Presentation on theme: "Modeling AMD Geochemistry in Underground Mines"— Presentation transcript:
1Modeling AMD Geochemistry in Underground Mines Bruce Leavitt PE PG, Consulting HydrogeologistJames Stiles PhD PE, Limestone EngineeringRaymond Lovett PhD, Shipshaper LLC
2Limitations of existing AMD Prediction Methods Only considers Acid and Base PotentialDoes not consider Latent AcidityDoes not consider Oxygen DepletionDoes not consider Solute TransportDoes not consider Recharge Water Chemistry and Volume
3Study PurposeTo 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.
5Effect of Flooding on Mine Water Chemistry Rapid dissolution of acidic saltsExclusion of oxygen from the mineChemical reaction with recharging ground water.
6TOUGHREACT 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.
8Mineral Assemblage Mineral Volume Concentration K25 (mol/m2/s) Ea (kJ/mol)calcite0.001equilibriumgypsum0.0001melanterite0.002rhodochrosite0.0103.55x10-640.0illite0.4006.9185x10-1322.2jarositeAl(OH)3 (amorphous)gibbsitepyrolusite
14Pyrite Kinetic DataNeutral 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-1Acidic 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
15Ferrous Ferric Oxidation Fe+2 + 1/4O2 + H+ > Fe+3 +1/2 H2OOxidation rate is pH dependant.Model holds ferrous and ferric iron in equilibrium.Model overstates ferric iron concentration leading to excess pyrite oxidation.
24Modeling Difficulties Ferrous iron oxidationInsufficient aluminum productionCO2 partial pressure spikes at full mine floodingMine complexity is limited by computational capacityHomogeneous mineral distributionMine atmosphere composition
25Other 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.
26Future 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.
27ConclusionsThe 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.