Resistive Barriers vs. Water Balance Covers: Tales of Two Cover Designs in
Seasonal Semi-Arid Climates
Craig H. Benson, PhD, PE, NAEUniversity of Virginia/CRESP
Cover Strategy:Resistive Barrier vs. Water Balance Covers
Conventional Cover Water Balance Cover
Surface
Layer
Interim
Clay
Barrier
GeomembraneStorage
Layer
Interim
Capillary Break
Clay RadonBarrier
GravelAmended
Water Storage
& Protection
BiotaBarrier
Sand
200 mm
900 mm
300 mm
300 mm
300 mm
Tailings
300 mm
150 mm
450 mm
450 mm
Clay RadonBarrier
FrostProtection
Bedding
Riprap
Tailings
Sand
Clay RadonBarrier
GravelAmended
Water Storage
& Protection
200 mm
1000 mm
910 mm
Tailings
ClayRadonBarrier
Interim
Monticello Disposal Facility
White MesaMill Tailings Facility
CheneyDisposal Facility
Cheney DisposalFacility
Monticello DisposalFacility
White MesaMill
Utah Colorado
Validate Performance with ACAP LysimeterACAP lysimeters used to confirm a design meets performance goal by directly monitoring percolation and to understand, when necessary,
the hydrologic processes controlling percolation.
Cover
Soils
Instrument
NestDiversion
Berm
Geomembrane
Liner
Geocomposite
DrainInterim
Cover
Soil
Root
Barrier
Surface
Runoff
Percolation
Diversion
Berm
Benson,C.,Albright,W.,Fratta,D.,Tinjum,J.,Kucukkirca,E.,Lee,S.,Scalia,J.,Schlicht,P.,Wang,X.(2011),EngineeredCoversforWasteContainment:ChangesinEngineeringProperties&ImplicationsforLong-TermPerformanceAssessment,NUREG/CR-7028,OfficeofResearch,U.S.NuclearRegulatory
Commission,Washington.
ResistiveWater Balance
Blackfoot Bridge, August 2013
Formwork
Installing GCLL
Base Geomembrane
Filling Alluvium
GCLL
July 2015
Elevation: 1500 mmPrecipitation: 230 mmClimate: Seasonal semi-aridCover Thickness: 1.4 m
Armored Cover at Cheney Disposal Facility
300 mm
150 mm
450 mm
450 mm
Clay RadonBarrier
FrostProtection
Bedding
Riprap
Tailings
Cheney Water Balance Record
0
500
1000
1500
2000
2500
0
20
40
60
80
100
10/31/07 6/12/09 1/24/11 9/6/12 4/20/14 12/2/15 7/15/17
Cu
mula
tive
Pre
cip
ita
tion
an
d E
vap
otr
asn
pir
ation
, a
nd S
oil W
ate
r S
tora
ge
(m
m)
Cu
mula
tive
Ru
noff a
nd P
erc
ola
tion
(mm
)
Soil Water Storage
Surface Runoff
Perc
On-Site Precipitation
On-Site ET
NWS ET
NWS Precipitation
• Most precipitation becomes ET
• Perc much higher later in record.
• Subtle variation in soil water storage.
• Very little runoff.
Cheney Water Content Record
0.00
0.05
0.10
0.15
0.20
0.25
0.30
0.35
0.40
Jan
/1/0
8
Jan
/1/1
0
Jan
/1/1
2
Jan
/1/1
4
Jan
/1/1
6
Bedding
Frost Protect
Radon Upper
Radon Lower
Sand
Vo
lum
etr
ic W
ate
r C
onte
nt (-
)
• Bedding and frost protection layer vary seasonally
• Radon barrier saturated at end of record
• Increase in water content of sand consistent with percolation
Cheney Annual Water Balance
• ET largest fraction, but usually < precip
• Perc relatively low, except during wetter years.
• Perc tied directly to integrity of clay radon barrier.
0
200
400
600
800
1000
0
20
40
60
80
100
20
08
20
09
2010
2011
20
12
20
13
2014
2015
20
16
2017
Evapotranspiration
Precipitation
Percolation
Pre
cip
ita
tion &
Evap
otr
an
sp
iratio
n (
mm
)
Pe
rcola
tion
(mm
)
Note: Y2 scale 10x < than Y1
Cheney: Percolation & Precipitation• Most of
precipitation becomes ET
• Perc much higher during wetter years
• Barrier integrity critical for managing wet conditions.
