Designs, Natural Succession, and LTS&M of Disposal Cell Covers for Uranium Mill Tailings WJ Waugh

Designs, Natural Succession, and LTS&M of Disposal Cell Covers for

Uranium Mill Tailings

WJ WaughS.M. Stoller Corporation

LTS&M ConferenceNovember 16-18, 2010

U.S. Department of Energy Office of Legacy Management (LM) Sites

3

Remedies at most LM sites include disposal cells for U mill tailings.

Broad range of climates, soils, and ecology.

U.S. Department of Energy Office of Legacy Management (LM) Sites

4

Presentation Topics

Purpose of disposal cell covers Cover designs, natural succession, and

performance Cover renovation – improving sustainability

by accommodating natural succession Summary

5

Presentation Topics

Purpose of disposal cell covers Cover designs, natural succession, and

performance Cover renovation – improving sustainability

by accommodating natural succession Summary

6

UraniumMill Tailings

GROUND WATERGROUND WATER

PLUMEPLUME

U TAILINGSU TAILINGS

VADOSE ZONEVADOSE ZONE

RADON GASRADON GAS

Uranium Mill Tailings Radiation Control Act (UMTRCA) of 1978

♦ limit radon escape♦ contain tailings source♦ clean up and protect ground water (came later)♦ last for 200-1000 years!

7

Remedy: EngineeredCover

GROUND WATERGROUND WATER

VADOSE ZONEVADOSE ZONE

RADON GASRADON GAS

Cover

PLUMEPLUME

U TAILINGSU TAILINGS

♦ Slow radon flux < 20 pCi m-2 s-1

(< 0.74 Bq m-2 s-1)♦ Control percolation and

mobilization of contaminants —satisfy GW standards

♦ Control erosion and bio-uptake♦ Last for 200-1000 years

8

Presentation Topics

Purpose of disposal cell covers Cover designs, natural succession,

and performance Cover renovation – improving sustainability

by accommodating natural succession Summary

Early Disposal Cell Cover Design

Tailings

Low-Permeability Radon Barrier

15 cm

30 cm

60 cm

Bedding

Rock Riprap

Lesson 1: Rock covers increase water storage and create habitat for deep-rooted woody plants for a broad range of climates and ecology Accumulation of water in the bedding layer and low-permeability radon barrier favors germination and establishment of shrubs and trees.

Natural Succession and Performance

11

Burrell, PAPrecip > 1000 mm/yr

(> 40 in/yr)

SycamoreTree-of-heavenJapanese knotweed

TailingsTailings

Low-Permeability Radon Barrier

15 cm

30 cm

60 cm

Bedding

Rock Riprap

12

Shiprock, NMPrecip ~ 180 mm/yr

(~ 7 in/yr)

Russian thistleKochiaTamariskRabbitbrushSaltbush

Rock Riprap30 cm

Low-PermeabilityRadon Barrier

200 cm

Tailings

Bedding

13

Grand Junction, CO Precip < 200 mm/yr

(< 8 in/yr)

Fourwing SaltbushShadscaleSpiny HopsageRabbitbrushHalogeton

Fourwing saltbushFourwing saltbushTailings

Bedding15 cm

Rock Riprap30 cm

45 cm ProtectionLayer

45 cmLow-Permeability Radon barrier

14

Tailings

15 cm30 cm

45 cm

15 cm Soil

Low-Permeability Radon barrier

Bedding

Rock Riprap

Lakeview, OR Precip ~ 380 mm/yr

(15 in/yr)

RabbitbrushSagebrushBitterbrush

Lesson 2: Roots of woody plants can penetrate compacted soil layers overlying tailings

Plant roots were excavated at several sites to determine rooting depths.

Natural Succession and Performance

♦ Primary roots extend vertically through rock and bedding layers and then branch laterally at the radon barrier surface

♦ Secondary and tertiary roots extend vertically in the radon barrier as root mats following soil structural planes

16

Lakeview, ORSagebrush

17

Lakeview, ORSagebrush

Test Dye and Sagebrush Roots in Radon Barrier

18

Burrell, PAJapanese knotweedGrand Junction, COFourwing Saltbush

19

Burrell, PAJapanese knotweedGrand Junction, COFourwing Saltbush

Saltbush Root Mat in Radon Barrier

20

Burrell, PAJapanese knotweed

21

Burrell, PAJapanese knotweed

Lesson 3: Windblown dust in semiarid West and organic litter in humid East are creating soils in rock riprap and drainage layers

