Unsaturated-Zone Case Study at the Idaho National Engineering and Environmental Laboratory: Can Darcian Hydraulic Properties Predict Contaminant Migration?

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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 America Denver,
Colorado November 9, 2004
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Idaho
Eastern Snake River Plain
INEEL
Subsurface Disposal Area (SDA)
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Subsurface Disposal Area
Fractured Basalt Interbedded with Thin Layers of
Coarse To Fine Sediments
200 m to Water Table
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Big Lost River
Diversion
SDA
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Chemical Tracer

• Previously applied in geothermal applications
• Conservative in subsurface materials
• Detectable to 0.2 ppb

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June 21-23, 1999 Applied 725 kg of tracer
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Subsurface Disposal Area
Spreading area
A-B Interbed
Depth to aquifer approximately 200 meters
B-C Interbed
Basalt
C-D Interbed
Ground water mound
Snake River Plain Aquifer
Prevailing ground water flow direction
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Sampling Results
B-C (34 m) Detection Non-detect
C-D (73 m) Detection Non-detect
Aquifer (200 m) Detection Non-detect
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C-D and Aquifer Well Detections
Aquifer (200 m depth 0.2 km away)
CD Interbed (73 m depth 1.3 km away)
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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

30 ( 10) m/day
(7 2) days
35 ( 17) m/day
(60 30) days
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Numerical modeling by Richards Equation (VS2DT
code)
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Model Sensitivity
Parameter 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 and 0.17 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 a 0.1216 0.2432 Low
Combination of Surficial Sediment and Basalt Van Genuchten a 0.1216 and 0.0384 0.242 and 0.0768 Low
Surficial Sediment Van Genuchten n 1.36 1.72 Low
Combination of Surficial Sediment and Basalt Van Genuchten n 1.36 and 1.474 1.72 and 1.948 Low
Surficial Sediment Thickness (m) 2 Cases 0.5 2.0 and 0 High
Ponding Depth (m) 2.0 4.0 Low
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Driving Force in Fractured BasaltExample
Spreading Area A to SDA on CD Interbed
2.1 km
SAA
Land Surface
Perched Water
Well USGS-92
9.4 m
Sloping Interbed
Gradient 9.4 m / 2100 m 0.0045
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Horizontal Flow Along Sloping Interbeds
Distance From Spreading Area (km)
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1.1
1.2
1.3
1.4
1.5
1.6
1.7
0.00E00
B-C Interbed, No Detection
B-C Interbed, Tracer Detected
-1.00E-03
C-D Interbed, No Detection
C-D Interbed, Tracer Detected
-2.00E-03
Average Gradient of Interbed from Spreading
Area to Detection Point
-3.00E-03
-4.00E-03
-5.00E-03
-6.00E-03
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Darcys 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

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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 gt 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
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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 Darcys law calculation predicts tracer
  arrival as well as, or better than, detailed
  numerical modeling based on Richards equation.