For the purpose of modeling solute transport, soil water has often been simply
divided into an essentially mobile fraction, q m , which
is active in solute transport, and an apparently immobile fraction, q
im . Distinction between q m and q
im was sought using the disc permeameter technique. This
study examines unsaturated estimates of mobile water content at suction heads,
h, of 20, 40, 80, and 120 mm for several soils ranging
in texture from sand to clay. Following infiltration of 35 mm depth of 0.01 M
KBr into initially dry soils, soil samples were collected from below the base
of the disc permeameter and analysed for tracer concentrations which enabled
partitioning of mobile and immobile water. Hydraulic conductivity and
sorptivity were also derived from the infiltration data. The results show the
expected non-linearity of hydraulic conductivity and sorptivity with suction.
The mobile water expressed as a fraction, f, of the
volumetric water content q (f = q
m / q ) was generally found to range from 0.7 to
0.95, with an average of 0.85. The exception was one site for which
f was ª 0.50. These values of
f are comparable to those derived from leaching studies
reported in the literature. An important finding of this work is that within
the range of suctions measured, the mobile fraction was independent of
suction. A possible explanation for this observation is that the soil
capillary forces were dominant during the time scale of the experiment and
therefore rapidly drew the invading solution. This finding could have
important implications for fertiliser application. Furthermore, this result
suggests that the assumption of a negligible solute transfer coefficient, a ,
between the mobile and immobile domains may be valid within the time scale of
this method of measuring the mobile water content.
Miscible Displacement in Porous Media Influenced by Mobile and Immobile Water
