Modeling AMD Modeling AMD Geochemistry in Geochemistry in

Underground MinesUnderground Mines

Bruce Leavitt PE PG, Consulting Hydrogeologist James Stiles PhD PE, Limestone EngineeringRaymond Lovett PhD, Shipshaper LLC

Limitations of existing AMD Limitations of existing AMD Prediction MethodsPrediction Methods

Only considers Acid and Base Potential Does not consider Latent Acidity Does not consider Oxygen DepletionDoes not consider Solute TransportDoes not consider Recharge Water Chemistry and Volume

Study PurposeStudy Purpose

To investigate the suitability of the model to underground mine discharges.

To determine the appropriate mineral assemblage and mass concentration.

To compare the model in different hydrologic settings.

To evaluate the sensitivity of the model to variations in input values comparable to typical field variations.

Three Hydrologic SettingsThree Hydrologic Settings

Flooded High Dilution

River

overburden

Mine Discharge

Flooded Mine Low Dilution

River

overburden

MinePump

No Discharge

Unflooded, Free Draining

River

overburden Mine Discharge

Effect of Flooding on Mine Effect of Flooding on Mine Water ChemistryWater Chemistry

Rapid dissolution of acidic saltsExclusion of oxygen from the mineChemical reaction with recharging

ground water.

TOUGHREACTTOUGHREACT Earth Sciences Division, Lawrence Berkeley Earth Sciences Division, Lawrence Berkeley

National Laboratory National Laboratory

TOUGHREACT was designed to solve the coupled equations of sub-surface multi-phase fluid and heat flow,

solute transport, and chemical reactions in both the saturated and unsaturated aquifer zones. This program can

be applied to many geologic systems and environmental problems, including geothermal systems, diagenetic and

weathering processes, subsurface waste disposal, acid mine drainage remediation, contaminant transport, and

groundwater quality.

Model ConfigurationModel Configuration

Mineral Assemblage Mineral Assemblage

MineralVolume

ConcentrationK25 (mol/m2/s) Ea (kJ/mol)

calcite 0.001 equilibrium equilibrium

gypsum 0.0001 equilibrium equilibrium

melanterite 0.002 equilibrium equilibrium

rhodochrosite 0.010 3.55x10-6 40.0

illite 0.400 6.9185x10-13 22.2

jarosite 0.001 6.9185x10-13 22.2

Al(OH)3

(amorphous)0.001 6.9185x10-13 22.2

gibbsite 0.001 6.9185x10-13 22.2

pyrolusite 0.001 6.9185x10-13 22.2

Mineral Assemblage cont.Mineral Assemblage cont.

MineralVolume

ConcentrationK25 (mol/m2/s) Ea (kJ/mol)

ferrihydrite 0.001 6.9185x10-13 22.2

jurbanite 0.001 1.0233x10-14 87.7

quartz 0.001 1.0233x10-14 87.7

kaolinite 0.500Neutral 6.918x10-14

Acid 4.898x10-12

Base 8.913x10-18

22.265.917.9

chlorite 0.001Neutral 3.020x10-13

Acid 7.762x10-12

Base N/A

88.088.0N/A

pyrite 0.0015Neutral 2.818x10-6

Acid 3.020x10-9

Base N/A

56.956.9N/A

siderite 0.001Neutral 1.660x109-9

Acid 2.570x10-4

Base N/A

62.7636.1N/A

magnetite 0.001Neutral 1.260x109-11

Acid 6.457x10-9

Base N/A

18.618.6N/A

Archetype pHArchetype pH

Archetype IronArchetype Iron

Model Results pHModel Results pH

0 4 8 12 16 20S im ula tion T im e, years

1

2

3

4

5

6

7

8p

HpHF ree D ra in ingLow D ilu tionH igh D ilu tio nF lood ing T im e

Model Results IronModel Results Iron

0 4 8 12 16 20S im ula tion T im e, years

0

400

800

1200

1600

2000T

otal

Iron

, m

g/L

T o ta l IronF ree D ra in ingLow D ilu tionH igh D ilu tionF ill T im e

Pyrite Kinetic Data Pyrite Kinetic Data

Neutral 2.818 x 10-6 mol-m-2-s-1 McKibben and Barnes (1986a)

Neutral 3.167 x 10-10 mol-m-2-s-1 McKibben and Barnes (1986b), Nicholson (1994), and Nicholson and Sharer (1994)

Acidic 3.020 x 10-9 mol-m-2-s-1

Acidic 1.553 x 10-8 mol-m-2-s-1 McKibben and Barnes (1986b), Brown and Jurinak (1989), and Rimstidt, et al. (1994)

Acidic 6.0 x 10-10 mol-m-2-s-1 Calibrated

Ferrous Ferric OxidationFerrous Ferric Oxidation

Fe+2 + 1/4O2 + H+ > Fe+3 +1/2 H2O

Oxidation rate is pH dependant.Model holds ferrous and ferric iron in

equilibrium.Model overstates ferric iron concentration

leading to excess pyrite oxidation.

High Dilution pHHigh Dilution pHYear 5Year 5

1

1.5

2

2.5

3

3.5

4

4.5

5

5.5

6

6.5

7

7.5

8

High Dilution pHHigh Dilution pHYear 10Year 10

1

1.5

2

2.5

3

3.5

4

4.5

5

5.5

6

6.5

7

7.5

8

High Dilution pHHigh Dilution pHYear 15Year 15

1

1.5

2

2.5

3

3.5

4

4.5

5

5.5

6

6.5

7

7.5

8

High Dilution pHHigh Dilution pHYear 20Year 20

1

1.5

2

2.5

3

3.5

4

4.5

5

5.5

6

6.5

7

7.5

8

High Dilution IronHigh Dilution IronYear 5Year 5

1

1.5

2

2.5

3

3.5

4

4.5

5

5.5

6

6.5

7

7.5

8

0

100

200

300

400

500

600

700

800

900

1000

1100

High Dilution IronHigh Dilution IronYear 10Year 10

0

10 0

20 0

30 0

40 0

50 0

60 0

70 0

80 0

90 0

10 00

11 00

High Dilution IronHigh Dilution IronYear 15Year 15

0

10 0

20 0

30 0

40 0

50 0

60 0

70 0

80 0

90 0

10 00

11 00

High Dilution IronHigh Dilution IronYear 20Year 20

0

10 0

20 0

30 0

40 0

50 0

60 0

70 0

80 0

90 0

10 00

11 00

Modeling DifficultiesModeling Difficulties

Ferrous iron oxidationInsufficient aluminum productionCO2 partial pressure spikes at full mine

floodingMine complexity is limited by

computational capacityHomogeneous mineral distributionMine atmosphere composition

Other ResultsOther Results

Gypsum precipitation / dissolution in the mine

Goethite precipitation in the mine.Elimination of pryhotite and the reduction

of the pyrite kinetic rate has reduced the observed difference in water pH and iron between the high dilution and low dilution cases.

Future WorkFuture Work

Resolve the iron oxidation issueClosed mine atmosphere sampling.Sensitivity analysis of input parameters

including: recharge chemistry, mine geometry, initial melanterite and calcite concentrations.

Testing of in situ remedial options.

ConclusionsConclusions

The TOUGHREACT program allows chemical and hydrodynamic interaction in a flooded and unflooded underground mine environment.

TOUGHREACT is able to emulate the change in discharge chemistry with time.

It is a useful tool in understanding acid formation, solute transport, and discharge relationships.

Due to the extensive number of assumptions it is not, at this time, a suitable permitting tool.