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SUSTAINABLE LANDFILLING: EVALUATING THE POTENTIAL IMPACT OF LANDFILL MINING ON GROUNDWATER QUALITY
May 2017
Speaker:
William G. Soukup, P.G.
Presentation Goals
• Understand Risk
• Reduce Risk
• Manage Risk
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Advantages of Mining MSW
• Gain landfill space
• Remediation of a
poorly-functioning landfill
• Side Benefits • Recovery of soil for reuse as daily cover
• Optional recovery of metals, combustibles, etc.
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MSW operational face
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Trommel Screen Rotating Drum
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Trommel Rig
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Potential Downsides of Landfill Mining
• Time and costs
• Exposure to wastes and landfill gas
• Odors
• Impacts to groundwater quality
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Typical Mining Sequence
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Limited leachate generation on side slopes
No further leachate generation on bottom
MSW
Natural Formation GW Table
Cap
Factors for Leachate Generation
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Moisture Content (MC): the amount of water per unit volume of waste. (% volume/volume)
10 to 20 %
Field Capacity (FC): the moisture holding capacity of waste under gravity drainage. (% volume/volume)
30 to 40 %
Leachate Generation: occurs when moisture content exceeds field capacity
Example Leachate Generation
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Assumed Waste thickness = 27’
1’ 1’
Waste Volume = 27 ft3 = 1 yd3
35 gallons of recharge = 56 inches of precipitation
Up to 30% of recharge due to “channeling”
Groundwater Model Simulation
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• 80 acres, 50 feet of waste
• Underlying sand; k=50’/d
• Uniform groundwater horizontal gradient
• Leachate generation rate 48 inches per year
• Steep side slopes
• Non-reactive, conservative solute
• Leachate concentration = 100 m/v units
Groundwater Model Simulations
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Simulated operational factors
1. Mining areas (phases) 2. Sequencing phases 3. Steepness of side slopes 4. Mining rate
Metrics used to evaluate results
1. Maximum concentration beneath excavation 2. Maximum concentration at downgradient boundary 3. Total mass-flux at downgradient boundary
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Discharge Boundary
Mined Area
Scale:
48’
38’
40’
42’
44’
46’
0’ 2,000’
Model Domain - Single Phase Scenario
Predicted Concentrations – Single Phase
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10 years
40 years 30 years
20 years
100
30
1
50+
Predicted Concentrations - Single Phase
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Source Area Concentration
Discharge Area Concentration
Normalized Total Mass Flux = 100 mass-units
10-year Mining period
0 10 20 years 30 40 50
Three Phase Scenario
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Discharge Boundary
Mined Area
Scale:
48’
38’
40’
42’
44’
46’
0’ 2,000’
Phase 1
Phase 2
Phase 3
Predicted Concentrations – 3 Phases
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3.3 years
30 years 10 years
6.6 years
40+
70+
90+
80+
20+
60+
Predicted Concentrations - 3 Phases
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10-year Mining period
Source Area Concentration
Discharge Area Concentration
Normalized Total Mass Flux = 32 mass-units
0 10 20 years 30 40 50
9 Phases (upgradient sequencing)
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Discharge Boundary
Scale:
48’
38’
40’
42’
44’
46’
0’ 2,000’
1 3 2
5
8
6
7
4
9
Predicted Concentrations – 9 Phases
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1.1 years
4.4 years 3.3 years
2.2 years
Predicted Concentrations - 9 Phases upgradient sequencing
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10-year Mining period
Source Area Concentration
Discharge Area Concentration
Normalized Total Mass Flux = 11 mass-units
0 10 20 years 30 40 50
9 Phases (downgradient sequencing)
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Discharge Boundary
Scale:
48’
38’
40’
42’
44’
46’
0’ 2,000’
1 3 2
5
8
6
7
4
9
Predicted Concentrations – 9 Phases downgradient sequencing
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10-year Mining period
Source Area Concentration
Discharge Area Concentration
Normalized Total Mass Flux = 10 mass-units
0 10 20 years 30 40 50
Steep Sidewalls
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6% 6%
88%
Shallow Sidewalls
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33% 33%
33%
0
20
40
60
80
100
120
One Phase 3 Phases 9 PhasesUG
9 PhasesDG
9 PhasesShallowSides
9 Phases15 years
Nor
mal
ized
mas
s un
its
Max Conc.
Max Conc.
Total
12 17 10 11
100
Summary of Model Results
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32
In-situ Groundwater Sample Collection
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Validation Sampling Sequence
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Conclusions
• The impact to groundwater quality from landfill mining depends primarily on:
• Size and duration of the open excavation
• Precipitation rate
• Operational modifications that can reduce impacts include:
• Mining in smaller sub-areas
• Reaching waste bottom more quickly
• Keeping side slopes relatively steep
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Conclusions
• Groundwater travel time is slow allowing early warning of pending groundwater impacts.
• The pro-active evaluation of potential groundwater impacts (through modeling and validation sampling) is a cost effective means of reducing long-term risks.
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Contact Information
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William G. Soukup, P.G. Cornerstone Environmental Group, LLC
bill.soukup@tetratech.com
Phone: (845) 695-0202
Cell: (201) 274-6863
Predicted Concentrations – 9 Phases Shallow Side slopes
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Source Area Concentration
Normalized Total Mass Flux = 15 mass-units
10-year Mining period
Discharge Area Concentration
0 10 20 years 30 40 50
Predicted Concentrations - 9 Phases 15-year Operation
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Source Area Concentration
Normalized Total Mass Flux = 12 mass-units
15-year Mining period
Discharge Area Concentration
0 10 20 years 30 40 50
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