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Modeling AMD Geochemistry in Underground Mines

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Presentation on theme: "Modeling AMD Geochemistry in Underground Mines"— Presentation transcript:

1 Modeling AMD Geochemistry in Underground Mines
Bruce Leavitt PE PG, Consulting Hydrogeologist James Stiles PhD PE, Limestone Engineering Raymond Lovett PhD, Shipshaper LLC

2 Limitations of existing AMD Prediction Methods
Only considers Acid and Base Potential Does not consider Latent Acidity Does not consider Oxygen Depletion Does not consider Solute Transport Does not consider Recharge Water Chemistry and Volume

3 Study 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.

4 Three Hydrologic Settings

5 Effect of Flooding on Mine Water Chemistry
Rapid dissolution of acidic salts Exclusion of oxygen from the mine Chemical reaction with recharging ground water.

6 TOUGHREACT Earth Sciences Division, Lawrence Berkeley 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.

7 Model Configuration

8 Mineral Assemblage Mineral Volume Concentration K25 (mol/m2/s)
Ea (kJ/mol) calcite 0.001 equilibrium gypsum 0.0001 melanterite 0.002 rhodochrosite 0.010 3.55x10-6 40.0 illite 0.400 6.9185x10-13 22.2 jarosite Al(OH)3 (amorphous) gibbsite pyrolusite

9 Mineral Assemblage cont.
Volume Concentration K25 (mol/m2/s) Ea (kJ/mol) ferrihydrite 0.001 6.9185x10-13 22.2 jurbanite 1.0233x10-14 87.7 quartz kaolinite 0.500 Neutral x10-14 Acid x10-12 Base x10-18 65.9 17.9 chlorite Neutral x10-13 Acid x10-12 Base N/A 88.0 N/A pyrite 0.0015 Neutral x10-6 Acid x10-9 56.9 siderite Neutral x109-9 Acid x10-4 62.76 36.1 magnetite Neutral x109-11 Acid x10-9 18.6

10 Archetype pH

11 Archetype Iron

12 Model Results pH

13 Model Results Iron

14 Pyrite Kinetic Data Neutral x 10-6 mol-m-2-s-1 McKibben and Barnes (1986a) Neutral x mol-m-2-s-1 McKibben and Barnes (1986b), Nicholson (1994), and Nicholson and Sharer (1994) Acidic x 10-9 mol-m-2-s-1 Acidic 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 mol-m-2-s-1 Calibrated

15 Ferrous 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.

16 High Dilution pH Year 5

17 High Dilution pH Year 10

18 High Dilution pH Year 15

19 High Dilution pH Year 20

20 High Dilution Iron Year 5

21 High Dilution Iron Year 10

22 High Dilution Iron Year 15

23 High Dilution Iron Year 20

24 Modeling Difficulties
Ferrous iron oxidation Insufficient aluminum production CO2 partial pressure spikes at full mine flooding Mine complexity is limited by computational capacity Homogeneous mineral distribution Mine atmosphere composition

25 Other 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.

26 Future Work Resolve the iron oxidation issue
Closed mine atmosphere sampling. Sensitivity analysis of input parameters including: recharge chemistry, mine geometry, initial melanterite and calcite concentrations. Testing of in situ remedial options.

27 Conclusions 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.

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