Steven Kershaw explains the environmental risks left by coal mining in Britain and how they can be managed
Centuries of mining in Britain has left a legacy of abandoned underground mines that continues to represent a risk to public safety and environmental contamination that will exist for many centuries to come.
The Coal Authority, founded in 1994 to look after the legacy of the coal industry, owns the estate of coal seams and mines within Britain. In 2014 it will celebrate its 20th anniversary of addressing these risks on behalf of the nation. Its principal roles are the control and treatment of mine waters, treating the effects of subsidence and providing mining information to the public, as well as licensing the mining of coal.
Risks to the public
A principal risk to the public is the ground collapsing above abandoned mine workings. A thousand such incidents per year are reported to the Coal Authority, approximately half of which are mining related.
There is a 24/7 hazard line where the public can phone in to report mining hazards and there are regional staff who can respond as soon as possible after notification. The collapses are largely associated with old shallow workings at depths of less than 30 metres below the surface and mine entries, especially shafts, where collapses occur into voids.
These failures can cause damage ranging from something that might represent a trip hazard to something that might swallow buildings or people.
Most typically, the damage arising from shallow workings manifests itself as a crown hole, where the collapse of the mine roof into a void migrates upwards until it reaches the surface. After filling any voids, shafts are typically fitted with a reinforced concrete cap to prevent any further collapses.
The geographical scale of the areas potentially at risk is large. Abandoned coal mine workings in Britain cover thousands of square kilometres with a significant percentage of these being at shallow depth.
The shallowest workings tend to be those that were mined first and are therefore usually the oldest. But abandonment plans have only had to be legally submitted to the HM Inspectorate of Mines since 1872, so there are many old workings, abandoned before that date, where no documentary evidence exists.
Inspecting mine risks
There are 170,000 recorded mine entries within the coalfields. There is probably a similar number of shafts which were never recorded or archived and whose positions remain unknown. These mined areas are often close to urban centres which grew on the coalfields during the industrial revolution and afterwards. Records show that there are over 58,000 recorded mineshafts in urban areas.
This close relationship between potential mining hazards and the areas where people live creates particular problems for public safety. The large scale of the problem and the fact that collapses are very hard to predict, means that expensive proactive remediation of shallow mine workings and shafts is not carried out unless there is a clearly identified reason to do so.
Typically, remediation will occur if developers wish to build over land that might have instabilities. This will usually involve filling workings with grout (fly ash, cement, and water) and building concrete caps over the shafts.
Known shafts represent discrete points where failure could occur and therefore represent a clear identifiable risk to the public. In 2008 the Coal Authority began a programme of inspecting all the locations of recorded shafts, in order to identify whether there were any particular risks that needed to be addressed.
At present 85,000 mineshafts have been visited and between 1–3 per cent of these shafts have been found to require treatment; 151,000 shafts are the responsibility of the Coal Authority which has records of 170,000 shafts in total for the UK.
Gas escapes from abandoned mines to the surface represent a small, but significant risk to the public and have resulted in fatalities over the years. The principal risks are from explosions due to the escape of methane and suffocation due to the emission of low oxygen/high carbon dioxide mixtures. Both of these risks are greatly increased where gas is released into buildings and other confined spaces.
To address these risks, vents have been installed into workings known to be at risk of containing gas at pressure, and these are regularly monitored for any changes in gas composition. In addition, two active pumping schemes have been installed in the North East to keep the workings at sub-atmospheric pressure to protect properties at risk.
There is also a specialised response to potential gas-related hazards through the 24/7 hazard line.
The main environmental impact of abandoned coal mines arises from the issue of mine water. These mine waters are typically laden with soluble iron sulphate which on contact with oxygen at the surface precipitates iron ochre. The ochre will turn water bodies a very distinctive red/orange colour and can kill off flora and fauna in those streams and ponds.
Under the European Water Framework Directive (WFD), countries have an obligation to bring the quality of their water bodies to a ‘good’ status and the Coal Authority works with the Environment Agency, the Scottish Environmental Protection Agency and National Resources Wales to address a list of sites that require treatment to conform with the WFD.
To this end it has designed and built over 60 mine water treatment schemes in England, Scotland and Wales. The great majority of these schemes are systems to collect the ochre and reduce the effluent to acceptable levels (1mg/L iron) before being released into streams, rivers, and the sea.
The significant active treatment scheme in Britain is the plant at Dawdon in the north east of England which treats water high in iron and salt, then discharges the treated water into the North Sea. The Dawdon plant is used to treat water that is pumped from mine workings below an aquifer serving the north east region with drinking water.
Pumping is required to prevent the iron entering the aquifer and potentially causing iron rich waters to enter the public water supply. These actions are contributing to making significant progress towards the UK’s compliance with the WFD, and significantly improving the environment in coalfield areas of Britain.
Remediating mine waters
Waters from abandoned metal mines can contain dangerous levels of toxic metals. They typically do not contain high levels of iron and therefore often appear clean and even drinkable. But unlike water from coal mines there is no single body with responsibility for treating these waters.
As a result, the Department for Environment, Food and Rural Affairs (Defra) is funding the Coal Authority to look at the feasibility of treating metal mine waters, and building schemes to control contamination from the many metal mines around the country.
Two such schemes have already been constructed – one an ironstone mine water issue at Saltburn in Cleveland and the other a zinc and lead mine water issue in the Lake District.
Research and development projects are under way to reduce the risks from mine collapses and the environmental impacts of mine waters. These include using alternative materials for repairing ground collapses, understanding ground failure mechanisms, and using remote sensing methods for inspecting mine entries.
Studies are also being conducted into how to build mine water treatment schemes where ground for construction is limited, and new methods of treating metal mine waters.
Assessing risk to the public and the Coal Authority’s liabilities are informed by the large body of mining information that is held by the Coal Authority. Coal mine abandonment plans have both been digitised and stored in the Coal Authority’s GIS system.
As well as being the source of mining reports, which the Coal Authority produces for house buyers, these records form the principal information base of the Coal Authority and are used for assessing liability and areas of particular risk.
Importantly, these records are vital for the role of the Coal Authority as a statutory consultee for local authority planning applications. They enable the Coal Authority to identify risks to development from mining so that such risks can be properly assessed.
Key plans identifying the 15 per cent of coal-mined areas that are at highest risk from mine entries and shallow mining, have been provided to all relevant local authority planning departments to assist in determining where past coal mining is an issue that needs to be addressed.
Steven Kershaw is Head of Research and Development for the Coal Authority. Contact Steven Kershaw: Steven.Kershaw@coal.gov.uk