Acid Mine Drainage Terms Acid Mine Drainage AMD

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 p. H 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 p. H

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

AMD Chemistry • Pyrite weathering pyrite water + air low p. H + metals

AMD Chemistry Iron oxide 4 Fe. S 2 + 14 H 2 O + 15 O 2 → 4 Fe(OH)3 + 8 SO 42 - + 16 H+ Overall acid producing

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

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 p. H – 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

“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 • • • Rio Grande Headwaters Elevation 12, 800’ Snowfall: 7 -11 m/ year Population: 700 112 stamping machines Abandoned in early 1900 s – 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. – HDPE liner ripped during placement, but not fixed.

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?

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

Summitville • 1992 – EPA assumes control, $20, 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

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