Natural Succession and Performance

♦ Soil development in rock enhances plant habitat and drives plant succession

♦ Soil development in drainage layer may limit lateral shedding of precipitation

23

Dust has filled the basalt riprap layer on leeward side of the cover

Grand Junction Cover

24

♦ Soil structure developing faster than expected♦ Plant roots and burrowing animals ♦ Freeze-thaw cracking and desiccation♦ Borrow soil structure retained after construction

Test dye shows structural planes Roots follow structural planes

Lesson 4: Different types of soil development (pedogenic) processes may be causing preferential flow in CSLs:

Lesson 5: Root intrusion and soil development increase the KS of the low-permeability radon barrierAssumed saturated hydraulic conductivity (KS):

Ks ≤ 1x10-7 cm/s

In situ Ks measured using air-entry permeameters

(AEPs) D.B. Stephens Air-Entry Permeameter

Cover Soil Development andSaturated Hydraulic Conductivity (Ks)

Burrell, PAIn-situ Ks1996 AEP Study

Tailings

Low-Permeability Radon Barrier

15 cm

30 cm

60 cm

Bedding

Rock Riprap

Lakeview, ORIn-situ Ks1998 AEP Study

Automated AEPs on topslope

Manual AEP on side slope

Tailings

15 cm

30 cm

45 cm

15 cm Soil

Low-Permeability Radon barrier

Bedding

Rock Riprap

Shiprock, NMIn-situ Ks1999 AEP Study

Rock Riprap30cm

Low-PermeabilityRadon Barrier

200 cm

Tailings

Bedding

Tuba City, AZIn-situ Ks 1999 AEP Tests

Tailings

Low-Permeability Radon barrier

Bedding15 cm

Rock Riprap30 cm

107 cm

Grand Junction, COIn-situ Ks2005 AEP Tests

Fourwing saltbushFourwing saltbushTailings

Bedding15 cm

Rock Riprap30 cm

45 cm ProtectionLayer

45 cmLow-Permeability Radon Barrier

31

1 x 10-8

1 x 10-7

1 x 10-6

1 x 10-5

1 x 10-4

1 x 10-3

Ks (

cm/s

)

Low-Permeability Radon Barrier Ks Means

Lakeview, OR

Burrell, PA

Grand Jct, CO

Shiprock, NM

Tuba City, AZ

Albany, GA

Apple Valley, CA

Altmont, CA

Polson, MT

Omaha, NE

EPA ACAP SitesBill Albright, Desert Research Institute

DOE LM Sites

ACAP initial Ks < 1 x 10-7 cm/s

32

Water fluxmeters Installed below CSL

Lakeview, Fall 2005

Lesson 6:High saturated hydraulic conductivity (Ks) may cause significant percolation through the cover

33

0

5

10

15

20

Nov-05

Jan-06

Mar-06

May-06

Jul-0

6

Sep-06

Nov-06

Jan-07

Mar-07

May-07

Jul-0

7

Sep-07

Date

Dai

ly P

reci

pita

tion

& P

erco

latio

n (m

m)

Lakeview Water Flux Meter Results(November 2005 – September 2007)

34

Lesson 7: Inadequate revegetation planning and poor soil edaphic properties can compromise performance

Thin soil layers overlying rock are poor habitat for grasses

edge of cover

Lakeview, OR

35

Lesson 8: Water balance cover designs accommodate natural succession (plant ecology and soil development) and perform better than low-permeability covers

Wat

er S

tora

ge L

ayer

(S

pong

e)

60 cm

60 cm

30 cm

38 cm

40 cm

30 cm

Monticello, Utah Disposal Cell Cover

36

Soil Water Balance Monitoring (3-hectare embedded lysimeter)

Drainage collection system

Percolation and Runoff: Dosing siphons

Soil Moisture Monitoring:- Water content TDR- Water potential HDU

CapillaryBarrier

Fine Soil

37

On-Site Evapotranspiration

0

500

1000

1500

2000

2500

3000

3500

4000

0

100

200

300

400

500

600

8/1/00 8/1/02 8/1/04 8/1/06 8/1/08 8/1/10Cum

ulat

ive

Pre

cipi

tatio

n an

d E

vapo

trans

pira

tion

(mm

)C

umulative P

ercolation and Surface R

unoff,and S

oil Water S

torage (mm

)

