Field water balance of final landfill covers: The USEPA’s Alternative Cover

Assessment Program (ACAP)

William H. Albright Desert Research Institute, University of Nevada

and

Craig H. BensonUniversity of Wisconsin-Madison

Final covers - the issues

• Lack of field-scale performance data• Excessive uncertainty in modeled predictions• No specified design process

Presented here...

• Field data from ACAP• A suggestion for acceptable use of models• A design process for engineers and regulators

ACAP: The Field Program

• Nationwide: 11 sites, 7 states• Large (10 X 20 m) drainage lysimeters• Conventional covers

– Composite– Soil barrier

• Alternative covers– Evapotranspiration (ET)– Capillary barrier

• Side-by-side demonstration at most sites

ACAP Site Locations

Apple Valley, CA Albany,GAMonticello,

UT

Boardman,OR

Polson,MT

Helena, MT

Cedar Rapids, IAOmaha, NE

Monterey,CA

Altamont, CA

Sacramento, CA

Conventional Composite Designs

0

0.3

0.6

0.9

1.2

1.5

m

OmahaNE

AltamontCA

MontereyCA

PolsonMT

Cedar Rapids

IA

BoardmanOR

Apple Valley

CA

Geomembrane over geosynthetic clay layer

Geomembrane over fine-grained soil layer

Water Balance ComponentsConventional Composite Cover, Cedar Rapids IA

1500

1600

1700

1800

5/1/03 5/16/03 5/31/03

(mm

wa

ter)

0

40

80

120

Precipitation Lateral flow

Percolation

Surface flow

• Percolation rate Percolation rate correlated withcorrelated with

– Heavy Heavy precipitation precipitation eventsevents

– Surface flowSurface flow

– Lateral flow on Lateral flow on geomembranegeomembrane

Water Balance ComponentsConventional Composite Cover, Marina CA

• Percolation coincides with Percolation coincides with precipitation, surface and precipitation, surface and lateral flowlateral flow

• Relatively high rate of Relatively high rate of percolationpercolation

• No cushion between the No cushion between the geomembrane and the geomembrane and the soil, punctures likely in soil, punctures likely in geomembranegeomembrane

600

700

800

900

8/22/02 10/11/02 11/30/02 1/19/03 3/10/03

0

50

100

150

Precipitation

Lateral flow

Percolation

Surface flow

• Illustrates importance of careful geomembrane installationIllustrates importance of careful geomembrane installation

Conventional Composite CoversDiscussion

• Perform well at all locations• Average percolation typically <1.5% of

precipitation <1.5 mm/yr at arid/semi-arid/subhumid sites <12 mm/yr at humid locations

• Percolation often linked to heavy precipitation events and lateral flow

• Damage to geomembrane greatly increases percolation rate

• Construction practice and quality control are very important

Conventional Soil Barrier Designs

0

0.3

0.6

0.9

1.2

1.5

m

Cedar Rapids

IA

AlbanyGA

Apple Valley

CA

Water Balance ComponentsConventional Soil Barrier Cover, Albany GA

50

300

550

800

7/1/00 9/1/00 11/2/00 1/3/01

0

75

150

225

Precipitation

Percolation

Soil water storageNo rain

3/4/02

mm

wa

ter• Soil dried for first Soil dried for first

time during 6-week time during 6-week droughtdrought

• Change in response Change in response of percolation to of percolation to precipitation eventsprecipitation events

– QuantityQuantity

– ““Stair step” Stair step” responseresponse

• No evidence that defects in clay barrier healed when soil water No evidence that defects in clay barrier healed when soil water increasedincreased

Change in saturated hydraulic conductivity in a compacted clay barrier

77,5003.1x10-3TSB - 3

8003.2x10-5TSB - 2

13005.2x10-5TSB - 1

50002.0x10-4SDRI

1.04.0x10-8As-Built

Kf/Ko

Hydraulic

Conductivity

(K)

(cm/s)

Test• Albany GAAlbany GA

• Cover installed Cover installed March 2000March 2000

• Final sampling Final sampling Feb. 2004Feb. 2004

Conventional Soil Barrier CoversDiscussion

• Percolation at humid locations 52 - 195 mm/yr 6 – 17 % of precipitation

• Percolation response to precipitation events changed at both humid sites– Percolation quantity increased – Temporal response increased

