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.