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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. - PowerPoint PPT Presentation
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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?