Formation and Treatment Acid Mine Drainage Formation and Treatment

Introduction AMD is an environmental concern many states face 2nd largest polluter in PA streams PA has over 5000+ miles of streams impacted 1910—AMD research began 1977—passage of Surface Mine Control and Reclamation Act (SMCRA)

AMD Formation AMD occurs due to the exposure of overburden during mining Function of geology, hydrology and mining technology Neutralizing compounds are absent from soil and the water becomes acidic Contaminated with high concentrations of Fe, Mn, Al, and SO4

Chemistry Pyrite + Oxygen + Water  “Yellowboy” + Acid 4FeS2 + 15O2 + 14H2O  4Fe(OH)3 + 8H2SO4

Effects of AMD Oxygen removal Depletes buffering capacity Decreases light penetration Smothers bottom dwellers Directly toxic Economic: PA loses $67 million each year

Treatment of AMD 1968—PA instituted strict effluent discharge limits Coal companies used chemical treatment to meet standards Calcium carbonate, sodium hydroxide, anhydrous ammonia Raise pH and precipitate metals

Active Treatment Mechanized procedure for addition of alkaline material Needs constant monitoring and maintenance Can exceed $200,000 per year at one site Produces large volumes of sludge Potentially harmful chemicals are used Caustic soda, soda ash, sodium hydroxide

Passive Treatment Mid 1980s No input of manufactured chemicals Lower operation costs Longer retention times and larger treatment areas First passive system was natural Sphagnum wetland

Constructed Wetland Encourage oxidation processes to precipitate metals and increase pH Aerobic Rely on oxidation Shallow, vegetated Surface flow Anaerobic No oxygen Organic layer with sulfate reducing bacteria

Anoxic Limestone Drain Alkalinity produced by AMD reacting with calcium carbonate Constructed as shallow channel sealed from atmosphere Used of oxidation basin after for metals to precipitate

Vertical Flow Wetland Combine ALDs and anaerobic wetlands Constructed as ponds with grid like piping system, 3’ of limestone, 2’ of mushroom compost, and 2-3’ of standing water Adds alkalinity through limestone dissolution and sulfate reduction

Open Limestone Channels Simplest passive treatment method Construct drainage ditch and fill with limestone Or, add limestone directly to the stream High velocity and turbulence enhance performance, steep slopes

Diversion Wells Well of limestone Hydraulic force of the water causes the limestone to mix and abrade particles of limestone Diverted back to the stream

Problems Use depends on water chemistry and flow rates Must calculate loadings! High Fe may “coat” limestone, less efficient High Al will precipitate and plug pore space

Other Solutions Remining Reclamation Removal of “spoil” piles Injection of alkaline material into mines Sealing of deep mine portals Fix at source instead of band-aid!