 0.5 FTU.)  Turbidity is a measure of “cloudiness” due to particulates in water, however, the levels of turbidity being measured in this paper are not easily seen with the naked eye (in other words, instruments are required to measure these levels).
  6. The paper provides background (pre-spill) data that indicate that the turbidity signal they observed at depth is at or below the level of natural turbidity events in the West Basin in the past (for example, a plume from the Horsefly River in May 2008 increased the turbidity in surface water of the West Basin to greater than 2.0 FTU as seen in Figure 3 in the paper). Natural turbidity events, such as are associated with heavy rains, spring freshet (snowmelt) or high-water floods, can generate similar or higher levels of turbidity. This summer, high creek and river levels generated muddy, debris-laden, flows into Quesnel Lake. 
  7. The paper postulates suspension of material from an unconsolidated layer of particulates at or near the bottom of the lake. While the unconsolidated layer identified in core samples is interesting, there is no data in the paper on what the particulates are that make up this layer. MPMC has reached out to the authors with an offer to either do this work on their samples or contribute funding to fill this information gap.  Note that the paper does not say that tailings are resuspending off the bottom of the lake. Note also that MPMC sediment monitoring has observed natural material, with organic carbon, settling into sediment traps placed on the bottom of Quesnel Lake and presumably covering tailings.
  8. There is no data in the paper that indicates that the particulates associated with their turbidity signal are contaminated with any metals or chemicals of concern. MPMC’s monitoring shows that water quality in Quesnel Lake is below the BC Water Quality Guidelines, except during spring freshet when area creeks naturally discharge elevated turbidity and copper.
  9. MPMC supports the Hamilton et al observation of no visible colour change in the lake since 2014. This confirms MPMC’s observations.  
  10. Mount Polley’s water discharge is permitted by the BC Government and is within strict permit guidelines that are protective of sensitive aquatic life. The paper noted a small increase in specific conductance associated with the MPMC treated water discharge in 2016, but also noted that there was no turbidity signal associated with this discharge. These data agree with Mount Polley’s monitoring data. MPMC’s monitoring continues to show that water quality in Quesnel Lake is below the BC Water Quality Guidelines except during spring freshet when area creeks naturally discharge elevated turbidity and copper and when MPMC are typically not discharging because of restrictive permit requirements.
  11. The paper expresses concern about the potential resuspension of spill material from Quesnel Lake and its impacts on juvenile sockeye salmon, while not including data DFO collected on juvenile salmon in the West Basin in 2014, the year of the spill, nor acknowledging that the 2014 juveniles were the cohort that “returned in droves” to the Quesnel Lake watershed in 2018. This juvenile salmon cohort would presumably have been the most impacted as they were feeding in Quesnel Lake the year of the spill, yet there has been no indication that the tailings spill had a deleterious effect on their feeding or their returns four years later.
  12. Mount Polley is very pleased to see that the paper noted that the MPMC remediation of Hazeltine Creek reduced sediment loads as no turbidity signal >0.2 FTU above background was detected near its mouth from 2015 through 2017”.

Quesnel Lake Water Quality

  • There is no evidence of pollution being caused in Quesnel Lake related to the Mount Polley spill. This is affirmed by MPMC monitoring and by BC ENV comments to the MPMC’s Public Liaison Committee.
  • Results of the Comprehensive Environmental Monitoring Program (CEMP) – Sediment and Aquatic Life (Minnow, March 2020) monitoring using DGT instruments in Quesnel Lake indicate:
    • copper concentrations in 2019 “were well below [freshwater aquatic life] effects thresholds”
    • there is “strong evidence of … post-depositional stability of the sediments impacted by the breach”, i.e. there is no indication that metals are leaching out of tailings into the water in Quesnel Lake, and
    • “… analytes in 2019 were all below BCWQG’s”, i.e. all metals analyzed using the DGT’s were below the BC Water Quality Guideline thresholds for protection of freshwater aquatic life.

Discussion of Turbidity from https://lamotte.com/technical-tips/post/turbidity

(website accessed 2020-09-01)

The definition of Turbidity is the cloudiness or haziness of a fluid caused by suspended solids that are usually invisible to the naked eye. The measurement of Turbidity is an important test when trying to determine the quality of water. It is an aggregate optical property of the water and does not identify individual substances; it just says something is there. Water almost always contains suspended solids that consist of many different particles of varying sizes. Some of the particles are large enough and heavy enough to eventually settle to the bottom of a container if a sample is left standing (these are the settleable solids). The smaller particles will only settle slowly, if at all (these are the colloidal solids). It’s these particles that cause the water to look turbid.


[1] Hamilton, A. K., B. E. Laval, E. L. Petticrew, S. J. Albers, M. Allchin, S. A. Baldwin, E. C. Carmack, et al. 2020. “Seasonal Turbidity Linked to Physical Dynamics in a Deep Lake Following the Catastrophic 2014 Mount Polley Mine Tailings Spill.” Water Resources Research 56. doi:https://doi.org/ 10.1029/2019WR025790.