NWS Precipitation

Surface Runoff

Percolation

Soil Water Storage

NWS Evapotranspiration

Monticello Water Balance Cover On-Site Precipitation

Embedded Lysimeter Water Balance

Upper Storage Limit

AveragePercolation ~ 0.5 mm/yr

On-Site Evapotranspiration

38

Cover Percolation Comparison

Low- Albany, GA 849 265.0 26.0Permeability Apple Valley, CA 61 2.5 4.1Cover

Cedar Rapids, IA 449 39.5 8.8Lakeview, OR 319 30.1 9.4

Water Apple Valley, CA 167 0.5 0.3Balance Boardman, OR 181 0 0.0Cover

Polson, MT 349 0.2 0.1Monticello, UT 387 0.5 0.1

Average PercolationPrecipitation Percolation as % of

(mm) (mm) Precipitation

SiteCoverType

EPA ACAPDOE LM

Average

39

Cover Water Balance: Role of Plants

Estimated Ranges of Annual Recharge (mm/yr)

Loam Soil

380 mm (15 in) 100-200~380Shrubs

SoilDepth(1.5 m)

100-20020-1000-20<1

CheatgrassWheatgrassBare / Rock

40

0

5

10

15

20

25

30

35

40

45

50Pe

rcen

t Cov

er

2001 2002 2003 2004 2005 2006 20070

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

Leaf

Are

a In

dex

2008 2009 20100

25

50

75

100

125

150

175

200

225

250

Shru

b D

ensi

ty (#

/Acr

e)Monticello Vegetation Monitoring

Preliminary LAI Map for Monticello CoverJune 2008 June 2008

5.432

0

LAI

John GladdenSRNL

42

Presentation Topics

Purpose of disposal cell covers Cover designs, natural succession, and

performance Cover renovation – improving

sustainability by accommodating natural succession

Summary

43

Shrub encroachment and soil development may be the solution, not the problem!

Grand Junction, Colorado

44

Shrub encroachment and soil development may be the solution, not the problem!

Without intervention, natural succession processes may eventually transform conventional low-permeability covers into ET-type covers.

LTSM Options:♦ Control plant growth♦ Let plants grow♦ Enhance soil

development and ecological succession

CCover Renovation!over Renovation!Grand Junction, Colorado

45

Cover Renovation Research Goal: Enhance natural transformation of conventional

covers into ET covers♦ Reduce soil bulk density (compaction)♦ Increase soil water storage capacity♦ Blend soil and rock to imitate natural analogs♦ Enhance establishment of favorable vegetation

Test: Construct pair of large drainage lysimeters, identical to actual cover, and compare water balance of existing and renovated designs

Renovation Concept: Rip the rock, drainage, and protection layers on the contour, and transplant native shrubs in rip rows

46 46

Control Renovate

Cover RenovationLysimeters 2008

Fourwing saltbushFourwing saltbushTailings

Bedding15 cm

Rock Riprap30 cm

45 cm ProtectionLayer

45 cmLow-Permeability Radon Barrier

47

Baseline Water Balance Monitoring: ‘Renovate’ and ‘Control’ Lysimeters

Control Lysimeter

0

100

200

300

400

500

600

700

0

5

10

15

20

10/31/07 5/1/08 10/31/08 5/1/09 10/31/09 5/1/10 10/31/10

Cum

ulat

ive

Pre

cipi

tatio

n an

d E

vapo

trasn

pira

tion,

an

d S

oil W

ater

Sto

rage

(mm

)

Cum

ulative Runoff and P

ercolation (mm

)

Soil Water Storage

Surface Runoff

Percolation

Evapotranspiration

Precipitation

Renovate Lysimeter

0

100

200

300

400

500

600

700

0

5

10

15

20

10/31/07 4/30/08 10/30/08 5/1/09 10/30/09 5/1/10 10/31/10

Cum

ulat

ive

Pre

cipi

tatio

n an

d E

vapo

trasn

pira

tion,

an

d S

oil W

ater

Sto

rage

(mm

)

Cum

ulative Runoff and P

ercolation (mm

)

Soil Water Storage

Surface Runoff

Percolation

Evapotranspiration

Precipitation

48

Presentation Topics

Purpose of disposal cell covers Cover designs, natural succession, and

performance Cover renovation – improving sustainability

by enhancing natural succession Summary

49

Cover LTSM Questions 1. Why did we cover uranium mill tailings?2. How were the covers designed to work and how were

they constructed?3. How have natural succession processes altered

cover performance?4. What are the risks to HH&E if the covers are not

performing as designed?5. What types of maintenance are required—and at what

cost—to keep covers performing as designed? 6. Could we design sustainable repairs or renovations if

needed to reduce LTSM costs and risks?7. Can we expect covers to continue working as

designed for the long term—200 to 1000 years?