Acid Mine Drainage

1. Acid Mine Drainage

2. Terms • Acid Mine Drainage (AMD) • Water that is polluted from contact with mining activity • Acid Rock Drainage (ARD) • Natural rock drainage that is acidic • Both produce acidic waters

5. Sources of Acid Mine Drainage(AMD) Mine Effluent

6. Mine Dump Mill Tailings

7. pyrite water + air low pH + metals AMD Chemistry • Pyrite weathering

8. Acid Mine Drainage Water - from rain and snowmelt + Oxygen - from the air + Pyrite- from the mine Reaction =Sulfuric Acid

9. AMD Chemistry Iron oxide 4FeS2 + 14 H2O + 15 O2 → 4Fe(OH)3 + 8 SO42- + 16 H+ Overall acid producing

10. Fe2+ + 2SO4 +2H+ Reaction 1: FeS2(s) + H2O + 7/2O2 Fe(OH)3(s) + 3H+ Reaction 4: Fe3+ + 3H2O THE CHEMISTRY ofACID MINE DRAINAGE pyrite water sulfate acid Reaction 2:* Fe2+ + 1/4O2 +H+ Fe3+ + 1/2H2O Reaction 3: FeS2(s)+ 8H2O + 14Fe3+ 15Fe2+ + 2SO4 + 16H+ * catalyzed by bacteria

11. AMD Chemistry • Surface area • more surface area, faster rate • smaller grains, more surface area

12. Characteristics • Increased acidity = decreased pH • Increased metal concentrations • Increased sulfate • Increased suspended solids All four don’t necessarily occur at the same time

13. Stream Effects Colored waters: • “Yellow boy” • Iron oxides, basically rusting the stream floor • White • Aluminum • Black • Manganese Determined by shifts in pH

14. Extent of Problem • Colorado • 20,000+ mines • 1,300 miles of streams • Montana • 20,000+ mines • 1,000 miles of streams • Arizona • 80,000+ mines • 200 miles of streams

15. Treatment • Active v. Passive • Active • physical addition of alkalinity to raise pH • High cost • effective • Passive • Naturally available energy sources • Little maintaince • Driven by volume

16. Passive Treatment

17. Active Treatment • Typical treatment processes (“ODAS”) • -oxidation • -dosing with alkali • -sedimentation

18. Iron Mountain, California Active Treatment

19. Shift in Mining Techniques • “Old school” • Abandoned mines • Tailings/waste rock piles • ARD • “New School” • Cyanide heap leach mining

20. “New School” • Cyanide Heap Leach • Extract gold from low grade ore • Ore crushed, placed in open air leach pads • Cyanide sprayed on top • Leaches gold as migrates through ore • Solution drained, gold recovered • Pretty huh?

21. Summitville, Colorado

22. Summitville Mine • Rio Grande Headwaters • Elevation 12,800’ • Snowfall: 7-11 m/ year • Population: 700 • 112 stamping machines • Abandoned in early 1900s • Gold prices fell, diminishing returns, weather issues

23. Summitville • 1984 • Application for mining permit • 1985 • Large scale open pit gold mine • Cyanide leaching • 1986 • Construction. Problems.

24. Summitville • 1987-1991: Heap Leach Pad • 73 acres • One pile >190’ • No outlet for water

25. Summitville • 1987-1991 cont • Permit to discharge excess water. Limits in concentrations • Could not meet limits • Fish kills downstream for 17 miles in Alamosa River

26. Summitville • 1992 • EPA assumes control, $20,000,000 to ‘fix’ • Heap leach pad near overflow, discharging 3,000 gallons/minute through leaks • 200 million gallons of cyanide laced water • Not last till spring snowmelt

32. Costs • To date: $185 million • Annually: $1.5 million • Taxpayers foot bill • Mine owner cost: $3 million bond

33. Conclusions • Acid rock drainage is generated at mines and naturally where sulfide minerals are present and the buffering capacity of the water is exceeded. • AMD degradation can be acute because: • 1) Mines act as collectors of groundwater • 2) Water is in contact with high grade ore minerals • 3) Mine dumps and tailings provide dramatically increased surface areas for the interaction of water, oxygen, and sulfide minerals.