Designs, Natural Succession, and LTS&M of Disposal Cell Covers for Uranium Mill Tailings
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Designs, Natural Succession, and LTS&M of Disposal Cell Covers for Uranium Mill Tailings WJ Waugh S.M. Stoller Corporation LTS&M Conference November 16-18, 2010. U.S. Department of Energy Office of Legacy Management (LM) Sites.

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

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

WJ Waugh

S.M. Stoller Corporation

LTS&M Conference

November 16-18, 2010


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


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

Remedies at most LM sites includedisposal cells for U mill tailings.

Broad range of climates, soils, and ecology.


Presentation Topics

  • Purpose of disposal cell covers

  • Cover designs, natural succession, and performance

  • Cover renovation – improving sustainability by accommodating natural succession

  • Summary


Presentation Topics

  • Purpose of disposal cell covers

  • Cover designs, natural succession, and performance

  • Cover renovation – improving sustainability by accommodating natural succession

  • Summary


UraniumMill Tailings

  • 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!

RADON GAS

U TAILINGS

VADOSE ZONE

PLUME

GROUND WATER


Remedy: EngineeredCover

  • 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

RADON GAS

Cover

U TAILINGS

VADOSE ZONE

PLUME

GROUND WATER


Presentation Topics

  • Purpose of disposal cell covers

  • Cover designs, natural succession, and performance

  • Cover renovation – improving sustainability by accommodating natural succession

  • Summary


30 cm

Rock Riprap

15 cm

Bedding

Low-Permeability Radon Barrier

60 cm

Tailings

Early Disposal Cell Cover Design


Natural Succession and Performance

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.


Rock Riprap

30 cm

15 cm

Bedding

Low-Permeability Radon Barrier

60 cm

Tailings

Burrell, PA

Precip > 1000 mm/yr

(> 40 in/yr)

Sycamore

Tree-of-heaven

Japanese knotweed


Rock Riprap

30 cm

Bedding

200 cm

Low-

Permeability

Radon Barrier

Tailings

Shiprock, NM

Precip ~ 180 mm/yr

(~ 7 in/yr)

Russian thistle

Kochia

Tamarisk

Rabbitbrush

Saltbush


Rock Riprap

30 cm

15 cm

Bedding

45 cm

Protection

Layer

Low-Permeability Radon barrier

45 cm

Tailings

Fourwing saltbush

Grand Junction, CO

Precip < 200 mm/yr

(< 8 in/yr)

Fourwing Saltbush

Shadscale

Spiny Hopsage

Rabbitbrush

Halogeton


15 cm

Soil

Rock Riprap

30 cm

15 cm

Bedding

Low-Permeability Radon barrier

45 cm

Tailings

Lakeview, OR

Precip ~ 380 mm/yr

(15 in/yr)

Rabbitbrush

Sagebrush

Bitterbrush


Natural Succession and Performance

Lesson 2:

Roots of woody plants can penetrate compacted soil layers overlying tailings

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

  • 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


Lakeview, OR

Sagebrush


Test Dye and Sagebrush Roots in Radon Barrier

Lakeview, OR

Sagebrush


Burrell, PA

Japanese knotweed

Grand Junction, CO

Fourwing Saltbush


Burrell, PA

Japanese knotweed

Grand Junction, CO

Fourwing Saltbush

Saltbush Root Mat in Radon Barrier


Burrell, PA

Japanese knotweed


Burrell, PA

Japanese knotweed


Natural Succession and Performance

Lesson 3:

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

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

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


Grand Junction Cover

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


Test dye shows structural planes

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

  • Soil structure developing faster than expected

  • Plant roots and burrowing animals

  • Freeze-thaw cracking and desiccation

  • Borrow soil structure retained after construction

Roots follow structural planes


Cover Soil Development andSaturated Hydraulic Conductivity (Ks)

Lesson 5:

Root intrusion and soil development increase the KS of the low-permeability radon barrier

Assumed saturated hydraulic conductivity (KS):

Ks≤ 1x10-7 cm/s

In situ Ks measured using air-entry permeameters (AEPs)

D.B. Stephens Air-Entry Permeameter


Rock Riprap

30 cm

15 cm

Bedding

Low-Permeability Radon Barrier

60 cm

Tailings

Burrell, PA

In-situ Ks

1996AEP Study


Lakeview, OR

In-situ Ks

1998 AEP Study

15 cm

Soil

Rock Riprap

30 cm

15 cm

Bedding

Low-Permeability Radon barrier

Manual AEP on side slope

Automated AEPs on topslope

45 cm

Tailings


Rock Riprap

30cm

Bedding

200 cm

Low-

Permeability

Radon Barrier

Tailings

Shiprock, NM

In-situ Ks

1999 AEP Study


Tuba City, AZ

In-situ Ks

1999 AEP Tests

Rock Riprap

30 cm

Bedding

15 cm

Low-Permeability Radon barrier

107 cm

Tailings


Rock Riprap

30 cm

15 cm

Bedding

45 cm

Protection

Layer

Low-Permeability Radon Barrier

45 cm

Tailings

Fourwing saltbush

Grand Junction, CO

In-situ Ks

2005 AEP Tests


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

Burrell, PA

Polson, MT

Omaha, NE

Albany, GA

Altmont, CA

Tuba City, AZ

Shiprock, NM

Lakeview, OR

Grand Jct, CO

Apple Valley, CA

EPA ACAP Sites

Bill Albright, Desert Research Institute

Low-Permeability Radon Barrier Ks Means

1 x 10-3

1 x 10-4

1 x 10-5

Ks (cm/s)

