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Acid Mine Drainage. 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 How to distinguish?. Characteristics. Increased acidity = decreased pH

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terms
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
  • How to distinguish?
characteristics
Characteristics
  • Increased acidity = decreased pH
  • Increased metal concentrations
  • Increased sulfate
  • Increased suspended solids

All four don’t necessarily occur at the same time

stream effects
Stream Effects
  • Colored waters:
  • “Yellow boy”
    • Iron oxides, basically rusting the stream floor
  • White
    • Aluminum
  • Black
    • Manganese
  • Determined by shifts in pH
shift in mining techniques
Shift in Mining Techniques
  • “Old school”
    • Abandoned mines
    • Tailings/waste rock piles
    • ARD
  • “New School”
    • Cyanide heap leach mining
amd chemistry

pyrite

water

+ air

low pH

+ metals

AMD Chemistry
  • Pyrite weathering
amd chemistry7
AMD Chemistry

Iron oxide

4FeS2 + 14 H2O + 15 O2 → 4Fe(OH)3 + 8 SO42- + 16 H+

Overall acid producing

amd chemistry8
AMD Chemistry
  • Surface area
    • more surface area, faster rate
    • smaller grains, more surface area
extent of problem
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
treatment
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
active treatment
Active Treatment
  • Typical treatment processes (“ODAS”)
    • -oxidation
    • -dosing with alkali
    • -sedimentation
new school
“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?
summitville mine
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
summitville
Summitville
  • 1984
    • Application for mining permit
  • 1985
    • Large scale open pit gold mine
    • Cyanide leaching
  • 1986
    • Construction. Problems.
    • HDPE liner ripped during placement, but not fixed.
summitville18
Summitville
  • 1987-1991: Heap Leach Pad
    • 73 acres
    • One pile >190’
    • No outlet for water, only lost through ET
    • Snowfall underestimated, ET overestimated

Now What?

summitville19
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
summitville20
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
costs
Costs
  • To date: $185 million
    • Annually: $1.5 million
  • Taxpayers foot bill
  • Mine owner cost: $3 million bond
summary
SUMMARY
  • 4 characteristics of AMD
  • Pyrite weathering (Fool’s gold) source of acidity in many AMD problems
  • “Old School” and “New School” mining
  • Cyanide heap leach technology
  • Summitville example