Remediation schemes to mitigate the impacts of abandoned mines

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Remediation schemes to mitigate the impacts of
abandoned mines

• Brian Bone
• Environment Agency for England and Wales

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Presentation aims content

• To illustrate the approach to remediation in
  England Wales through 3 case studies
• Remediation drivers
• Remedial strategy case studies
• active treatment of minewater
• passive treatment of minewater
• stabilisation of tailings
• Conclusions

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Key drivers for remediation of abandoned mines

• EU Dangerous Substances Directive - requires
  consented discharges for all sites abandoned
  after 1981 where the minewater contains listed
  substances
• EU Groundwater Directive - requires consented
  discharges from mine waste where leachate
  contains listed substances
• UK Contaminated Land Regulations - requires
  remedial action where a significant pollutant
  linkage is identified

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Future drivers

• EU Water Framework Directive - consolidates a
  number of directives, including the dangerous
  Substances and Groundwater Directives.
  Environmental objectives will need to be set for
  ALL water bodies in terms of chemical and
  ecological quality.
• Future EU Mining Wastes Directive - will require
  exchange of technical information on best
  available techniques with a view to developing
  methods to identify and remedy closed waste
  facilities

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Remedial strategies

• Single or combination of options to prevent
  pollution and/or treat, selected from
• active treatment
• passive treatment
• prevention control of discharge
• Each approach is highlighted by a case study

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Active treatment

• Advantages
• Track record available expertise
• Process control
• Consistent effluent quality
• Disadvantages
• Cost (op cap)
• Sludge disposal
• Energy consumption

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Wheal Jane Incident

• Tin/zinc mine in Cornwall
• closed early 1990s after 100s-1000s years mining
• in 1 year minewater rapidly recovered following
  closure
• temporary pump and treat scheme set up in Nov
  1991
• Jan 1992 sudden release of 50,000m3 of water and
  sediment containing large quantities of Fe, As,
  Cu, Cd and Zn into the Fal estuary

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Wheal Jane
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Drivers objectives for remediation

• EU Dangerous Substances Directive - applies to
  mines abandoned after 1981
• Minimise the polluting effects of minewater
  discharges from Wheal Jane
• Monitor changes in water quality and the effects
  on the aquatic environment
• Determine the most cost-effective long-term
  remediation strategy for Wheal Jane

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Impact of Treatment - Wheal Jane
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Active treatment system

• Designed to treat 350 l/s (average 200 l/s)
• Lime-dosing with sludge recirculation
• Pre-settlement sludge density design of 20 w/w
  solids
• Metals removal to satisfy discharge consent to
  local stream
• Three key stages

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Stage 2
Stage 1
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Active treatment system - summary

• State-of-the-art active treatment plant
  commissioned in 2000, cost 20M to build and
  operate for 10 years
• High density sludge system is very successful,
  solid content of 50 w/w achieved in tailings dam
• Tertiary filters presses not needed - saving
  1.7M
• 1st 22 months of operation gt12 Mcu.m of water
  treated and gt3200te of metals removed (overall
  removal efficiency 99.2)

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Passive treatment

• Advantages
• Low maintenance
• Aesthetically pleasing
• Sustainable?
• Disadvantages
• Large land take
• Lack of process control
• Relatively new (track record)
• Cost (capital)?

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Objectives of Wheal Jane pilot passive treatment

• Examine the feasibility of passive treatment as a
  long-term solution for Wheal Jane minewater
• develop an understanding of the key geochemical
  and microbiological processes
• model the system to aid design of a permanent
  system for Wheal Jane and elsewhere

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Wheal Jane passive treatment site
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Aerobic cells
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Wheal Jane passive treatment plant anaerobic cell
Anaerobic cell
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Rock filters
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Passive system - conclusions

• The conclusion that Wheal Jane drainage could not
  be treated with passive technology was tested.
• Further multidisciplinary studies have been
  carried out to understand the processes involved
  in passive treatment systems
• The studies indicate that a passive system could
  be redesigned to treat Wheal Jane drainage
• Proposal to establish an international research
  centre at Wheal Jane passive site

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Prevention and control of discharges

• Encapsulation of mining waste
• Advantages
• Track record of civil engineering approaches
• Low maintenance
• Disadvantages
• Surface area limits
• Durability

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Greenside Mine, Cumbria
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Remediation Scheme (1)

• Objective - to prevent mobilisation of
  contaminated tailings from a collapsed dam
• Control infiltration
• run-off into dam
• groundwater flow into dam
• infiltration through surface
• Reduce gradient
• regrade slope
• retaining walls

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Remediation Scheme (2)

• Stabilise retaining structures
• walls
• underpin revetments
• Stabilise soil
• geosynthetic pre-seeded matting

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August 2003
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Summary

• Case studies from two sites were presented,
  included active and passive treatment of acid
  minewater and a civil engineering approach to
  deal with tailings
• Minewater treatment cases highlight the need for
  good planning, including treatability studies to
  understand key chemical and biological processes
• Current and future legislation pose significant
  challenges to cost-effective remediation in
  meeting chemical and ecological objectives and
  disposal of treatment wastes