Repairing damage caused by abandoned mines

Repairing damage caused by abandoned, contaminated mines

Latest approaches to mine-remediation planning and implementation are helping the environment and communities.

Alex Watson’s interest in abandoned mines and contaminated land began with the UK Government in the 1990s. Watson, a geochemist, completed investigations of Royal Air Force stations including Second World War bomber stations and an old nerve-gas plant in Cornwall in southwest England.

The Cornwall site represented a range of problems. It had been constructed in an area subject to intense historical lead and zinc mining, with some of the old shafts used for the disposal of wastes from the nerve-gas plant when it was decommissioned and demolished.

Watson became interested in abandoned sites, contaminated land and mining, and began a professional journey that would later take him to South Africa and then Sydney, Australia.

In 2008, Watson joined SRK Consulting as an environmental consultant. Today, he is a principal environmental scientist at SRK, specialising in environmental management, site assessment and remedial design. Watson also has extensive experience in geochemical characterisation, mine closure, water quality and water-balance modelling.

“Identification and remediation of abandoned, contaminated mines are incredibly important,” Watson says. “An abandoned mine could range from a shaft in a field from centuries ago that nobody knows about to a large, well-known project.

“Governments worldwide are paying more attention to the risks of old abandoned mines. Population growth and urban sprawl in cities are exposing more people to the risks of abandoned mines. Environmental expectations are also much higher and communities are better informed, allowing them to become more involved when old mines present risks.”

Abandoned mines are a timely topic. More than 600 historic and abandoned mine sites in New South Wales (NSW) are listed on the Legacy Mines Program (LMP). The LMP is a NSW Government initiative that focuses on public safety and improving the environment through remediation of abandoned mines.

The NSW Government is investing more than $100 million over the next 10 years to remediate abandoned mines that represent the greatest risk to public health and safety, and the environment. Remediation works include detailed site assessments, preparation of remediation action plans and engineering design works.

Other Australian states and territories are increasing their focus on historic abandoned mines.

“This issue is steadily gaining momentum,” Watson says. “There’s a lot of open dialogue between governments and industry on abandoned mines and liaison with communities. It’s an issue that mining stakeholders are passionate about fixing.”

RISKS FROM HISTORIC MINES

Abandoned mines can have significant risks. From a public-safety perspective, an old mine shaft covered by corroded corrugated iron could lurk in a field, unknown to the public. Some people have been badly injured after falling down unmarked mine shafts in recent years.

Environmentally, some abandoned mines have leached toxic materials into surface water and groundwater on-site, and into nearby waterways. That can kill aquatic life and affect downstream communities that rely on the waterways for work or recreation. Dust blown off abandoned mine sites is another risk.

A slag-lined drain at an abandoned mine.

Financial liabilities from remediation of abandoned mines can be large and persist for years or decades. Often, governments must fund the clean-up because the person or company with direct responsibility for the mine’s rehabilitation can no longer be traced.

Watson says the new remediation plan for the old Rum Jungle mine in the Northern Territory highlights increasing government focus and collaboration on this issue.

The Rum Jungle mine operated from 1953 to 1971, producing a reported 3530 tonnes of uranium oxide and 20,000 tonnes of copper concentrate. The mine created legacy landform, groundwater and surface water contamination problems.

The Australian Government rehabilitated the site from 1983 to 1986, but recent studies found gradual deterioration from the original works. The Australian and Northern Territory governments are partnering on the stage-three implementation of a rehabilitation plan for Rum Jungle that is expected to be a 15-year project.

Watson has presented on Rum Jungle at industry conferences.

“There’s a lot of interest in this project,” he says. “Rum Jungle highlights the complexities and environmental risks of large historical mines – and why stakeholders need to continue working together and share learnings on complex mine remediation as it is happening.”

NSW CASE STUDY

Watson’s works on an abandoned copper mine in the Central West region of NSW reinforces the benefits of latest approaches to mineremediation planning and implementation.

Covering approximately 15 hectares, the mine operated between 1875 and 1905. Copper was the primary resource, and silver and gold were also recovered. Lead and zinc were present at the site but discarded during its operation. The base metals were present as sulfides such as chalcopyrite, galena and sphalerite.

