b(x,y) x

y zFracture

Colloid

PorousRock

Matrix

Contaminant

Colloids and contaminants in a porous fractured rock Processes Considered Important sites and interested partiesIntroduction

Colloid/Contaminant Co-Transport in a Variable Aperture Fracture Scott C. James and Constantinos V. Chrysikopoulos1

Sandia National Laboratories, Geohydrology Department, Albuquerque, NM 87185–0735.

Sandia National Laboratories is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy under Contract DE-AC04-94AL85000.

1University of California Civil & Environmental Engineering Irvine, California 92697-2175, USA

The Geological Society of America Annual Meeting and Exposition – Seattle, Washington. November 2–5, 2003.

Colloids:

•Advection

•Diffusion

•Irreversible filtration onto fracture surfaces

Numerical model Representative fracture Particle tracking Reaction equations

References:1 James, S. C. and C. V. Chrysikopoulos, Transport of polydisperse colloids in a saturated fracture with spatially variable aperture, Water Resour. Res., 36(6), 1457 – 1465, 2000.2 Chrysikopoulos, C. V. and S. C. James, Transport of neutrally buoyant and dense variably sized colloids in a two-dimensional fracture with anisotropic aperture, Transp. Porous Media, 51(2), 191 – 210, 2003.1 James, S. C. and C. V. Chrysikopoulos, Effective velocity and effective dispersion coefficient for finitely sized particles flowing in a uniform fracture, J. Colloid interface Sci., 263(1), 288–295, 2003.

Yucca Mountain Project – USANevada Test Site – USAGrimsel Underground Laboratory – SwedenÄspö Hard Rock Laboratory – SwitzerlandUnderground Research Laboratories – Japan

Results Uniform aperture fracture validation Co-transport processes only All reaction processes Conclusions

Colloids are subject to advection, diffusion, and filtration on the fracture walls. They may not diffuse into the surrounding porous matrix because colloids are larger than the matrix pore size. Colloids travel by advection in the x- and y-directions according to the local parabolic velocity profiles, and diffuse isotropically in all directions.3

Monodisperse (dp=5×10-6 m) colloid and contaminant analytical (solid curves) and numerical (dashed curves) breakthrough curves at x=8 m in a uniform aperture fracture with b=5×10-5 m and ū=6×10-5 m/s. Here, =, and c=Kn=0.

Contaminants:

•Advection

•Taylor dispersion

•Diffusion into the matrix

•Equilibrium sorption onto the matrix

•Irreversible sorption onto fracture surfaces

•Irreversible sorption onto mobile and filtered colloids

•Colloids, ubiquitous in the subsurface, can act as a mobile third phase in saturated media.

•High surface area to mass ratio makes colloids likely candidates for cotransport phenomenon.

•Colloids tend to travel faster and farther than contaminants.

•A colloid with sorbed contaminant becomes a contaminant.

•Colloids can enhance contaminant transport, particularly be reducing the impact of other retarding mechanisms.

One realization of a variable aperture fracture and its corresponding flow field. The mean fracture aperture is 510-5 m with a log-variance in the aperture fluctuations of 0.037 and isotropic correlation length of 1 m. There are no-flow boundaries at the bottom (y=0) and top (y=0) of the fracture and a head gradient of 0.31 induces flow from left to right.1,2

( )

( )

211 1 1

1

211 1 1

2

13

3, 1 4 2

2

3, 1 4 2

2

2

p

p

p

mm m m m

x d

mm m m m

y d

m md

zx x U x y t Z t

b

zy y U x y t Z t

b

z z Z t

-- - -

-- - -

-

é ùæ ö÷ê úç= + - D + D÷çê ÷ú÷ççè øê úë ûé ùæ ö÷ê úç= + - D + D÷çê ÷ú÷ççè øê úë û

= D

D

D

D

( )

( )

( ) ( )

1 1 11 Taylor

1 1 12 Taylor

2 21 1 1 1,

Taylor

, 2

, 2

, ,

210

m m m mx

m m m my

m m m mx y

x x U x y t Z D t

y y U x y t Z D t

U x y b x yD

- - -

- - -

- - - -

= + D + D

= + D + D

= +DD

Contaminants move through the fracture by advection and dispersion in the x- and y-directions and their transport is retarded by sorption onto the fracture walls and diffusion into and sorption onto the surrounding porous rock matrix. Contaminants may sorb onto colloids, thereafter adopting the transport properties of the carrier colloid.

Contaminant equations Colloid equationsAperture Velocity Colloid filtration

expap kTæ öF ÷ç= - ÷ç ÷çè ø

Contaminant sorption onto the matrix

( )( ) ( )

**,

, ,mc m m c m

C x yC x y C C x y t

tk k

¶= Þ D = D

¶Contaminant sorption onto colloids

( )

( )( )

( )( )

*

*,

, ,, ,

f n nff f

C x y K KC x y C C x y t

t b x y b x y

¶= Þ D = D

¶

Colloid and contaminant breakthrough curves resulting from inclusion of matrix diffusion, wall sorption, and co-transport processes in a variable aperture fracture. Colloids have one sorption site per particle. Here, =9kT, and =0.1, c = 1×10-3 1/s.

Colloid and contaminant breakthrough curves showing the effect of variation of the co-transport partition coefficient, Kn. Here, =, and c=0.

Why are colloids important?Enhanced transport!