Transport of Tracer Br in Frozen Morin Clay
in Response to Temperature Gradients
YOSHISUKE NAKANO
INTRODUCTION
front has attracted the attention of researchers. Mahar et
The transport of solutes in frozen or freezing soils is
al. (1983) investigated salt redistribution during freez-
important for both engineering and scientific reasons.
ing of gravel saturated with a salt solution of a known
The presence of salts is known to drastically alter the
concentration. They found that a distinct freezing inter-
mechanical properties of frozen soils (Sego et al. 1982,
face did not exist. Instead, a zone of partially frozen
Hivon 1991). It is also known that the presence of salts
gravel with a high unfrozen water content beginning at
tends to reduce the rate of frost heave in fine-grained
the depressed freezing point isotherm (freezing front)
soils (Konrad 1990). The migration of fertilizer and
was observed. It was also found that the salt concentra-
pesticides during annual freezing is a subject of con-
tion in unfrozen water increases toward the lower tem-
siderable interest in agronomy (Gray and Granger
peratures and that the content of salt in the frozen part of
1986).
gravel was less than the initial content (Mahar et al.
The phenomenon of freezing soils that contain sol-
1983).
utes is complex and poorly understood. Inasmuch as
When the solidification occurs from the melt con-
solutes tend to be excluded from growing ice, they are
taining impurities, the composition of the solid phase
confined to the domains of unfrozen (interfacial) wa-
generally differs from that of the coexisting melt. This
ter. The transport process of solutes depends mainly on
phenomenon, called segregation or redistribution
the transport of unfrozen water, but it also is influ-
among researchers of crystal growth, has been studied
enced by the diffusion or dispersion of solutes and
extensively. Analyzing the process of redistribution,
complex interactions of solutes with the surfaces of
Burton et al. (1953) obtained a quantitative description
soil particles and ice. The amount of unfrozen water in
(BPS theory) of an effective segregation coefficient k
frozen soils generally increases with the increasing
given as
concentration of solutes (Banin and Anderson 1974).
Since the mobility of unfrozen water tends to increase
[
]
k = Cs / Cl = k0 / k0 + (1 - k0 )e-bu
with the increasing unfrozen water content (Nakano
(1)
1991), the two transport processes of solutes and un-
frozen water mutually depend on each other. Because
where Cs and Cl = solute concentrations in the solid
of this mutual dependence, the transport of solutes in
and liquid phases, respectively, ad-
frozen soils is more complex than that in unfrozen
jacent to the solid/liquid interface
soils.
k0 = value of k when the growth rate u of
The rate of solute transport is anticipated to be great
the solid phase is zero
where the rate of water transport is great. Since the
b = positive number that depends on a
transport of water is closely coupled with the phase
given system.
change of water in freezing soils and solutes tend to be
excluded from growing ice, the solute redistribution is
Weeks and Lofgren (1966) showed the validity of BPS
expected to be most rigorous near a freezing front.
theory applied to the problem of freezing salt so
Therefore, the solute redistribution near a freezing
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