Wetlands for remediation of acid-mine drainage

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Wetlands for remediation of acid-mine drainage

• Alyssa Shiel

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Overview

• Define acid-mine drainage
• Constructed wetlands as passive treatment systems
• Acid-tolerant aquatic plants
• Benhar ironstone mine spoil in Central Scotland

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Acid-mine drainage

• Drainage flowing from or caused by mining that
  is typically highly acidic with elevated metal
  levels
• Increased concentrations of cadmium, nickel,
  lead, zinc, copper and sulfates
• The primary source of freshwater metal
  contamination
• Associated with land contamination and sparse
  vegetation
• May leach into surrounding watercourses

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Acid-mine drainage

• Lowers pH, between 2.0 and 3.0
• Contaminates water with oils, cyanides, acids and
  alkalies
• Reduces buffering capacity
• Alters phosphorus availability
• Solubilizes some heavy metals

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Constructed wetlands as treatment systems

• Viable treatment system for many mining regions
  of the world
• Passive, low-cost remediation
• Goals
• Removal of iron from the water column
• Removal of other metals
• Sulfate reduction
• Alleviation of extreme acidic conditions

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Acid tolerant aquatic species

• Typha sp. (cattails)
• Sphagnum dominates
• Found to accumulate iron
• Green algaes
• Chlamydomonas aplanata (lt3)
• Blue-green algaes
• Spirulina nordstedtii (4.9)
• Diatoms
• Navicula nivalis (3.0)

(M. Kelly, 1988)
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Benhar ironstone spoil heap in Central Scotland

• Acidic metal-rich drainage from reclaimed
  ironstone mine spoil
• 12 month evaluation of a constructed wetland
  system

(Heal and Salt, 1999)
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Benhar before remediation
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Ironstone spoil

• Characterized by pH 2.7 and 247 mg/l total Fe
• Highly acidic with elevated metal concentrations
• Acidity may mobilize iron, manganese, aluminum
  and other metals
• Although mining ceased early in the 20th Century,
  the Bing has remained mostly unvegetated due to
  the highly acidic and infertile spoil
• Mine drainage was polluting a small watercourse
  which leads into the River Almond

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Ironstone spoil

• Iron pyrites in ironstone and coal spoil are
  oxidized to form sulfuric acid when exposed to
  air and water
• At pH above 4, acid production proceeds slowly
  and is controlled by the rate of oxygen diffusion
  into the spoil
• Below pH 4, an alternative oxidation pathway
  exists involving the bacterium Thiobacillus
  ferrooxidans

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Treatment

• Creating an aesthetically-pleasing landform
• Treatment with dried pelleted sewage sludge,
  limestone and peat
• After which trees were planted Betula pubescens
  and Alnus glutinosa (common alder)
• Construction of settling pond and surface flow
  wetland to receive spoil drainage
• Mushroom compost substrate
• Cattails (Typha latifolia).

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Reclamation

• Acidity, iron, manganese and aluminum
  concentrations declined
• Removed on average 33 of acidity and 20-40 of
  metal inputs
• Removal rates decreased in the winter
• Enabled vegetation establishment
• Effect was spatially variable

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Summary

• Constructed wetlands offer an inexpensive,
  passive treatment for acid-mine drainage
• Several acid-tolerant aquatic species including
  Sphagnum and Typha sp.
• Benhar mine spoil
• Site history
• Treatment
• Results