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

Sources of Acid Mine Drainage (AMD) Mine Effluent

Mine Dump Mill Tailings

AMD Chemistry Pyrite weathering pyrite water + air low pH + metals

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

AMD Chemistry 4FeS H 2 O + 15 O 2 → 4Fe(OH) SO H + Iron oxide Overall acid producing

THE CHEMISTRY of ACID MINE DRAINAGE Reaction 1: FeS 2(s) + H 2 O + 7/2O 2 2H + Fe SO 4 + 2H + H + Reaction 2:* Fe /4O 2 + H + Fe /2H 2 O Reaction 3: FeS 2(s) + 8H 2 O + 14Fe 3+ 16H + 15Fe SO H + Reaction 4: Fe H 2 O 3H + Fe(OH) 3(s) + 3H + pyrite water sulfate acid * catalyzed by bacteria

AMD Chemistry Surface area –more surface area, faster rate –smaller grains, more surface area

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

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

Passive Treatment

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

Active Treatment Iron Mountain, California

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

“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, Colorado

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

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

Summitville : Heap Leach Pad –73 acres –One pile >190’ –No outlet for water

Summitville cont –Permit to discharge excess water. Limits in concentrations –Could not meet limits –Fish kills downstream for 17 miles in Alamosa River

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 To date: $185 million –Annually: $1.5 million Taxpayers foot bill Mine owner cost: $3 million bond

Conclusions 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. Acid rock drainage is generated at mines and naturally where sulfide minerals are present and the buffering capacity of the water is exceeded.