50
100
150
200
250
300
350
400
0
10
20
30
40
50
60
50 100 150 200 250 300 350 400
Evapotranspiration
Percolation
Eva
potr
anspira
tio
n (
mm
)
Perc
ola
tion (m
m)
Precipitation (mm)
Water Balance Cover at Monticello Disposal FacilityElevation: 2150 mmPrecipitation: 370 mmClimate: Seasonal semi-aridCover Thickness: 2.0 m
Clay RadonBarrier
GravelAmended
Water Storage
& Protection
BiotaBarrier
Sand
200 mm
900 mm
300 mm
300 mm
300 mm
Tailings
Monticello Water Balance Record
On-Site ET
0
2000
4000
6000
8000
0
100
200
300
400
500
600
8/1
/00
12
/12
/02
4/2
3/0
5
9/3
/07
1/1
3/1
0
5/2
5/1
2
10/5
/14
2/1
5/1
7
Cum
ula
tive
Pre
cip
ita
tion
an
d E
va
po
tra
nsp
ira
tio
n (
mm
)
Cu
mu
lativ
e P
erc
ola
tion
an
d S
urfa
ce
Ru
no
ff,a
nd
So
il Wa
ter S
tora
ge
(mm
)
NWS Precipitation
Surface Runoff
Percolation
Soil Water Storage
NWS ET
Monticello Water Balance Cover
On-Site Precipitation
• Nearly all precip becomes ET
• Storing & releasing water each year
• Runoff very small fraction (minimal erosion)
• Avg percolation rate < 0.5 mm/yr
Monticello Water Content Record
0.00
0.05
0.10
0.15
0.20
0.25
0.30
0.35
0.40
Jan
/1/0
0
Jan
/1/0
5
Ja
n/1
/10
Jan
/1/1
5
Topsoil
Lower Storage
Deep Storage
Sand
Vo
lum
etr
ic W
ate
r C
onte
nt (-
)
• High frequency variation near surface.
• Dampens with depth
• Runoff very small fraction (minimal erosion)
• Avg percolation rate < 0.5 mm/yr
Monticello Annual Water Balance
• ET largest fraction, ≈ precip
• Perc very low, elevated only during wettest year
Note: Y2 scale 100x < than Y1
0
200
400
600
800
1000
0
2
4
6
8
10
2001
2002
2003
2004
2005
2006
2007
2008
2009
2010
2011
2012
2013
2014
2015
2016
2017
Precipitation
Evapotranspiration
Percolation
Pre
cip
itation &
Evapotr
anspiration (
mm
)
Perc
ola
tion (m
m)
Cheney: Percolation & Precipitation
• Nearly ALL of precipitation becomes ET
• Perc elevated in very wet year only. Not dependent on magnitude of precipitation.
200
300
400
500
600
0
1
2
3
4
5
200 300 400 500 600
Evapotranspiration
Percolation
Eva
potr
anspira
tio
n (
mm
)
Perc
ola
tion (m
m)
Precipitation (mm)
White Mesa MillTailings FacilityElevation: 1900 mPrecipitation: 360 mmClimate: Seasonal semi-aridCover Thickness: 2.0 m
Clay RadonBarrier
GravelAmended
Water Storage
& Protection
200 mm
1000 mm
910 mm
Tailings
ClayRadonBarrier
Interim
White Mesa Water Balance Record
0
200
400
600
800
0
50
100
150
200
250
300
350
9/20/16 11/21/16 1/23/17 3/26/17 5/28/17 7/29/17 9/30/17
White Mesa, UT
Soil Water Storage
On-Site Precipitation
On-Site Evapotranspiration
Surface RunoffPercolation
NWS Evapotran-
spiration
NWS Precipitation
Lateral Flow
VaultFlooded
So
il W
ate
r S
tora
ge
, C
um
ula
tive
R
un
off
, L
ate
ral F
low
, a
nd P
erc
ola
tio
n (
mm
)
Cu
mula
tive
Pre
cip
itatio
n a
nd
Eva
po
tran
sp
iratio
n (m
m)
• Large seasonal change in soil water storage
• Vegetation being established, allowing longer residence of water in profile
• Perc, lateral flow, and runoff very low. Little erosion potential
Lessons Learned• Water balance covers very effective in managing water
balance at semi-arid sites for broad range of conditions. Percolation typically < than 0.5 mm/y on average.
• Water balance covers consistent with natural ecological setting, promoting long-term sustainable performance with minimal maintenance.
• Armored resistive barrier functions well with modest precipitation, but long-term integrity of the barrier layer critical to proper functioning.
• At higher precipitation, percolation from cover with resistive earthen barriers increases substantially.