Categories
Mining facts

Mining and Mineral Processing at the Mount Polley mine

In the Mount Polley Mine, run-of-mine ore from the open pits and underground is hauled to the crusher.  The crusher has three stages of crushing involving five crushers, twenty conveyors and four sets of screens.  Ore is dumped by the surface mining fleet into the feed pocket of the primary gyratory crusher, and is then crushed in three stages to produce a product at finer than 16 mm for the grinding circuit. Periodically, the crusher also used for the production of aggregates used in tailings construction and other tasks.

The grinding circuit consists of two parallel rod mill/ball mill circuits and a pebble mill circuit. Crusher product is first split between two rod mills where water is added to form slurries.  The rod mill discharge is pumped to the primary hydrocyclones that classify the particles by size.  The larger particles flow to feed the ball mills while the fine particles report to two flash flotation cells. The ball mills are in “closed circuit”, meaning that the discharge is pumped to the classifying units (primary hydrocyclones) and the particles will not pass to the next grinding stage until they are fine enough to feed through the flash flotation cells.  The underflow from the flash flotation cells is pumped to the secondary hydrocyclones, the flash flotation product can report directly to the concentrate circuit or to regrind for further upgrading.

The coarse particles classified by the secondary hydrocyclones reports to three pebble mills for further size reduction. The pebble mills are in “closed circuit” with the secondary hydrocyclones and product that is sized at 65% finer than 200-mesh is fed to the flotation circuit. Pebbles obtained from the triple deck screen in the crusher are used as grinding media in the pebble mills.

The flotation circuit separates the valuable minerals from the rest of the crushed rocks. With the addition of reagents, the valuable minerals, mostly in the form of sulphide minerals chalcopyrite and bornite, are separated by flotation and are collected and upgraded to produce a concentrate. Initial separation is completed in a rougher/scavenger circuit, where the remaining minerals are discarded as tailings (which flow by gravity to the Tailings Storage Facility).  Rougher concentrate is reground in a regrind mill and further upgraded in a cleaner circuit to produce the final concentrate product. Cleaner tailings are recycled to the scavenger circuit.

The concentrate from the flotation circuit is dewatered in two stages: the thickener settles particles and decants water so that the settled particles form a sludge by sedimentation and have a reduced water content of roughly 25%-30%; pressure filtration further reduces water content to approximately 8%. The water removed is utilized as process water. The filtered concentrate is stored in the load-out building and loaded onto 40-tonne trucks for shipping. Tailings materials generated by mill operations are piped via gravity to the TSF.

Categories
Mining facts

Tailings – What are they and what is in the Mount Polley tailings?

First, what are tailings?

Tailings are essentially crushed rock, and are the leftover material after the minerals containing the “elements of interest” have been removed. At Mount Polley the elements of interest are copper, gold and silver. The minerals containing the copper, gold and silver are released by crushing and grinding the mined rock down to sand and silt sized particles.

At Mount Polley, a process known as flotation is then used to separate the important copper-bearing minerals from the rest of the crushed ground rock. The remaining crushed rock is considered waste (gangue) and is what makes up the tailings. No cyanide is used at Mount Polley.

Read more about Tailings on the Mining Association of BC’s website here.

What is the in the Mount Polley tailings?
At Mount Polley, the valuable elements are copper, gold and silver and they are found most commonly in the sulphide minerals, chalcopyrite and bornite. The leftover minerals found in the waste are piped as a slurry with water to the tailings storage facility. [ref: Community Updates 2017 Issue 3; 2016 Apr Issue 2]

The rocks that are mined at Mount Polley are around 200 million years old and represent ancient volcanic rocks and magma that intruded into these rocks. The intrusive rocks host the copper, gold and silver mineralization.

Let’s talk rocks!
The rocks which host most of the ore are made up primarily of the minerals orthoclase (potassium feldspar), albite (sodium plagioclase), magnetite (iron oxide), Ca-plagioclase (calcium plagioclase), diopside (pyroxene), garnet, biotite (mica), epidote and calcite (calcium carbonate). These minerals are all common rock-forming minerals, and represent 90% of what ends up in the Mount Polley tailings pond.

Of the other 10 percent, most are also common minerals, with a minor amount of sulphide minerals, including a little bit of chalcopyrite (0.17%) that didn’t get captured in the mill and a small amount of pyrite (0.04%).

What is unusual about Mount Polley is that, when compared to many other copper deposits (and the reason why these tailings are considered by geochemists to be chemically quite benign) there is very little pyrite (iron sulphide) and a fair amount of calcite (calcium carbonate) in the tailings.

Due to this, Mount Polley’s tailings do not generate “acid rock drainage”. This is the process that happens when sulphide minerals, especially pyrite, are exposed to the atmosphere and react to form sulphuric acid, which then can leach metals out of tailings and lead to metal mobility and potential contamination.

Mount Polley’s tailings do not have this “acid rock drainage” problem, as there is very little pyrite, and calcite acts as a neutralizing agent if any of the minor amounts of sulphide in the tailings breaks down. The vast majority of the rest of the minerals in Mount Polley’s tailings does not react easily with air or water, and are very similar to natural sand.