Site infrastructure included underground shafts (up to 65 metres deep), four smelting furnaces, spoil dumps and three sediment dams. Waste rock/overburden (from developing the mine shafts) and smelter slag were disposed of on site.

Some remediation works were completed around 1990, but the remaining waste rock and slag remained as contamination sources.

“The mine required a lot of work to understand the extent of the problem and develop a remediation plan,” Watson says.

SRK’s work on the project so far includes a desktop review of site information to develop a sampling and analysis plan. There has also been a visual inspection of the site, mapping of element concentration using X-ray fluorescence (XRF), and collection and analysis of surface water samples.

Watson says the XRF mapping identified high concentrations of zinc, sulfur, lead and copper in some areas around the mine’s main shaft, and smaller concentrations north of the site.

“Concentrations of arsenic were also high,” Watson says. “It’s possible that the arsenic was associated with ore material from the mine.”

Old mine shafts pose significant risks to public safety.

“This issue is steadily gaining momentum. There’s a lot of open dialogue between governments and industry on abandoned mines and liaison with communities. It’s an issue that mining stakeholders are passionate about fixing.

Moreover, sampling found the water was mildly acidic (pH 4) and contained elevated compositions of heavy metals, such as cadmium, copper and lead. Water samples collected upstream and downstream of the site suggested the transportation of contaminants offsite, but concentrations were low and within stock-water guideline values.

Watson says the project’s next steps involve investigating the amount of mining waste remaining on the surface, collecting samples of mineralised wastes to assess their contamination potential, and assessing the volume of material that may be obtained from borrow pits for use in rehabilitation activities.

“This information will be used to characterise the site and assess the risks it poses,” says Watson. “This data can then be used to identify possible remedial options for the site, which may range from doing nothing to carrying out onsite engineering works or removal of contamination for off-site disposal.”

SRK Consulting is a leading, independent international consultancy that advises clients mainly in the earth and water resource industries. Its mining services range from exploration to mine closure. SRK experts are leaders in fields such as due diligence, technical studies, mine waste and water management, permitting, and mine rehabilitation. To learn more about SRK Consulting, visit www.srk.com

Mine remediation is critical to environmental conservation. EIGHT IDEAS FOR BEST PRACTICE IN THE REHABILITATION OF ABANDONED MINES 1. Site history: Conduct a desktop study of the historical mine site. Identify what was mined and any heavy metals of concern. The presence of sulfides, for example, poses a risk of acid and metalliferous drainage that could leach into nearby waterways.

2. Environmental screening: Understand the geography of the site and its surrounding area. Identify any natural or physical features of an area that could be affected by the mine – and any nearby communities.

3. Sampling and analysis plan: Using the information from the desktop study and environmental screening, develop a sampling and analysis plan to inform a site investigation. The plan should identify areas of potential concern at the historical mine, which could include waste-dump sites, slag heaps, tailings dams, quarries or areas of other possible contaminants. It should also identify the type of sample to collect, such as soil, rock or water, as well as the location from which to collect it.

4. Site investigation: Having identified areas of potential concern, complete a site walkover prior to collecting samples. The samples may range from solids collected from trial pits and drill holes to water from creeks and ponds. The site investigation provides an opportunity to confirm learnings from the desktop review, modify the sampling locations based on ground conditions and gain visual insight into the extent of potential contamination.

5. Laboratory sampling: Send samples collected during the site investigation to a laboratory for a range of analytical tests to understand the composition and reactivity of the materials.

6. Risk assessment: Use information from the desktop study, site investigation and laboratory analysis to develop a detailed risk assessment of the mine. Document key environmental risks, their magnitude and potential longevity. This could include leaching of contaminants into groundwater or surface water from the mine or dust blown off the site.

7. Site-remediation plan: Having identified and quantified site environmental risks, determine if remedial action is required. If so, develop a remediation plan to address the problems.

8. Share learnings: Where possible, document learnings from a mine rehabilitation project and share the results with industry and governments through case studies, papers and/or industry presentations.

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