After the flood: Investigating vanadium geochemistry following a mine tailings dam failure

 

Electron microprobe X-ray maps showing V-bearing titanite and magnetite in Mount Polley tailings (POL-5) and V-bearing Fe oxide in the Fe oxyhydroxide (Fe oxyhyd) (POL-13).

Tracking the fate of an emerging contaminant released into a river environment

Mount Polley, British Columbia, Canada is a copper and gold mine, producing waste (tailings) containing mostly silicate minerals (feldspars, ferro-magnesian and Ca-Ti-silicates, and muscovite), oxides such as magnetite and rutile, carbonates, and small amounts of copper sulfides and pyrite. The mine tailings have elevated concentrations of vanadium compared to local background soils, and following the failure of the tailings dam in 2014, there was concern about vanadium contamination downstream. Vanadium occurs naturally in four oxidation states (V2+, V3+, V4+, and V5+). Although vanadium is an essential element for humans and animals, high concentrations can be carcinogenic and toxic. V5+ is considered to be the most toxic vanadium species, as it can inhibit or replace phosphate. There is, however, a lack of information about, and understanding of, the geochemical–mineralogical cycling of vanadium in mining-affected environments, which is required to determine health effects and to develop management and remediation schemes. In work recently published in Environmental Science & Technology, a team of researchers present the first evidence that dissolution of vanadium-bearing magnetite and titanite may contribute to aqueous vanadium, and that natural chemical processes can result in an attenuation of vanadium levels. 

     

Mount Polley mine disaster

On 4 August 2014, an earthen dam at Mount Polley Mine in British Columbia failed, releasing more than 10 million cubic metres of water (enough to fill 4,000 Olympic-sized pools) and 4.5 million cubic metres of tailings sand into local lakes. The waste travelled down Hazeltine Creek, swelling it from its normal size of about a metre to over 150 metres in width. The release of these mine wastes led to a water-use ban in nearby towns, which was not lifted until 12 August 2014. At the time, this was the second largest tailing dam failure (by volume).

Image of Mount Polley from 29 July, 2014, before the dam breach. Image Credit: NASA Earth Observatory images by Jesse Allen, using Landsat data from the U.S. Geological Survey.

Image of Mount Polley from 5 August, 2014, after the dam breach. Image Credit: NASA Earth Observatory images by Jesse Allen, using Landsat data from the U.S. Geological Survey.

 

Professor Karen Hudson-Edwards has been part of a multi-disciplinary team studying tailings dam failures since 1998. The team received NERC Urgency Grant funding to study the environmental aftermath of the Mount Polley dam failure, looking at both the physical and chemical impacts. The mine had identified the elements of concern - copper, selenium and vanadium. As Prof. Hudson-Edwards explains, 

Vanadium is an emerging contaminant. We don’t know much about its toxicity in the ecosystem and how it behaves in the environment. So this situation was quite new, and a challenge to understand it.

Vanadium species

The team were interested in the redox (reduction–oxidation reaction) chemistry, and the different species of vanadium that were formed in the environment. They focused their investigations on Hazeltine Creek, which had been scoured by the released wastes as they flowed down to the lake, uprooting trees on the way. Mine tailings mixed with organic matter under water, burying organic waste and causing reducing conditions. The aim of the project was to understand the geochemical cycling of vanadium in the Hazeltine Creek catchment and its implications for the environmental impact of vanadium in other river systems.

Using an electron microprobe, the team identified vanadium in two minerals present in the creek - magnetite and titanite. Under normal conditions, these two minerals are considered to be very resistant to weathering, i.e. are not thought to break down in the environment. When they analysed individual magnetite and titanite grains using microfocus X-ray absorption near-edge structure (XANES) on beamline I18, they found that the vanadium present in the minerals was, as expected, a mixture of V3+ and V4+.
 
 
 
A) K-edge XANES spectra collected using beamline I18 from Mount Polley mineral samples and selected V-containing standards; B) blot of pre-edge intensity vs pre-edge peak energy derived from V K-edge XANES spectra
 
XANES analysis of secondary iron oxyhydroxides formed in the creek showed the presence of V5+ species, and PHREEQC modeling suggested that the stream waters mostly contain V5+. The data show peaks in vanadium at a depth of around 10 cm, suggesting that magnetite and titanite were breaking down in the reducing conditions formed just below the water–sediment interface by the influx of materials from the dam. The XANES and electron microprobe data suggest that the V5+ was ‘mopped up’ by the iron oxyhydroxides that form deposits on rocks and colloids in the water. This is an entirely natural process, by which the chemistry of the creek is ridding itself of the vanadium contamination. Prof. Hudson-Edwards said:

 

Being able to investigate the vanadium speciation at Diamond really helped us to finish this story. We couldn’t have done it without the invaluable assistance of the beamline staff, who particularly helped with processing the data, which isn’t an everyday task for most scientists.

Mining for the future

The results of this work will inform restoration and management schemes for river systems receiving vanadium from natural and anthropogenic sources.

 

Location of the study area showing the Hazeltine Creek stream (HC-), inflow sample, pore water (PW-) and tailings, sediment, and Fe oxyhydroxide (POL-) sample sites for materials collected in 2014 and 2015. Labels shown are for those samples discussed in this work.

 

Since this research was undertaken, extensive cleanup of the mine spill has removed most of the spilled tailings from the creek, and restored it. The aim of this remedial work was to restore ecosystem habitats through the establishment of a new rock-lined channel, reducing remobilisation of the remaining tailings and exposed natural sediments and decreasing turbidity.

By studying tailings dam failures wherever they occur internationally, UK scientists are building up a knowledge base that will benefit the UK in the future. A long history of mining has left the UK with a legacy of tailings that can’t yet be cleaned up, and we need to understand their potential impacts on the environment. Cornish Lithium will begin drilling this summer, and there are also prospects for mining tin, tungsten and copper in Cornwall, all of which would contribute greatly to renewable energy technologies and a low carbon economy. There are also exciting new ideas arising with the aim of bringing mining into the circular economy – reducing mine wastes and using them to make useful products.
 

To find out more about the I18 beamline, or to discuss potential applications, please contact Principal Beamline Scientist Konstantin Ignatyev: konstantin.ignatyev@diamond.ac.uk.

 

Related publications:

Hudson-Edwards KA et al. Origin and Fate of Vanadium in the Hazeltine Creek Catchment following the 2014 Mount Polley Mine Tailings Spill in British Columbia, Canada. Environmental Science & Technology 53, 4088−4098 (2019). DOI:10.1021/acs.est.8b06391

Byrne P et al. Water quality impacts and river system recovery following the 2014 Mount Polley mine tailings dam spill, British Columbia, Canada. Applied Geochemistry 91, 64-74 (2018). DOI:10.1016/j.apgeochem.2018.01.012.