Wetlands for remediation of acid-mine drainage

1. Wetlands for remediation of acid-mine drainage Alyssa Shiel

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

3. 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

4. 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

5. 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

6. Acid tolerant aquatic species • Typha sp. (cattails) • Sphagnum dominates • Found to accumulate iron • Green algaes • Chlamydomonas aplanata (<3) • Blue-green algaes • Spirulina nordstedtii (4.9) • Diatoms • Navicula nivalis (3.0) (M. Kelly, 1988)

7. 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)

8. Benhar before remediation

9. 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

10. 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

11. 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).

12. 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

13. 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