John R. Nimmo, Kim S. Perkins, and Kari A. Winfield USGS, Menlo Park, California
description
Transcript of John R. Nimmo, Kim S. Perkins, and Kari A. Winfield USGS, Menlo Park, California
Unsaturated-Zone Case Study at the Idaho National Engineering and Environmental
Laboratory: Can Darcian Hydraulic Properties Predict Contaminant Migration?
John R. Nimmo, Kim S. Perkins, and Kari A. Winfield
USGS, Menlo Park, California
Geological Society of AmericaDenver, Colorado
November 9, 2004
Idaho
Eastern Snake River PlainINEEL
Subsurface DisposalArea (SDA)
Subsurface Disposal Area
200 m toWater Table
Fractured BasaltInterbedded with Thin Layersof Coarse To Fine Sediments
June 21-23, 1999:Apply tracer to spreading areas.
1999-2000:Sample available wellsin unsaturated zone and aquifer (symbols).
2 km
Diversion
SDA
Chemical Tracer
• Previously applied in geothermal applications
• Conservative in subsurface materials
• Detectable to 0.2 ppb
June 21-23, 1999: Applied 725 kg of tracer
Sediment
Basalt Aquifer
Perched Water
Snake River Plain AquiferGround water mound
Subsurface Disposal AreaSpreading area
BasaltB-C Interbed
A-B Interbed
C-D Interbed
Prevailing ground water flow direction
Depth to aquiferapproximately200 meters
1 km
SDA
B-C (34 m)
Detection
Non-detect
C-D (73 m)
Detection
Non-detect
Aquifer (200 m)
Detection
Non-detect
Sampling Results
0
1
2
3
4
5
0 50 100 150 200 250
Days post tracer application
Co
nce
ntr
atio
n (p
pb
)
USGS 92
USGS 120
C-D and Aquifer Well Detections
Aquifer(200 m depth;0.2 km away)
CD Interbed(73 m depth;1.3 km away)
Speed of Travel
• Vertical (at edge of SAB): 200 m
qvertical* = 3 10-2 cm/s
• Horizontal (SAA to SDA): 2.1 km
qhorizontal* = 4 10-2 cm/s
* Flux density for effective porosity of 0.3
(7 ± 2) days= 30 (± 10) m/day
(60 ± 30) days= 35 (± 17) m/day
Numerical modeling by Richards’ Equation
(VS2DT code)
Water Content
X (m)
Z (
m)
Basalt Ksat= 1.7 cm/sPorosity= 0.33
SedimentKsat= 5.8 x 10-3 cm/s
104 days
Model SensitivityParameter Initial Value Modified Value Sensitivity
Surficial Sediment Ksat (cm/s) 5.79 x 10-4 5.79 x 10-3 High
Combination of Surficial Sediment and Basalt Ksat (cm/s)
5.79 x 10-4
and0.17
5.79 x 10-3
and 1.7
High
Basalt Porosity .23 .33 Low
Basalt Residual Moisture Content 0 0.1 None
Surficial Sediment Van Genuchten 0.1216 0.2432 Low
Combination of Surficial Sediment and Basalt Van Genuchten
0.1216and
0.0384
0.242and
0.0768Low
Surficial Sediment Van Genuchten n 1.36 1.72 Low
Combination of Surficial Sediment and Basalt Van Genuchten n
1.36and
1.474
1.72and
1.948Low
Surficial Sediment Thickness (m): 2 Cases
0.52.0
and 0
High
Ponding Depth (m) 2.0 4.0 Low
Driving Force in Fractured Basalt
Example: Spreading Area A to SDA on CD Interbed
Gradient: 9.4 m / 2100 m = 0.0045
Perched Water
Sloping Interbed
SAA
9.4 m
WellUSGS-92
2.1 km
Land Surface
Horizontal Flow Along Sloping Interbeds
-6.00E-03
-5.00E-03
-4.00E-03
-3.00E-03
-2.00E-03
-1.00E-03
0.00E+001 1.1 1.2 1.3 1.4 1.5 1.6 1.7
Distance From Spreading Area (km)
Ave
rag
e G
rad
ien
t o
f In
terb
ed f
rom
Sp
read
ing
A
rea
to D
etec
tio
n P
oin
t
B-C Interbed, No Detection
B-C Interbed, Tracer Detected
C-D Interbed, No Detection
C-D Interbed, Tracer Detected
Darcy’s law calculation
Example: Spreading Area A to SDA on CD Interbed
q = 4 10-2 cm/s, inferred from observation
Gradient = 0.0045, based on interbed elevation data
K 9 cm/s
Estimated Maximum Effective Hydraulic Conductivity
Medium & Source Method Khoriz (cm/s)
1-cm Gravel
(Fayer and others, 1992)Lab measurement 0.35
INEEL UZ
(this study)Darcy calculation 9
INEEL UZ
(this study) RE numerical model > 1.7
INEEL UZ
(Wood & Norrell, 1996)Large-Scale Infiltration Test
of 1994 0.09
INEEL UZ (Magnuson & Sondrup, 1998) TETRAD calibration 0.009
INEEL Sat. Zone
(Anderson and others, 1999)Single-well aquifer tests 11
Conclusions for Prediction of Long-Range Horizontal UZ
Transport
There is a feature of the INEEL UZ, probably associated with basalt-sediment interfaces, that conducts fast and continuous flow over km-scale distances.
The INEEL UZ must have extreme anisotropy, in excess of previous estimates.
A simple Darcy’s law calculation predicts tracer arrival as well as, or better than, detailed numerical modeling based on Richards’ equation.