the bioremediation of mine-site pit lakes: considerations...
TRANSCRIPT
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The Bioremediation of Mine-Site Pit Lakes: Considerations, Limitations and Case Studies
Alan Martin, Jay McNee, Jan Gerits and Robert Goldblatt
Lorax Environmental Services Ltd.(604) [email protected]
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Outline
• Introduction
• Data requirements (basis of any action)
• Take-Away Messages
• Considerations/limitations for pit lake bioremediation
• Case Studies
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Bioremediation - Introduction
• Typically involves:1. Addition of nutrients (inorganic or organic)
and/or2. Addition of organic matter
WATER QUALITY OBJECTIVES
• Offers attractive alternative to traditional in-situ treatments (liming) and active treatment given potential lower costs, “benign nature” and sustainability (passive or semi-passive level of effort)
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Beneficial Mechanisms
1. Increased particle concentration andbiological scavenging
Zooplankton feacal pellets
Algal detritus (organic flocs)2. Particle settling
Sorption to algal surfaces
Biological assimilation
DECREASED [TOTAL] AND [DISSOLVED]
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Beneficial Mechanisms3. Oxygen Demand• Increased sediment/water oxygen demand which can lead
to suboxic/reducing conditions
Alkalinity generation – acid neutralizing potentiale.g., (CH20)106(NH3)16(H3PO4) + 53SO4
2-↔ 39CO2 + 16NH4++
HPO42- + 39H2O + 53 HS- + 67HCO3
-
Sulphate reduction – metal sulphide precipitation(e.g., Cu (aq) + HS-↔ CuS (s))
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Algal Growth
• Phytoplankton assimilate C and P at a ratio of ~ 106:1
• Therefore, only trace quantities of P addition are required to generated “eutrophic” conditions
• Algal growth limited by available phosphorus
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BioremediationWho does the work?
Algae AND Bacteria
Take advantage of incredible metabolism and assimilative capacity
Pit Lakes Well Suited to Bioremediation• natural algal communities• respond well to nutrient amendments
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Bioremediation:Considerations/limitations
• Water quality (parameters of concern)• Lake morphometry• Water/Chemical mass balance (flow and chemistry
of all inflows and outflows)• Physical Limnology (e.g., stratification)• Waste management (backfill, tailings, ARD,
treatment sludges)
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Considerations –Water Quality
• Parameters of ConcernParameters such as pH , NH3, Zn, Cu, Ni, Cd, sulphate are amendable to bioremediationParameters such as Se, Mo, and Ra less so
• Toxicity may limit effectiveness of bioremediationpHTrace elementsAmmonia
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WindMixing Depth
Pit LakeShield Lake
• Vertical structure has important chemical implications
Considerations - Lake Mixing
In pit lakes, vertical mixing is restricted due to:•Deep water column•Small fetch•Topographic sheltering (pit walls)•Seasonal and/or permanent stratification
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Considerations – BackfillTailings/Wasterock/Treatment Products
Mixing
• Physical Mixing• turn over more likely• limits on geochemical evolution
• Chemical stability of solids• Water Quality in shallow water columns is more susceptible to exchanges across waste-water boundary
Exchange
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Considerations Disposal of treatment sludges
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oxygenheat
• Progressive increase in deep watertemperature through spring and summer
Oxygen:Slurry
• Surface water does not have to cool as much for lake to be isothermal = Earlier mixing of surface and deep waters (“turn-over”)
• Replacement of deep water oxygen via slurry
• No depletion of secondary oxidants = limits redox gradient
Heat:
Considerations –Sludge/ARD DisposalConceptual Model
• Induced Circulation
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Case Studies – Field Scale Using Enclosures
•Carbon Sources• Sewage sludge• Straw• Green wastes • Carbokalk: by product of sugar industry• Others: pyruvate, ethanol, whey, molasses, potatoes
• Nutrient sources:• Liquid fertilizers (inorganic P and N)• Organic fertilizers• Pellet-based fertilizers• Phosphate rock
• Results:• Increase in algal biomass, organic carbon, total phosphorus • Increased sediment/water oxygen demand• Development of suboxic conditions• Mitigation of acidic pH (acid neutralizing potential)• Metal removal
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Case Studies – Whole Lake
• Mt. Nansen Tailings Pond - Yukon
Bioremediation Considered –not implemented
Bioremediation Implemented• Island Copper Mine – B.C.• Colomac – North West Territories• Lake Koyne 113 – Germany• Rävlidmyran Pit Lake - Sweden• Grum Pit Lake – Yukon
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Case Study 1 – Mt. Nansen
• Mt. Nansen Tailings Pond• 4 ha tailings facility• Contaminants of concern: ammonia (>10 mg/L)• Objective: enhance ammonia removal through stimulation of algal growth
However, numerous secondary arsenic-bearing phases were identified which will become more soluble under more reducing sediment conditions, including:• Fe oxyhydroxides present as isolated particles• Fe oxyhydroxides present as alteration rims on sulphide grains• Fe-arsenate (FeAsO4) as oxidation rims on arsenopyrite
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Mt. Nansen – Tailings Pond
Summary: The prevalence of redox-sensitive secondary phases in the tailings deposits, and the abundance of associated arsenic, precluded therecommendation of fertilization as a viable form of bioremediation for the tailings pond.
