water balance and the influence of soil structural changes on final covers for landfill closure

School of Civil and Environmental Engineering

Life Impact The University of Adelaide

Water Balance and the Influence of Soil

Structural Changes on Final Covers for Landfill

ClosureMelissa Salt, University of AdelaideMark Jaksa, University of Adelaide

Jim Cox, CSIROPaul Lightbody, Tonkin Consulting



Research Objectives

• Measure and compare the drainage to determine if phytocovers reduce drainage to the same extent as conventional covers

• Correlate changes in drainage patterns over time from the phytocover and conventional cover changes with changes in bulk density, soil water characteristic curve and permeability

• Assess the tendency for anthropomorphic soil to tend towards the natural profile of the borrow source or toward a new stable profile

• Determine the effect of changes in bulk density, soil water retention curve and permeability on the predictability of the water balance as estimated from pre-construction laboratory testing



Water Balance

• P = ET + R + L + D + ΔS

WASTE

COVER

Precipitation (P)

Runoff (R)

Leachate

Drainage (D)

Lateral flow (L)

Soil moisture storage (S)

Evapotranspiration (ET)



Methodology – Field Scale

• Precipitation - Weather station

• Runoff - Flow meters

• Drainage and lateral flow - Tipping bucket rain gauges

• Soil moisture content – MP406

• Soil suction – CS229

Drainage layerRoot barrier

Runoff collection

Drainage collection

Soil cover layer1.5 mm LLDPE geomembrane

Interim cover

Earthen berms

not to scaleMonitoring nest



Methodology – Small Scale

• 1 m x 1 m x 1.5 m deep

• Replicate conventional and phytocover from Adelaide, including plants

• Irrigation and measure drainage

• Destructively sample 1 box of each cover type every 6 months

• Analyse samples for bulk density, soil water characteristic curve and permeability



Methodology – Core samples

• To measure any change in permeability or bulk density as a result of alternate saturation and then drying the soil layers proposed to use in the Adelaide A-ACAP trial.

• Prepared core samples at known bulk density of phytocover soil and clay barrier

• Wet using falling head permeability apparatus until saturated and hydraulic conductivity measured

• Dry in oven at 30 oC until equilibrated and observe shrinkage and cracking

• Repeat process



Water Balance Modelling

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

0.45

0.5

0 200 400 600 800 1000 1200

Days since modelling commenced

Volu

met

ric

moi

stur

e co

nten

t

0.1 m 0.5 m

1 m 1.5 m

Moisture content at depth through conventional profile over first 3 years modelled



Water Balance – Adelaide Site

Water balance

Phytocover (mm/yr & %) Conventional (mm/yr & %)

1 m 1.5 m 2 m As placed Dried

P 515 515 515 515 515

I 117 (23) 123 (24) 123 (24) 103 (20) 125 (24)

ET 349 (68) 357 (69) 364 (71) 412 (80) 369 (72)

R 0.7 (0.1) 0.7 (0.1) 0.7 (0.1) 0.8 (0.1) 0.8 (0.1)

L 0 0 0 0.5 0

D 48 (9) 34 (7) 28 (5) 0.2 (0) 23 (4)



Water Balance Predictions

0

20

40

60

80

100

1957 1962 1967 1972 1977 1982 1987 1992 1997 2002

Year

Runoff

(m

m)

0

200

400

600

800

1000

Rai

nfa

ll (m

m)

Conventional Phytocover Rainfall

Predicted annual runoff volumes when surface soil permeability is reduced by one order of magnitude



Results – Adelaide Soil Moisture

0

5

10

15

20

25

30

35

40

31/10/2007 0:00

2/11/2007 0:00

4/11/2007 0:00

6/11/2007 0:00

8/11/2007 0:00

10/11/2007 0:00

12/11/2007 0:00

14/11/2007 0:00

16/11/2007 0:00

18/11/2007 0:00

Date and Time

Mo

istu

re c

on

ten

t (%

vo

l)

upslope 150 mm

upslope 700 mm

upslope 1350 mm

centre 150 mm

centre 700 mm

centre 1350 mm

downslope 150 mm

downslope 700 mm

downslope 1350 mm

RG201017



Results – Adelaide Topsoil Moisture Content

0

50

100

150

200

250

300

350

400

450

500

17/07/20070:00

6/08/20070:00

26/08/20070:00

15/09/20070:00

5/10/20070:00

25/10/20070:00

14/11/20070:00

4/12/20070:00

Date and time

Cum

ulat

ive

Rai

nfal

l (m

m)

0.00

5.00

10.00

15.00

20.00

25.00

30.00

35.00

40.00

45.00

50.00

Soi

l moi

stur

e (%

vol

)

Rainfall Phytocover 150 mm Conventional cover 150 mm



Results – Melbourne Soil Moisture

0

5

10

15

20

25

30

35

40

12/02/070:00

3/04/070:00

23/05/070:00

12/07/070:00

31/08/070:00

20/10/070:00

Date Time

Mo

istu

re c

on

ten

t (%

vol)

0

100

200

300

400

500

600

700

800

Cu

mu

lativ

e R

ain

fall

(mm

)

Phytocover 150 mm Phytocover 800 mm Phytocover 1550 mmConventional 150 mm Conventional 500 mm Conventional 850 mmCumulative Rainfall



Outcomes

• Water balance comparison of phytocaps for Australian environment

• Variability of water balance predictions from selected models

• Quantification of soil structural changes in the short to medium term

• Impact of soil structural changes on sustainability of the phytocaps

• Determination of best soil input parameters for pre-construction modelling

water balance and the influence of soil structural changes on final covers for landfill closure

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