In-Situ Bioremediation Demonstration of Coal-Based Acid Mine Drainage Tide Mine Site Indiana County, Pennsylvania Presented to: 26 th West Virginia Surface Mine Drainage Task Force Symposium Kelly S. Houston ARCADIS G&M, Inc. 20 April 2005
AMD Chemistry Initiation Reaction: FeS 2(s) + H 2 O + 3.5 O 2 Fe 2+ + 2 SO 4 2- + 2 H + Propagation Reactions: Fe 2+ + 0.25 O 2 + H + 0.5 H 2 O + Fe 3+ 14 Fe 3+ + FeS 2(s) + 8 H 2 O 15 Fe 2+ + 2 SO 4 2- + 16 H + (Singer & Stumm, 1970) In-situ treatment can eliminate the causative agents of AMD 2
Breaking the AMD Triangle Provide an alternative sink for oxygen and ferric iron (e.g., a carbon source) Oxidation of the carbon source generates carbon dioxide/alkalinity Eliminating/minimizing water flushing - not required Water Oxygen/Ferric Iron Sulfide 3
Coal spoil pile IW-4 Tide Mine Schematic IW-3 IW-2 IW-1 Flow to Yellow Creek: 90 gpm ph = 3 300 PPM acidity No alkalinity Discharge Tunnel A Yellow Creek 125 acres of unsaturated mine 8 acres of saturated mine 4
Demonstration Project Plan 3 months of recirculation system operation with caustic and molasses injection to affect net change in biogeochemistry Bromide tracer study to characterize mine pool hydraulics Carbon dioxide gas injection to alter mine atmosphere Additional 3 months of passive methanol injection to demonstrate sustainability of in-situ treatment Fenced enclosure photo of recirc system 5
Bromide Tracer Test to Evaluate Mine Pool Hydraulics 16 14 12 Measured Predicted, CSTR First Moment Analysis t = (t i C C i i ) = 18 days Bromide (mg/l) 10 8 6 C = C o - t t e CSTR Model Analysis C 1 R LN = - (t) t R = 18 days C o t R 4 2 0 9/14/04 9/28/04 10/12/04 10/26/04 11/9/04 11/23/04 6
Elevated TOC Results in Sulfate Reducing Conditions 800 600 Sulfate TOC Sulfide Current Amount of Sulfate Reduction 5.0 4.0 Sulfate and TOC (mg/l) 400 3.0 2.0 Sulfide (mg/l) 200 1.0 First Molasses Injection 0 June-04 August-04 October-04 December-04 February-05 0.0 7
Metals Performance Monitoring Data 200 20 Ferrous Fe Ferric Fe Ferrous Iron (mg/l) 150 100 50 15 10 5 Aluminum, Ferric Iron, Manganse (mg/l) 0 June-04 August-04 October-04 December-04 February-05 0 Aluminum concentrations decreased greater than 50% during treatment function of ph 8
Continuing Sources of Iron in Mine Workings Reductive dissolution of solid-phase ferric hydroxide in the saturated portion of the mine workings Controlled through FeS precipitation and maintaining sulfate reduction in the mine pool Saturated portion accounts for approximately 6% of mine workings AMD-impacted water from the unsaturated portions of the mine workings Gas phase oxygen provides continuous source of AMD causative agents (i.e., dissolved oxygen and ferric iron) Requires gas phase treatment for AMD prevention or continuous liquid phase treatment for control Unsaturated portion accounts for approximately 94% of mine workings 9
Carbonate Alkalinity Is Now Dominant In Previously Acidic Mine Pool 400 Alkalinity ORP ph 7 350 300 6 ORP (mv), Alkalinity (mg/l) 250 200 150 5 4 ph 100 3 50 0 2 June-04 August-04 October-04 December-04 February-05 10
Re-Oxidation of Observed FeS? In-Situ Treatment: FeS + 2O 2 FeSO 4 + no acidity Natural Aerobic State: FeS 2 + 3.5O 2 + H 2 O FeSO 4 + H 2 SO 4 At the same operating re-dox state, oxidation of ferrous sulfides will not generate acidity in contrast with pyritic sulfides. Consistent with this expectation, alkalinity is observed from Tide Mine during the treatment period after washout of the NaOH neutralization effect 11
Carbon Dioxide Injection - Why Control the Gas Phase? Oxygen Molar Balance Pyrite oxidation stoichiometry indicates O 2 :SO 4 is 2:1 Oxygen-saturated water holds 8 ppm or 0.25 mm O 2 Assuming full conversion, 0.25 mm of O 2 creates 0.125 mm SO 4 Tide mine water has 8 mm sulfate Therefore, at a maximum, 1.5% of the sulfate produced by the mine is generated by aqueous phase oxygen 98.5% of the sulfate (and acidity) is being generated by gas phase molecular oxygen With O 2 consumed by AMD reactions, resulting gas is buoyant maintaining 5% CO 2 through direct injection or biological activity generates a dense anaerobic blanket 12
CO 2 addition to the mine atmosphere broke the Oxygen supply lines 25 Carbon Dioxide Injection Period 20 Oxygen, Carbon Dioxide (% ) 15 10 Oxygen Carbon Dioxide 5 0 June-04 August-04 October-04 December-04 February-05 13
Results Tide Mine project demonstrated the efficacy of insitu treatment proof of concept No dissolved oxygen in the mine pool or ferric iron in the mine pool Sustained increased ph in the mine pool, continued alkalinity detection Decreasing overall iron concentrations, reduced aluminum concentration Gas exchange occurs over large footprint of unsaturated mine workings; therefore, sustained CO 2 injection required for gas phase control at Tide Mine 14
How Does In-Situ Treatment Fit with the Big Picture? Economics of prevention vs. collection and treatment System-wide application of in-situ technology to stop or reduce AMD, minimize risk with blowout or mine failure Low cost O&M system implemented insitu Sludge settling occurs in mine workings Use of local agricultural products and waste streams Compatible with passive polishing systems 15