• Clay barrier properties changed significantly over a relatively short time

2.4

2.7

0

0.3

0.6

0.9

1.2

1.5

1.8

2.1

m

SacramentoCA

BoardmanOR

PolsonMT

MonticelloUT

Alta-montCA

HelenaMT

Apple Valley

CA

MarinaCA

Alternative Designs: Arid/Semi-Arid/Sub-Humid Locations

Capillary barrier designs

Water Balance ComponentsAlternative Cover, Helena MT

• Seasonal Seasonal precipitation precipitation patternpattern

• Seasonal Seasonal fluctuations in soil fluctuations in soil water contentwater content

• No percolationNo percolation

0

200

400

600

800

8/1/99 7/31/00 7/31/01 7/31/02 7/31/03

(mm

wa

ter)

0

75

150

225

300

Precipitation

Soil water

Percolation

Surface Runoff

Water Balance ComponentsAlternative Cover, Marina CA

0

250

500

750

1000

1/2/00 1/1/01 1/1/02 1/1/03 1/1/04

(mm

wat

er)

0

100

200

300

400

Percolation

Soil Moisture

Precipitation

• Water storage Water storage capacity lower than capacity lower than expectedexpected

• Effective storage Effective storage capacity (300 mm) capacity (300 mm) lower than lower than calculated (385 calculated (385 mm) mm)

• Drainage when Drainage when storage capacity storage capacity exceededexceeded

0

0.3

0.6

0.9

1.2

1.5

m

OmahaNE

Cedar Rapids

IA

AlbanyGA

Alternative Designs: Humid Locations

Water Balance ComponentsAlternative Cover, Omaha NE

0

400

800

1200

1600

10/1/00 10/1/01 10/1/02

(mm

wa

ter)

0

150

300

450

600

Precipitation

Soil water

Percolation

• Moderate Moderate precipitationprecipitation

• Percolation occurs Percolation occurs late springlate spring

• Improvements in Improvements in design and factor-design and factor-of-safety of-safety considerations may considerations may provide acceptable provide acceptable performanceperformance

Water Balance ComponentsAlternative Cover, Cedar Rapids IA

(mm

wat

er)

• High precipitationHigh precipitation

• Extended periods Extended periods when precipitation > when precipitation > ETET

• Probably exceeds Probably exceeds capacity of soil/plant capacity of soil/plant system to achieve system to achieve low percolation rateslow percolation rates

0

500

1000

1500

2000

6/3/00 6/3/01 6/3/02 6/3/03

0

150

300

450

600

No data

Precipitation

Soil water

Percolation

Alternative DesignsDiscussion

• Very low (<2mm/yr) percolation rates at 7 of 10 covers at arid/semi-arid/sub-humid locations– Annual variation in transpiration capacity at Sacramento

CA cause of anomalous behavior– Insufficient soil water storage capacity at Marina CA

• Higher (33-160 mm/yr) percolation rates at humid locations.

• Preliminary calculations of water holding capacity can underestimate apparent capacity by 0-25%

• Successful design requires careful attention to:– Site characterization– Water balance mechanisms

The problem with models:excessive uncertainty in results

Sensitivity analysis as a design tool

• Design sensitivity analysis (DSA) is performed by

comparing results from systematic variation of a single

parameter

• DSA helps designer and regulator understand relative

contribution of various design features or environmental

stresses to cover performance

• DSA can provide valuable information for negotiations in

a regulatory environment

DSA exampleEvaluate the effect of cover thickness

0

0.5

1

1.5

2

30 60 90 120Cover thickness (cm)

Avg

ann

ual p

erco

latio

n (c

m)

A design process from the Interstate Technology Regulatory Council (ITRC)

1. Define performance criteria• No flux• Bioreactor operation

2. Select and validate design concept• natural analogs• lysimeter data (ACAP)

3. Characterize site (soil, plants, climate)

4. Model with DSA to understand important design parameters and environmental stresses

5. Final design considerations (final land use, etc)• www.itrcweb.org