surface water quality studies and effects assessment in ... · 7 water quality assessment:...
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Surface Water Quality Studies and Effects Assessment in Mining: Methods and ApplicationsSergei TouchinskiWaterTech 2010Banff, April 21-23, 2010
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Environmental Issues
Water diversion / dewatering / lakes drawdownMine site water management – Water Management Pond (WMP)Mine water managementWastewater Treatment Plant discharge (WTP)Sewage Treatment Plant discharge (STP)Water releases from tailings facilities
Construction, Operations, Closure/Reclamations
Changes in downstream streams and/or lakesEnd-Pit Lake water quality issues
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Assessment Methods
Flow and transport modelling: AQUASEA
Mass balance analysis: Loadings calculations (spreadsheet or Goldsim shell)
Physical limnology of stratified and meromictic lakes: Hydrodynamic criteria
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Flow and Transport Modelling
Relative concentration C/Co of conservative substance (no chemical reactions, no decay, but settling for TSS):
Runs made separately for different flow regimes:
Winter - no inflow or outflowSummer – natural inflow varied monthly – wind varied daily Basin inflow/outflow vary for construction and operation
Concentrations found at the end of winter used as initial condition at the start of summer runs.
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Transport Modelling
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Beginning of summer End of summer
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Mass Balance Modelling: Assumptions
The following assumptions were made for water quality modelling:complete mixing;constituents are considered as conservative substances (no chemical reactions, biological uptake or degradation or precipitation occur – except TSS); waste rock (WRD) leachate loadings are not affected by the different runoff scenarios; andtotal annual loadings provided for each water quality constituent, which prorated monthly from May to October, based on the monthly runoff distribution.
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Water Quality Assessment: Scenarios
The feed streams for discharge to the environment: discharge from the Water Treatment Plant (WTP); discharge from the Sewage Treatment Plant (STP); and natural runoff from surrounding areas.
Three different runoff scenarios following climatic conditions: median and two extreme conditions of consecutive wet or dry years represent the worst case scenarios.
1-in-100 year Dry;1-in-2 year Wet (median); and1-in-100 year Wet.
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2011 2012 2013 2014 2015 2016 2017 2018 2019 2020 2021 2022 2023 2024 2025 2026 2027 2028 2029 2030
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n (%
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1:2 Wet Conditions 1:100 Wet Conditions 1:100 Dry Conditions*
*Peak concentrations for the 1:100 dry scenario are representative of prevalent dry conditions.
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2011 2012 2013 2014 2015 2016 2017 2018 2019 2020 2021 2022 2023 2024 2025 2026 2027 2028 2029 2030
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cent
ratio
n (%
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1:2 Wet Conditions 1:100 Wet Conditions 1:100 Dry Conditions*
*Peak concentrations for the 1:100 dry scenario are representative of prevalent dry conditions.
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2011 2012 2013 2014 2015 2016 2017 2018 2019 2020 2021 2022 2023 2024 2025 2026 2027 2028 2029 2030
Year
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cent
ratio
n (%
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1:2 Wet Conditions 1:100 Wet Conditions 1:100 Dry Conditions*
*Peak concentrations for the 1:100 dry scenario are representative of prevalent dry conditions.
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2011 2012 2013 2014 2015 2016 2017 2018 2019 2020 2021 2022 2023 2024 2025 2026 2027 2028 2029 2030
Year
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cent
ratio
n (%
)
1:2 Wet Conditions 1:100 Wet Conditions 1:100 Dry Conditions*
*Peak concentrations for the 1:100 dry scenario are representative of prevalent dry conditions.
