Geology, Mining, and Water Quality

by Matthew A. Sares

Acid Mine DrainageAcid Mine Drainage

WaterWater - from rain and snowmelt

++OxygenOxygen - from the air

++PyritePyrite - from the mine

Reaction =Reaction =

Sulfuric AcidSulfuric Acid

Sources of Acid

Mine Drainage(AMD)

• Mine Effluent

Burbank Mine, San Juan Co.Burbank Mine, San Juan Co.

Lewis Mine and Mill, San Miguel Co.Lewis Mine and Mill, San Miguel Co.

• Mill TailingsMill Tailings

• Mine DumpMine Dump

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

Lewis Mine and Mill, San Miguel Co.Lewis Mine and Mill, San Miguel Co.

Background Water QualityBackground Water Quality

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

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

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.

(mod. from Guilbert and Park, 1986)

Hydrothermal Alteration

Feldspars >> ClaysIron and Sulfur >> Pyrite

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.

Red Cone, Park Co.Red Cone, Park Co.

Red Mountain, Chaffee and Pitkin Co.Red Mountain, Chaffee and Pitkin Co.

Springs

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

Lookout Mountain, Conejos Co.Lookout Mountain, Conejos Co.

Iron Creek Lower Spring, Iron Creek Lower Spring,

Conejos Co.Conejos Co.

Ferrosinter deposits are

characteristic of springs in

hydrothermally altered areas.

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.

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

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.

• 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.