Influences of Flow Transients and Porous Medium Heterogeneity on Colloid-Associated Contaminant Transport in the Vadose Zone Page: 2 of 6
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(Objective 3). During the first nine months of Year 3, we have focused on accomplishing
Objectives 4 - 6.
Part of our work in Year 3 involved testing our inferences on mechanisms that govern
colloid deposition and mobilization through analysis of pore-scale visualization experiments
(Objective 4). In these visualization experiments, a transparent flow cell packed with a thin layer
of unsaturated sand was placed on a stage of an inverted microscope to permit direct observation
of deposition and mobilization. Our results show that multiple mechanisms contribute to colloid
deposition. Insular air bubbles scavenged colloids, which is consistent with results of
Figure 1. Pore-scale visualization of
colloid immobilization: (a) colloids
retained within thin films and
pendular rings of a partially saturated
pore (air occupies the center of the
pore) and (b) colloids stored within
stagnant-water regions that branch
off from mobile-water regions (note
colloid accumulation along perimeter
of stagnant water zone at air-water
experiments conducted with etched micromodels [Wan and
Wilson, 1994; Sirivithayapakorn and Keller, 2003]. Other
deposited colloids were held within thin films of water that
stretched between pendular rings of unsaturated pores (Figure
la). These observations confirm previous speculation of the
significance of film straining in influencing colloid mobility
[Wan and Tokunaga, 1997; Lenhart and Saiers, 2002; Saiers
and Lenhart, 2003]. Colloids also were effectively
immobilized upon entry into zones of stagnant water (Figure
lb). This concept of stagnant-water zones has long been used
to account for the storage of solutes, but, with very few
exceptions [Gamerdinger and Kaplan, 2001; Cherrey et al.,
2003], has been ignored in descriptions of colloid transport
and, prior to our work, no confirmatory evidence for stagnant-
water storage of colloids existed.
The visualization experiments were especially useful
in yielding insight into the mechanism that control colloid
mobilization. During transient porewater flow, characterized
by temporal increases in moisture content, two mechanisms
dominated the mobilization response. Film-strained colloids
were released as the films were abruptly eliminated when
partially saturated pores filled spontaneously with water.
Porous-medium imbibition also promoted the release of
colloids held within stagnant-water zones as water invaded
air-filled regions of the porous medium and fragmented
stagnant-water zones reconnected to areas of bulk fluid flow.
The increases in flow rate that accompanied the moisture-
content increases during the transient-flow experiments did
not shear detectable quantities of colloids from the surfaces of
the sand grains or insular air bubbles. Knowledge gained
from these experiments is currently being used to refine the structure of our mathematical models
that account for coupled transient porewater flow, colloid mobilization, and transport.
Even in our well-controlled laboratory systems, the porous medium is non-ideal,
consisting of non-spherical mineral grains with considerable surface roughness. Published
theoretical descriptions of colloid filtration, which were developed for water-saturated systems,
are not designed to account for these non-idealities. One aspect of our ongoing research is
devoted to modifying colloid-filtration theory to account for non-idealities associated with real
geologic materials (Objective 5). Through statistical analysis of pore-scale simulations of
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Saiers, James & Ryan, Joseph. Influences of Flow Transients and Porous Medium Heterogeneity on Colloid-Associated Contaminant Transport in the Vadose Zone, report, June 1, 2005; United States. (digital.library.unt.edu/ark:/67531/metadc892910/m1/2/: accessed February 22, 2019), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.