effect of perched water conditions in msw landfills
TRANSCRIPT
Effect of Perched Water Conditions in MSW Landfills:
Considerations for Landfill Operators
Timothy Townsend and Pradeep JainDepartment of Environmental Engineering
University of Florida
2005 SWANA Landfill SymposiumBolder, Colorado
Motivation
• At last year’s landfill symposium, several presentations and audience comments described the issue of saturated waste layers in the deeper parts of landfills.
• The presence and cause of these saturated layers can be interpreted differently.
• The question that we asked: “what should one expect?”
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MSW
Consider a MSW Landfill
Install Gas Wells
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MSW
Consider a MSW Landfill
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MSW
Landfill GasWell
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Landfill GasWell
WaterSurface
Implications of Perched Water
• Problems with gas recovery?
• Slope stability concerns?
• Leachate collection system problems?
• Future side slope seepage issues?
Landfill gas well equipped with liquidpumping system
Pump repairand maintenance
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Landfill GasWellSlope stability
concerns
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Landfill GasWellSlope stability
concerns
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Landfill GasWellSlope stability
concerns
The implications of the perched liquids depend on their true nature
within the landfill
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Landfill GasWell
WaterSurface
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Landfill GasWell
Phreaticsurface
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Landfill GasWell
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Landfill GasWellSlope stability
concerns
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Landfill GasWellSlope stability
concerns
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Landfill GasWellSlope stability
concerns
Let’s examinethe scenario where only
waste around the well is saturated
Some source of water is added to the well at a rate greater that it can drain out. Possible sources:
• Gas condensate• Perched zones of
leachate in the landfill• Short circuiting from
liquids addition
Gas Well
Soil Layer
PerchedLiquids
Flow1 Flow2 Flow3
Vertical Injection Wells at New River Regional Landfill
Modified Version of Richard’s Equation
( )t
St
C
kz
kz
Krr
kKr
kr
K
s
zr
r
∂∂
+∂
∂=
=
+
∂∂
∂∂
+∂∂
+
∂∂
∂∂
ψψψ
ψψψ
Richard’s equation was solvedusing a USGS program called
SUTRA
r0 5 10 15 20
z
0
5
10
15
20
S=1
S=0.2
r0 5 10 15 20
z
0
5
10
15
20
ψ=0
ψ=2 m
ψ=4 m
r (ft)0.0 1.0 2.0 3.0 4.0
z (ft
)
0
10
20
30
0 ft1 ft2 ft3 ft4 ft5 ft
Simulation Parameters
K = 10-5 cm/sec
Q = 17 gallons/day
Duration of Moisture Addition = 10 days
Head in the well ~ 8 ft
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Landfill GasWell
Phreaticsurface
• If the liquids are added to the landfill at a rate greater than the hydraulic conductivity, saturated conditions will result
Let’s examinethe scenario where saturated conditions will develop in the
landfill even if barrier layers are not present
Consider a Liquids Infiltration Pond
• The waste underneath the pond will become saturated
• In the absence of cover soil layers, a saturated zone will extend to the leachate collection system
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MSW
Consider a MSW Landfill with an Infiltration Pond
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MSW
Consider a MSW Landfill with an Infiltration Pond
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Consider a MSW Landfill with an Infiltration Pond
h
d
ddhi +
=
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Consider a MSW Landfill with an Infiltration Pond
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MSW
Consider a MSW Landfill with an Infiltration Pond
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MSW
Consider a MSW Landfill with an Infiltration Pond
WaterLevel
Can saturated conditions develop if the liquids are added at a rate less than the permeability of the waste?
• Yes, if the permeability of the waste is reduced with depth
DecreasingPermeability
Col 21 vs Col 22
Density (t/m3)
0.60 0.65 0.70 0.75 0.80 0.85 0.90 0.95 1.00
Hyd
raul
ic C
ondu
ctiv
ity (m
/sec
)
10-8
10-7
10-6
10-5
10-4
10-3
At ~1400 pcyK = 8x10-5 cm/sec
Air permeability of waste at NRRL at different depths
Air Permeability, k (X10-12 m2)
<0.1
0.1-
2.5
2.5-
5.0
5.0-
7.5
7.5-
10.0
10.0
-12.
5
12.5
-15.
0
15.0
-17.
5
17.5
-20.
0
20.0
-22.
5
22.5
-25.
0
>25.
0
Num
ber o
f Loc
atio
ns
0
3
6
9
12
15
Air Permeability, k (X10-12 m2)
<0.1
0.1-
2.5
2.5-
5.0
5.0-
7.5
7.5-
10.0
10.0
-12.
5
12.5
-15.
0
15.0
-17.
5
17.5
-20.
0
20.0
-22.
5
22.5
-25.
0
>25.
0
Num
ber o
f Loc
atio
ns
0
3
6
9
12
15
Air permeability of waste at NRRL at different depths
Air Permeability, k (X10-12 m2)
<0.1
0.1-
2.5
2.5-
5.0
5.0-
7.5
7.5-
10.0
10.0
-12.
5
12.5
-15.
0
15.0
-17.
5
17.5
-20.
0
20.0
-22.
5
22.5
-25.
0
>25.
0
Num
ber o
f Loc
atio
ns
0
3
6
9
12
15
Air permeability of waste at NRRL at different depths
Bottom Liner
Compacted MSW
Leachate Collection System
27 Gpd K=10-4 cm/s
K=5X10-5 cm/s
60 ft
Pressure (ft of w.c.)
-8 -6 -4 -2 0 2 4 6 8
Dep
th (f
t)
0
10
20
30
40
50
60
Simulation Parameters
Decreasing K = 10-5
cm/sec (top) to 5X10-6
cm/sec (bottom at 60 ft deep)
Q = 8.5 gallons/day
Duration of Moisture Addition = 10 days
Head in the well ~ 5 ft
r (ft)0.0 1.0 2.0 3.0 4.0
z (ft
)
0
10
20
30
0 ft1 ft2 ft3 ft4 ft
Review
• The existence of standing liquids in gas wells in landfills does not necessarily result from a phreatic liquid surface in the landfill.
• Liquids added to wells as a result of perched layers in the landfill, gas condensate or other sources can result in relatively large depths of water in the well.
Review
• The decreasing permeability of landfilled waste with depth should have impact.
• Saturated waste conditions may be present, but the pressure of this water may not be accurately reflected by the depth of water that would be measured if a well was installed.
• At large liquid addition rates, saturated conditions in deeper layers may develop.
Implications
• The presence of liquids in gas wells in “dry” landfills should not automatically assumed to represent a phreatic surface.
• In “wet” landfills, the liquid levels in wells may result from both situations.
• When evaluating slope stability, careful thought must be given to the pressures that truly occur.
• Leachate collection systems need to be designed and operated correctly.
New Experiment in FloridaBury piezometers in waste
vertical well and horizontal trench
25’ Between Wells
Well #1 Well #2
10’
VW Piezometer Well
Injection Wells
5’
Current Bioreactor
MSW
10’
5’
20’
30’
40’
VW piezometers
Data Station
Injection Well #1
Cover Soil
10’
20’
35’
Injection Well #2
