effect of perched water conditions in msw landfills

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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Consider a MSW Landfill with an Infiltration Pond

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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

Contact Info

• Tim Townsend – ttown@ufl.edu

• Pradeep Jain – pjain@ufl.edu