Post on 26-Mar-2015
transcript
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.