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Geology, Mining, and Water Quality
by Matthew A. Sares
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Acid Mine DrainageAcid Mine Drainage
WaterWater - from rain and snowmelt
++OxygenOxygen - from the air
++PyritePyrite - from the mine
Reaction =Reaction =
Sulfuric AcidSulfuric Acid
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Sources of Acid
Mine Drainage(AMD)
• Mine Effluent
Burbank Mine, San Juan Co.Burbank Mine, San Juan Co.
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Lewis Mine and Mill, San Miguel Co.Lewis Mine and Mill, San Miguel Co.
• Mill TailingsMill Tailings
• Mine DumpMine Dump
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THE CHEMISTRY THE CHEMISTRY ofof
ACID MINE DRAINAGEACID MINE DRAINAGE
THE CHEMISTRY THE CHEMISTRY ofof
ACID MINE DRAINAGEACID MINE DRAINAGE
Reaction 1: FeS2(s) + H2O + 7/2O2 Fe2+ + 2SO4 + 2H2H++
Reaction 2:* Fe2+ + 1/4O2 + HH++Reaction 2:* Fe2+ + 1/4O2 + HH++ Fe3+ + 1/2H2O Fe3+ + 1/2H2O
Reaction 3: FeS2(s)+ 8H2O + 14Fe3+ 15Fe2+ + 2SO4 + 16H16H++
Reaction 4: Fe3+ + 3H2O Fe(OH)3(s) + 3H3H++
pyrite water sulfate acid
* catalyzed by bacteria
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Lewis Mine and Mill, San Miguel Co.Lewis Mine and Mill, San Miguel Co.
Background Water QualityBackground Water Quality
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THE CHEMISTRY THE CHEMISTRY ofof
ACID MINE ACID MINE ROCKROCK DRAINAGE DRAINAGE
THE CHEMISTRY THE CHEMISTRY ofof
ACID MINE ACID MINE ROCKROCK DRAINAGE DRAINAGE
Reaction 1: FeS2(s) + H2O + 7/2O2 Fe2+ + 2SO4 + 2H2H++
Reaction 2:* Fe2+ + 1/4O2 + HH++Reaction 2:* Fe2+ + 1/4O2 + HH++ Fe3+ + 1/2H2O Fe3+ + 1/2H2O
Reaction 3: FeS2(s)+ 8H2O + 14Fe3+ 15Fe2+ + 2SO4 + 16H16H++
Reaction 4: Fe3+ + 3H2O Fe(OH)3(s) + 3H3H++
pyrite water sulfate acid
* catalyzed by bacteria
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Acid Rock Drainage (ARD) sources:Acid Rock Drainage (ARD) sources:
• Natural springs and drainages inNatural springs and drainages in
• Abandoned minesAbandoned mines
hydrothermally altered areashydrothermally altered areas
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Hydrothermal alteration is a process whereby hot water circulating within the earth changes the composition of rocks.
Intensely altered rocks have yellow, orange, and red colors from the oxidation of iron in pyrite.
East Mancos River headwaters, East Mancos River headwaters, Montezuma Co.Montezuma Co.
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(mod. from Guilbert and Park, 1986)
Hydrothermal Alteration
Feldspars >> ClaysIron and Sulfur >> Pyrite
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A number of streams in eleven
different headwater areas of Colorado have naturallynaturally high concentrations of
metals and/or acidity, upstreamupstream of of
any significant any significant human impacts.human impacts. Rocks in these
areas have been affected by intense
“hydrothermal alteration” in the
geologic past.
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Red Cone, Park Co.Red Cone, Park Co.
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Red Mountain, Chaffee and Pitkin Co.Red Mountain, Chaffee and Pitkin Co.
Springs
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Natural acid rock drainage dissolves metals present in the rock, affecting downstream water quality.
1 10 100 1000 10000
Factor Above Above Aquatic Life Standard
Co
nst
itu
ent
PEEKABOO GULCH SPRINGPEEKABOO GULCH SPRINGpH=2.17, conductance=2470 µS, Q=25 gpm
1,724 x
42 x
164 x
100 x
6 x
21 x
6 x
AlCdCuFeMnPbAgZn
SO4
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Lookout Mountain, Conejos Co.Lookout Mountain, Conejos Co.
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Iron Creek Lower Spring, Iron Creek Lower Spring,
Conejos Co.Conejos Co.
Ferrosinter deposits are
characteristic of springs in
hydrothermally altered areas.
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Some hydrothermally altered areas have high rates of erosion, and are prone to debris flows and landslides
because bedrock has been weakened.
Slide
Headwall
Red Mountain #2, Ouray Co.Red Mountain #2, Ouray Co.
Alum Creek, Conejos Co.Alum Creek, Conejos Co.
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Slumgullion landslide is in an area of hydrothermally altered rock. Movement and erosion continually expose acid-producing
minerals. Drainage from the slide area is very acidic (pH 3.4).
Slumgullion Landslide near Lake City, Mineral Co.Slumgullion Landslide near Lake City, Mineral Co.
HeadwallHeadwall
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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.
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• Both ARD sources must be characterized to determine realistic remediation goals.
• Both ARD sources must be characterized to determine realistic remediation goals.
Conclusions
• Natural ARD is important in areas of intense hydrothermal alteration because:
1) Rocks contain disseminated sulfide minerals.
2) Rocks are depleted of buffering minerals.
3) Large volumes of rock are affected.