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The water quality during end-pit lake filling will be determined by the following major substance sources:
natural runoff from surrounding watershedscontinuous surface drainage from waste rock storages into the lakegroundwater inflow
The water quality will depend on how fast the lake will be refilled:natural runoff and water diversion to the lakereduction in groundwater inflow
The final concentrations will depend on water balance and lake hydrodynamics
End-Pit Lake Water Quality Assessment
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The following assumptions are made for chemistry modelling: constituents are considered as conservative substances (no chemical reactions, biological degradation or precipitation occur);
nitrogen loadings from blasting residues are negligible during post-closure;
WRD leachate loadings are not affected by the different runoff scenarios; once the open pit is flooded, the constituents within the top 50m of the Pit are mixed
Water Quality Modelling in the Open Pit
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End-Pit Lake Overturn Assessment
The assessment of the development of stable meromictic conditions in the End-Pit Lake was completed by the following steps:
modelling the vertical TDS profile in the pit basin over time based on groundwater inflow and molecular diffusion;
examining the thermal characteristics of a similar lake to determine the natural depth of seasonal mixing/active zone; and
determining a likelihood of wind-driven mixus in the pit basin to see if mixing could extend to a depth that could cause upwelling of more saline water into the overlying fresh water layer.
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TDS Distribution in The End-Pit Lake
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TDS (mg/L)
Dep
th (m
) 26 yrs526 yrs1,000 yrs15,000 yrs
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Meromictic Lakes
Meromixis may be caused by:
decomposition of organic material leading to an elevated salt concentration in deeper water
input of salt from an outside source, such as an intrusion of seawater
submerged saline springs delivering dense water to deep portions of lake
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Method and Rationale
Dimensionless Numbers –hydrodynamic criteria.
Lake Number (introduced by Jorg Imberger) combines the strength of the water column stratification (Schmidt Number) with the destabilizing forces of the wind to provide a non-dimensional measure of how close the overall system is to turnover.LN = 1 indicates that the wind is just sufficient to force the seasonal thermocline (or chemocline in meromictic lake) to be deflected to the surface at the windward end of the lake. LN >>1, there is no upwelling of deep water. LN <<1, the deep water will reach the surface layer.
Lake Stability Criteria
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Assess density in the water column (both salinity and temperature).
Calculate static lake stability (Schmidt Number).
Calculate dynamic lake stability (Lake Number).
Determine stability for the seasonal thermocline.
Determine stability for the exposed chemocline (worst case).
Discuss stability for meromictic system.
Lake Stability Calculations
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End-Pit Lake Stability: Thermocline
Seasonal thermocline observed at the depth 20 m (control lake).Temperature gradient in the thermocline at 2-3oC per meter as per baseline.LN>>1: seasonal thermocline will not penetrate deeper than 20 m.
Lake Number Time Snapshots Wind 8 m/s Wind 25 m/s Wind 50 m/s
26 yrs 33 4 1 526 yrs 38 5 1 1,000 yrs 46 6 2 15,000 yrs 47 6 2
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End-Pit Lake Stability: Chemocline
Chemocline depth varies from 220 to 50 meters (depends on the time snapshots).No temperature stratification, homogeneous vertical distribution.LN>>1: chemocline will be not disturbed and no saline water upwelling will occur.
Lake Number Time Snapshots
Wind 8 m/s Wind 25 m/s Wind 50 m/s
26 yrs 127 13 3 526 yrs 182 19 5 1,000 yrs 262 27 7 15,000 yrs 235 24 6
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End-Pit Lake Stability Results
The chemocline interface is more stable compare to the thermocline.The upper twenty meters (baseline observations) of fresh water will ‘float’ on top of the more saline water. No mixing will occur between fresh and saline water due to density stability under average and extreme wind. The water mass will be a subject for summer seasonal stratification in the upper zone.Spring and fall lake overturn will affect only mixolimnion and no saline water will be exposed to the surface layer of the lake.This upper layer of the lake will be flashed by annual water inflow.
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Conclusions
Projects applications
Effects assessment and water management:ConstructionOperationsClosure
Comparative assessment of mining effects scenariosMethods to quantify and study potential effectsEffectiveness of mitigation measures Closure effects and rationale for end-pit lakes design and refillingMonitoring
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Acknowledgements
Ranee Lai, PhD, PEngThang Nguyen, MSc, PBioChristine Peters, MSc, PEngSukru Sumer, PhD, PEngShaun Toner. BSc, PBio
Thank you !Thank you !
AMEC Water Quality Group