Fundamentals of Groundwater Hydrogeology - Introduction to Phytoremediation of Contaminated Groundwater - page 104

the sand hills in the Coastal Plain geophysical province of
the eastern United States, the hydraulic gradient of the water
is less pronounced because the higher permeabilities of the
sandy materials lead to less frictional head losses such that
groundwater flow can occur with lower hydraulic gradients
(see also Fig. 4.3 ).
To simplify the definition of groundwater flow under
such conditions, Dupuit (1863) and Forchheimer (1930)
assume that the head at a vertical location is constant, all
groundwater-flow lines are horizontal across the entire thick-
ness of the water-table aquifer, vertical flow is eliminated,
and the velocity of groundwater flow is directly related to the
slope of the hydraulic gradient for each flow line. Although
not relative for small-scale simulations of groundwater flow,
the Dupuit-Forchheimer model is used widely for larger-
scale simulations. An alternative conceptualization is that
the water-table surface is controlled by the recharge poten-
tial of the sediments (Haitjema and Mitchell-Bruker 2005).

Fig. 4.9 Representations of how water behaves in the capillary zone
above the water table. In both ( ), representative large and small

diameter pipes with an open end were placed in a pan of water. The
larger pipe represents a well, and the smaller pipe represents
interconnected pore spaces. In (

a ) and ( b

), the water in the larger diameter
pipe is at the same level as the water in the pan. In the smaller diameter
pipe, the water level rises in the direction of the arrows to a maximum
level, h c , above the water level in the pan. This is caused by water
cohesion and adhesion to the inner wall of the pipe and reflects the
capillary zone. In (

a

), the same two pipes are filled with porous media,
but the water does not rise immediately in the smaller diameter pipe to
h c . Rather, it rises to some lower elevation, z , initially (

b

c ), and then over

time reaches h c . (Modified from Heath 1983).

4.4.2 Unsaturated Zone, Capillary Fringe, and
Capillary Zone

to a final height balanced by gravity and the smaller well's
inner diameter.
Even though the capillary zone is hard to directly
observe, it can be measured with certain devices. To assess
the water potential under tension, a tensiometer can be used.
The measurement of tension is crucial to understanding the
source of water to plants, because the initial entry of water
into root hairs is by capillary action, which then is continued
by osmosis. The very small diameters of root hairs are
designed to maximize diffusion and osmosis as well as
take advantage of the surface tension of water.
The thickness of the capillary zone and fringe, or height
above the saturated zone, can be estimated by using qualita-
tive and quantitative approaches. Qualitatively, fine-grained
sediments, such as silts and clays, have a thicker capillary
zone and fringe than coarse-grained sediments, such as sands
and gravels, assuming that soil moisture from previous pre-
cipitation is not confused with the uppermost location of the
capillary fringe. This is because water is found in the small
pores due to surface-tension attraction to the sediments; the
large pores do not permit this to occur and are mostly filled
with air. In most cases, however, the sediments are not
homogeneous. If pore sizes are uniform, the capillary zone
and fringe are larger than if the pore sizes were not uniform.
The use of a hand auger also can supply a reliable esti-
mate of the thickness of the capillary fringe during field
studies. The upper layer of the capillary fringe can be
detected when the removal of the auger from the hole
coincides with resistance, as well as accompanied by an
audible sucking sound. The energy needed to overcome the
tension and remove the auger from the borehole provides

The previous description of the water table often gives rise to
a common misconception that the surface of the water table
can be defined as a sharp interface between completely
saturated and completely dry sediments. This is not the
case at all, because the surface tension of water in contact
with porous media will cause water to rise under tension
above the fully saturated area where water is at atmospheric
pressure (Fig. 4.8 ). The thickness of sediments nearest the
water table where water completely saturates the pore spaces
but is held under tension, or tension saturated, is the capillary
zone. The thickness of the capillary zone is controlled by the
size of the pore spaces (Fig. 4.9 ); increased porosity results
in a thinner capillary zone and decreased porosity a thicker
capillary zone. As the pore spaces begin to be occupied by
more air than water above the capillary zone, water is still
under tension and is called the capillary fringe (Fig. 4.8 ).
Above this is the unsaturated zone, where water may or may
not be present (Fig. 4.8 ). In some sub-disciplines of hydro-
geology, the capillary zone and fringe are called either one
or the other term and meant to be synonymous.
One of the reasons the concept of the capillary zone and
fringe are often misunderstood is because they generally are
not encountered during groundwater investigations that
employ conventional site assessment techniques. For exam-
ple, if a 2-in. diameter observation or monitoring well is
installed through the capillary zone to the water table, the
water that flows into the well is from the saturated zone
where water is under pressure. If, however, a well of smaller
diameter is installed next to the first, conventional well,
groundwater will rise above the water table due to tension