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Remediation schemes to mitigate the impacts of abandoned mines

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Title: Remediation schemes to mitigate the impacts of abandoned mines


1
Remediation schemes to mitigate the impacts of
abandoned mines
  • Brian Bone
  • Environment Agency for England and Wales

2
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

3
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

4
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

5
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

6
Active treatment
  • Advantages
  • Track record available expertise
  • Process control
  • Consistent effluent quality
  • Disadvantages
  • Cost (op cap)
  • Sludge disposal
  • Energy consumption

7
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

8
Wheal Jane
9
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

10
Impact of Treatment - Wheal Jane
11
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

12
Stage 2
Stage 1
13
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)

14
Passive treatment
  • Advantages
  • Low maintenance
  • Aesthetically pleasing
  • Sustainable?
  • Disadvantages
  • Large land take
  • Lack of process control
  • Relatively new (track record)
  • Cost (capital)?

15
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

16
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17
Wheal Jane passive treatment site
18
Aerobic cells
19
Wheal Jane passive treatment plant anaerobic cell
Anaerobic cell
20
Rock filters
21
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

22
Prevention and control of discharges
  • Encapsulation of mining waste
  • Advantages
  • Track record of civil engineering approaches
  • Low maintenance
  • Disadvantages
  • Surface area limits
  • Durability

23
Greenside Mine, Cumbria
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26
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

27
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32
Remediation Scheme (2)
  • Stabilise retaining structures
  • walls
  • underpin revetments
  • Stabilise soil
  • geosynthetic pre-seeded matting

33
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35
August 2003
36
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
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