1 x 10-6

1 x 10-7

1 x 10-8

DOE LM Sites


Lesson 6:

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

Lakeview, Fall 2005

Water fluxmeters

Installed below CSL


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


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

Lakeview, OR

edge of cover

Thin soil layers overlying rock are poor habitat for grasses


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

60 cm

Water Storage Layer (Sponge)

40 cm

30 cm

30 cm

38 cm

60 cm

Monticello, Utah Disposal Cell Cover


Soil Water Balance Monitoring (3-hectare embedded lysimeter)

Fine Soil

Capillary

Barrier

Drainage collection system

Percolation and Runoff:

Dosing siphons

Soil Moisture Monitoring:

- Water content TDR

- Water potential HDU


Upper Storage Limit

Embedded Lysimeter Water Balance

On-Site

Evapotranspiration

Average

Percolation

~ 0.5 mm/yr


Cover Percolation Comparison

Average

Percolation

Site

Average

Cover

Precipitation

Percolation

as % of

EPA ACAP

Type

(mm)

(mm)

Precipitation

DOE LM

Low-

Albany, GA

849

26.0

265.0

Permeability

Apple Valley, CA

61

4.1

2.5

Cover

Cedar Rapids, IA

449

8.8

39.5

Lakeview, OR

319

9.4

30.1

Water

Apple Valley, CA

167

0.3

0.5

Balance

Boardman, OR

181

0.0

0

Cover

Polson, MT

349

0.1

0.2

Monticello, UT

387

0.1

0.5


Cover Water Balance: Role of Plants

Estimated Ranges of Annual Recharge (mm/yr)

~380

Shrubs

380 mm (15 in)

100-200

Wheatgrass

Cheatgrass

Bare / Rock

Soil

Depth

(1.5 m)

Loam Soil

<1

0-20

20-100

100-200


250

50

1.0

225

45

0.9

200

40

0.8

175

35

0.7

150

30

0.6

Percent Cover

Shrub Density (#/Acre)

Leaf Area Index

125

25

0.5

100

20

0.4

75

15

0.3

50

10

0.2

25

5

0.1

0

0

0

Monticello Vegetation Monitoring

2001

2002

2003

2004

2005

2006

2007

2008

2009

2010


June 2008

Preliminary LAI Map for Monticello CoverJune 2008

LAI

5.432

John Gladden

SRNL

0


Presentation Topics

  • Purpose of disposal cell covers

  • Cover designs, natural succession, and performance

  • Cover renovation – improving sustainability by accommodating natural succession

  • Summary


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

Grand Junction, Colorado


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

    Cover Renovation!

Grand Junction, Colorado


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


Rock Riprap

30 cm

15 cm

Bedding

45 cm

Protection

Layer

45 cm

Low-Permeability Radon Barrier

Fourwing saltbush

Tailings

Control

Renovate

Cover Renovation

Lysimeters 2008

46


20

700

Control Lysimeter

600

Precipitation

15

Soil Water Storage

500

400

Cumulative Runoff and Percolation (mm)

10

Cumulative Precipitation and Evapotrasnpiration,

and Soil Water Storage (mm)

Evapotranspiration

300

200

5

Percolation

100

Surface Runoff

0

0

10/31/07

5/1/08

10/31/08

5/1/09

10/31/09

5/1/10

10/31/10

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

20

700

Renovate Lysimeter

Precipitation

600

Soil Water Storage

15

500

400

Cumulative Runoff and Percolation (mm)

10

and Soil Water Storage (mm)

Cumulative Precipitation and Evapotrasnpiration,

Evapotranspiration

300

Percolation

200

5

100

Surface Runoff

0

0

10/31/07

4/30/08

10/30/08

5/1/09

10/30/09

5/1/10

10/31/10


Presentation Topics

  • Purpose of disposal cell covers

  • Cover designs, natural succession, and performance

  • Cover renovation – improving sustainability by enhancing natural succession

  • Summary


Cover LTSM Questions

  • Why did we cover uranium mill tailings?

  • How were the covers designed to work and how were they constructed?

  • How have natural succession processes altered cover performance?

  • What are the risks to HH&E if the covers are not performing as designed?

  • What types of maintenance are required—and at what cost—to keep covers performing as designed?

  • Could we design sustainable repairs or renovations if needed to reduce LTSM costs and risks?

  • Can we expect covers to continue working as designed for the long term—200 to 1000 years?


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