Presentation on theme: "Modeling AMD Geochemistry in Underground Mines Bruce Leavitt PE PG, Consulting Hydrogeologist James Stiles PhD PE, Limestone Engineering Raymond Lovett."— Presentation transcript:
Modeling AMD Geochemistry in Underground Mines Bruce Leavitt PE PG, Consulting Hydrogeologist James Stiles PhD PE, Limestone Engineering Raymond Lovett PhD, Shipshaper LLC
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
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.
Three Hydrologic Settings
Effect of Flooding on Mine Water Chemistry Rapid dissolution of acidic salts Exclusion of oxygen from the mine Chemical reaction with recharging ground water.
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.
Mineral Assemblage Mineral Volume Concentration K 25 (mol/m 2 /s)Ea (kJ/mol) calcite0.001equilibrium gypsum0.0001equilibrium melanterite0.002equilibrium rhodochrosite x illite x jarosite x Al(OH) 3 (amorphous) x gibbsite x pyrolusite x
Mineral Assemblage cont. Mineral Volume Concentration K 25 (mol/m 2 /s)Ea (kJ/mol) ferrihydrite x jurbanite x quartz x kaolinite0.500 Neutral 6.918x Acid 4.898x Base 8.913x chlorite0.001 Neutral 3.020x Acid 7.762x Base N/A 88.0 N/A pyrite Neutral 2.818x10 -6 Acid 3.020x10 -9 Base N/A 56.9 N/A siderite0.001 Neutral 1.660x Acid 2.570x10 -4 Base N/A N/A magnetite0.001 Neutral 1.260x Acid 6.457x10 -9 Base N/A 18.6 N/A
Model Results pH
Model Results Iron
Pyrite Kinetic Data Neutral x 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 mol-m -2 -s -1 Acidic x 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
Ferrous Ferric Oxidation Fe /4O 2 + H + > Fe +3 +1/2 H 2 O 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 pH Year 5
High Dilution pH Year 10
High Dilution pH Year 15
High Dilution pH Year 20
High Dilution Iron Year 5
High Dilution Iron Year 10
High Dilution Iron Year 15
High Dilution Iron Year 20
Modeling Difficulties Ferrous iron oxidation Insufficient aluminum production CO 2 partial pressure spikes at full mine flooding Mine complexity is limited by computational capacity Homogeneous mineral distribution Mine atmosphere composition
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.
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.
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.