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

1
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.
Colloid/Contaminant Co-Transport in a Variable
Aperture Fracture Scott C. James and
Constantinos V. Chrysikopoulos1 Sandia National
Laboratories, Geohydrology Department,
Albuquerque, NM 871850735.
1University of California Civil Environmental
Engineering Irvine, California 92697-2175, USA
Introduction
? Colloids and contaminants in a porous fractured
rock ? Processes Considered
? Important sites and interested parties

• 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

• Yucca Mountain Project USA
• Nevada Test Site USA
• Grimsel Underground Laboratory Sweden
• Äspö Hard Rock Laboratory Switzerland
• Underground Research Laboratories Japan

• Colloids
• Advection
• Diffusion
• Irreversible filtration onto fracture surfaces

Why are colloids important? Enhanced transport!
Numerical model
? Representative fracture
? Particle tracking
? Reaction equations
Aperture
Velocity
Contaminant equations
Colloid equations
Colloid filtration
Contaminant sorption onto the matrix
Contaminant sorption onto colloids
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
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.
One realization of a variable aperture fracture
and its corresponding flow field. The mean
fracture aperture is 5?10-5 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 (y0) and top
(y0) of the fracture and a head gradient of 0.31
induces flow from left to right.1,2
Results
? Uniform aperture fracture
validation ? Co-transport processes
only ? All reaction processes
?Conclusions

• 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.

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, F9kT, and
q0.1, kc 110-3 1/s.
Monodisperse (dp510-6 m) colloid and
contaminant analytical (solid curves) and
numerical (dashed curves) breakthrough curves at
x8 m in a uniform aperture fracture with
b510-5 m and u610-5 m/s. Here, F?, and
qkcKn0.
Colloid and contaminant breakthrough curves
showing the effect of variation of the
co-transport partition coefficient, Kn. Here,
F?, and qkc0.
References1 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), 288295, 2003.
The Geological Society of America Annual Meeting
and Exposition Seattle, Washington. November
25, 2003.