Sustainable ET Landfill Caps – A Closer Look at Hydraulic Equivalency

Milind V. Khire, Ph.D., P.E.Department of Civil & Environmental Engineering

Michigan State University, E. Lansing, MIkhire@egr.msu.edu

Megafills

Source: Qian et al.

Source: Qian et al.

Source: Benson et al. (2001)

Typical Lysimeter (ACAP)

1.5 mm LLDPEGeomembrane

(textured both sides)Percolation Pipe(25 mm PVC)

EarthenBerm

GeocompositeDrain

20 m

0.2 mEarthenBerm

Cover (thickness site specific)

Site InterimCover Soil

(thickness varies)

GeosyntheticRoot Barrier

Sump LeakTest Pipe LLDPE Cutoff

Welded to BaseGeomembrane

LLDPE CutoffWelded to BaseGeomembrane

EarthenBerm

(0.3 m)

Lysimeters vs. Actual Final Caps –Hydraulic Differences

Geomembrane(Zero Flux)

• Equivalency Demonstration Required • Percolation needs to be < few mm/year for an ET Cap

to receive permit

Numerical Simulation using Vadose/W

Source: Khire and Mijares (2008)

• Unit Gradient vs. Seepage Face• Simulation of drainage layer –

Capillary Break?

0

20

40

60

80

100

120

140

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

Aver

age

Mon

thly

Pre

cipi

tatio

n (m

m)

Ave. Annual Precipitation = 731 mm/yrEstimated Annual PET = 1,076 mm/yr

PET/P = 1.47

Precipitation Data - Detroit

Finite-Element Mesh in Vadose/W: 2-D

• Only Capillary flow considered• Flow through cracks needs to be simulated

Simulated Percolation: Upward vs. Downward

-400

-300

-200

-100

0

100

200

0 200 400 600 800 1000 1200 1400200

250

300

350

400

450

500

Time (days)

Cum

ulat

ive

Per

cola

tion

(mm

)

KWL

X 10

KWL

X 1

Lysimeter

KVL

X 10 and KSL

X 100

Source: Khire and Mijares (2008)

(Day 699)

Downward FluxUpward Flux (ET)

Simulated ψ Profiles: Day 699 (Downward Percolation)

1.0

2.0

3.0

4.0

5.0

6.0

-100-80-60-40-20

Matric Suction (kPa)

Ele

vatio

n (m

)

Lysimeter KVL

X 10 & KSL

X 100

KWL

=

X 1

00

X 10X 1

Day 699

SL

WL

VL

Simulated Percolation: Upward vs. Downward

-400

-300

-200

-100

0

100

200

0 200 400 600 800 1000 1200 1400200

250

300

350

400

450

500

Time (days)

Cum

ulat

ive

Per

cola

tion

(mm

)

KWL

X 10

KWL

X 1

Lysimeter

KVL

X 10 and KSL

X 100 UpwardFlux

due toEvaporation

UpwardFlux

absent

Source: Khire and Mijares (2008)

(Day 902)

1.0

2.0

3.0

4.0

5.0

6.0

-1200-1000-800-600-400-2000200

Pore Water Pressure (kPa)

Ele

vatio

n (m

)

Lysimeter

KWL

=

X 1

00X

10

X 1

SL

WL

VL

KVL

X 10 & KSL

X 100

Day 902

Simulated ψ Profiles: Day 902 (Upward Flux)

Simulated Relative Percolation(Capillary Flow Only)

TVL

= 0.3 mT

SL= 1.0 m

TWL

= 3.3 mBenson and Wang (1998) MSW

-1

-0.5

0

0.5

1

1.5

2

2.5

310-6 10-5 10-4 10-3 10-2 10-1 100

Rat

io o

f Ann

ual P

erco

latio

n (P

r CA

P/Pr LY

S)

Hydraulic Conductivity of Waste Layer, KWL

(cm/s)

Upward Flux

DownwardPercolation

K VL

/KSL

:

Commonly Reported Values

10-5 /10-7(CL)

10-4 /10-6(ML)

10-3 /10-5(SM-ML)

LysimeterOverestimatesPercolation

0.8 -1.4/11.7

0.2-0.3/0.40.1/0.001

Field-Scale Testing

25’

2.5’

Small-Scale Lysimeter

GeomembranePea

Gravel

~ Waste ~

14 ?

SensorNest 3

Sensor Nest 2

Nest 1

6’’

1’

2’

≥ 2.5’

TVL = 1 ft

TSL = 3.5, 5 ft

Cross Section BB’

Not to Scale

Water Content and Water Potential Sensors

Gas Pressure Sensor

Liquid Level Sensor

Legend:

Lysimeters

Lysimeters

Completed Test Sections

Field Data (Preliminary)

200

220

240

260

280

300

320

340

360

Sep/

1

Oct

/1

Nov

/1

Dec

/1

Jan/

1

Feb/

1

Mar

/1

Apr/1

Soil

Wat

er S

tora

ge (m

m)

2007 to 2008

NT

NM

NB

NORTH TEST SECTIONStorage Layer

Summary

• Percolation through lysimeters may not be representative of the percolation through an actual cap – Waste Layer Present

• Gas pressures in the landfill will also impact percolation for actual caps

• Saturated and unsaturated properties of waste and flow through cracks will influence actual percolation

Acknowledgements

• Waste Management, Inc.• Environmental Research &

Education Foundation