Evolution of Mine-Pool Chemistry at the Sydney Coal Mines, Nova Scotia, and Its Impacts on Decommissioning Strategy Y.T. John Kwong 13th British Columbia MEND ML/ARD Workshop Vancouver, November 29-30, 2006 Acknowledgement Bob MacDonald (Cape Breton Development Corporation) Steve Forgeron (Conestoga-Rovers & Associates, formerly Chief Geologist, CBDC) Keith Brady (Pennsylvania Department of Environmental Protection) Presentation Outline • General setting of the Sydney coal mines • History of mine water-related issues • Controls of mine water chemistry • Decommissioning alternatives • The Sydney Coalfield, located on Cape Breton Island, is the largest in Atlantic Canada • About 100 coal mines with varied lifespans have been opened since 1720 • Coal has been mined from the Glace Bay sub-basin since 1980s • Seams dip N & NE at -4o to -15o •Coal mined at >1200 m depth and 11 km from coastline • Coal is pyrite- bearing Mine locations and coal seams exploited Sequence of Coal Seams Hub Seam Prince Atlantic Ocean Harbour Seam Phalen Seam Point Aconi Emery Seam Florence No. 12 PrincessPrincess No.No. 1212 LinganLingan No. 26 No. 26 Queen No. 26 188 Sydney Mines No..1 PhalenPhalen N .18 No No.16 No.No.No. 1B 1B No. 20No. 2 New Waterford Donkin GlaceNo. 20 No. 9 Glace Tunnels 75 No. 9 No.1ANo.1A No. 8 No.Bay 9 Bay No. 8 No. 6 No.No.No. 55 5 No. 4 No. 10 3 Donkin o. No. 4 No. 24 No. 10 N No. 24 No. 11 Sydney (Courtesy: S. Forgeron) Connected mines of the “1B Hydraulic System” Sequence of Coal Seams Hub Seam Prince Atlantic Ocean Harbour Seam Phalen Seam Point Aconi “1B Hydraulic Emery Seam System” Florence No. 12 PrincessPrincess No.No. 12 LinganLingan No. 26 No. 26 Queen No. 26 188 Sydney Mines No..1 PhalenPhalen N .18 No No.16 No.No.No. 1B 1B No.1B No. 20No. 2 New Waterford Donkin GlaceNo. 20 75 1B Shaft GlaceNo. 20 Tunnels No. 9 9 No.1ANo.1A No. 8 No. 9 No.No. 8 2 BayBay No. 6 No.No.No. 55 5 No. 4 No. 10 3 o. No. 4 No. 24 No. 10 N No. 24 No. 11 Donkin Sydney Courtesy: S. Forgeron (MGI et al., 2003) SelectedSelected MineMine WaterWater ChemistryChemistry 1 B 1A No. 5 No. 3 PARAMETER Shaft Borehole Colliery Colliery pH 4.1 3.8 7.4 7.0 Fe (mg/L) 1,640 745 0.56 3.1 - HCO3 (mg/L) 8 1 285 213 2- SO4 (mg/L) 6,870 6,650 1,128 837 Conductivity 13,300 6630 2,560 2,130 (µS/cm) - Flooding of Lingan Colliery due to November 1992: breach of a 325m coal barrier adjacent to the flooded No.26 Colliery - Pumping AMD from No.26 to save Lingan gave rise to plume in the ocean - Disallowed pumping led to premature closure of Lingan In 2002 when discharge from the 1A Outfall appeared imminent, CBDC was prepared to pump and treat water from the 1B shaft prior to disposal at sea. 1B Rd, Glace Bay Lime silo Pumping shaft Settling pond Windy Craggy in advanced Detailed hydrogeologic studies, however, ledexploration to a new approach. stage (W. Price) Controls of Mine Pool Chemistry 1A Colliery • 92% room & pillar with minor pillar removal • 45% coal removal Limestone • AMD from stored acidity in efflorescent salts • Little flushing No.5 Colliery • 75% room & pillar with extensive pillar removal • 85% coal removal • Acidity neutralized by collapsed limestone • Flushed system Efflorescent salts observed in underground workings at CBDC Perceived Water Flow Paths Before and During Mine Flooding Importance of Local Controls on Water Chemistry Relatively good-quality water is B196? found in full extraction mining areas No. 5 Pumping System <= October 2002 Late fall 2003 => Well Discharge Locations <= Cadegan Brook Constructed pond at => Neville Street • Largely stratified, stagnant Scenario prior to water mine pool with very poor- extraction from No.5 Mine to quality water at the bottom control water level at 1A and progressively better water at shallow depth except locally near the 1A outfall • Except where locally broken, the coal pillars effectively prevent mixing and exchange of water from the No.1A and No.5 mine pools • The same presumably applies to pillar barriers between other mine pools Looking Ahead for Mine Decommissioning Strategy • Pumping water from No.5 has so far maintained water level at No.1A below sea level and saved $0.5M per year for treatment • Concern: Will No.5 water quality deteriorate with further pumping? • Opportunity: Will the 1A discharge be of acceptable quality if the system is allowed to fill up? Sulphate at Neville Street 5000 Monitoring Wells Monitoring Wells A B-183 4000 A A A B-177 AA A A D B-172 AA A AA A A A A B-180 3000 AA A A B-182 AA B-171 A D A A B-186 2000 AADA A SO4 (mg/l) B-175 ADDAAAA A AAA A DDAADAAADAAA A 1000 D A A A A AAA A AAAAAAAA AA A D AAAAAA A AA 0 A 20 20 20 20 20 02 03 04 05 06 Time Total Iron at Neville Street 100.00 A Legend Legend A D B-171 10.00 A A D B-172 A A A A A B-175 AA A A A A A A A B-177 AA A 1.00 AA A A B-180 AAAAAAAAAAAAAAAA A A A B-182 Fe (mg/l) A B-183 0.10 AAA A AAAA AAAA AAAAAAAAAA 0.01 20 20 20 20 20 02 03 04 05 06 Time Pumping at rates in excess of inflow rate at B-188 showed evidence of mixing Cl Will a fresh water lens form on top of the 1A mine pool with the elimination of the water level? - A “hydrologic bathtub” does not necessarily mean a homogeneous mine pool - Numerous pit lakes (e.g. Island Copper, the Berkeley Pit) are stratified A laboratory demonstration of maintaining a fresh water cover (dyed with fluorescein) on seawater without mixing Stratification is also apparent at monitoring wells near the 1A Outfall The shallowest hole (62’ deep) has higher pH and lower Fe and SO holes (123’ & 215’) 4 compared to the deeper Temporal changes in SO4 12000 10000 8000 6000 mg/L 4000SO4 2000 0 Temporal Change in pH Jan-03 pH 8 Jul-03 6 4 Jan-04 2 PBH01- 7 Jul-04 0 PBH01- 5 Jan-03 PBH01- 2 Jul-03 Temporal changesJan-04 in Fe(tot) 1500 Jul-04 1000 PBH01-7 mg/L Fe PBH01-5 500 PBH01-2 0 Jan-03 Jul-03 Jan-04 Jul-04 PBH01- 7 PBH01- 5 PBH01- 2 To determine if perpetual active treatment is unavoidable or a passive, walk-away solution is possible - Field monitoring coupled with laboratory simulation and modeling exercise to confirm mine pool stratification and its long-term stability - Research to establish if the mine pools can ever be flushed given its setting 100 50 East 0 West 1st Qtr2nd3rd Qtr4th Qtr North Qtr (ADI, 2002) Thank you for your attention! Questions?.