As-bearing Fe oxyhydroxide
Fe-arsenate rim on arsenopyrite
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Case Study 2
Lake Koyne 113: Whole-lake bioremediation
• Study Site: Lake Koyne 113• Lusatian mining lake in Germany• Open-cast lignite mine• Shallow (maximum depth of 2.5 m)• Acidic (pH=2.5), high concentrations of major ions and trace elements
• Treatment:• “Biobags”: jute bags (50 x 60 cm) filled with cut-up beer and water bottle labels (source of oganic carbon)- 5 tons per hectare• Objective: reversal of acidification (via formation of anoxic microbial reaction compartments and increase primary production)
• Results:• Increase in algal biomass, organic carbon, total phosphorus, • No change in pH• Unsuccessful in meeting objective in increasing pH• Limited by high acidity of inflowing groundwater
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Case Study 3
Rävlidmyran Pit Lake: Whole-lake bioremediation
• Study Site: Rävlidmyran Pit Lake• Skellefte ore district of northern Sweden• Open pit/underground mining (1953-1974). Flooded since 1975.• Area = ~5 ha, and mean depth of 11 m• Low pH (pH=3), oligotrophic, and meromictic
• Treatment:• Pre-treatment with lime (200 tonnes in a three week period)• Followed by addition of 300 tons of sewage sludge•Objectives: Metal sorption to sewage particles
Development of sulphate reduction in hypolimnion and metal removal• Results:
• Liming increased pH to 6 to 8, removal of dissolved trace elements (Fe, Mn, Zn, Cu)• Sewage treatment: No evidence of enhanced metal scavenging by particles
No evidence of sulphate reduction in bottom waters Unsuccessful in meeting objectives
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Surface area: 100,000 m2
Max. depth:50 m
Ice-cover in winter
Seasonal turnover
Oxygenated water column
10-12 mg/L Zn
Case Study 4 – Faro Mine Grum Lake Bioremediation
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• Characterization of Grum Pit Lake• physical limnology• geochemical limnology
• Remediation and Monitoring Program• one season – focus on Zn uptake by algal growth• fertilization with customized liquid nutrient best suited to lake (weekly)• semi-weekly sampling
Grum Lake (Bioremediation)
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Grum Lake (fertilization)
Existing algae
Algae in surface waters responded to nutrient addition
Response of Algal Community - Chlorophyll
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July SeptTime (weeks)1 2 3 4 5 6 7 8 9
Grum Lake (fertilization)
Existing algae
Algae in surface waters responded to nutrient addition
Response of Algal Community - Chlorophyll
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Grum Lake (fertilization)
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Grum Lake – Zinc Response1 m
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& D
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D-Zn
T-Zn
• Zn decrease to <100 ug/L at 1 m
• Largest net decrease at 3 m depth from 12 mg/L to ~1 mg/L
• No Zn loss at 5 m depth (no effect below mixed layer)
1 m
5 m
3 m
July SeptTime (weeks)1 2 3 4 5 6 7 8 9
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Data Requirements• Site-Specific Meteorology
• very affordable (< $10,000)• data are invaluable at closure
• Inflows and Outflows• WQ, temperature and salinity• Flows• Dewatering program (assess chemistry)
• Pit Geometry/Bathymetry
• Pit Lake Elevation Data• (once pit begins to fill)
• These data are Required to Make Robust Predictions• Proxy data must be found where data do not exist• Weakens accuracy of predictions
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• Pit-lake management strategies are strongly dependent on site-specific factors, including
• Pit geometry• Physical mixing• Climate• Water balance• Parameters of concern• Management practices• Water quality objectives
Emphasis –Site Specific Management
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MainZonePit
WaterlinePit
Equity Silver Mine
Example of within-region pit lake contrasts
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Pit Lake ComparisonEquity Silver Mine
Waterline Pit
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Pit Lake ComparisonEquity Silver Mine
Main Zone Pit
Sludge
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Conclusions - Bioremediation
• Bioremediation through fertilization (enhancing algal growth) has been shown to provide effective management of pit lakes at whole-lake scale.
• Successful bioremediation requires careful consideration of site specific physical and biogeochemical variables (enormous degree of pit-lake specificity, based on both regional and within-region variables).
• Bioremediation through other means (addition of organic matter)has shown mixed results at full scale.