Fundamentals of Groundwater Hydrogeology - Introduction to Phytoremediation of Contaminated Groundwater - page 103

and the groundwater in the hydropher the groundwater body
(see Clebsch 1994). In the time before Darcy, the water

produced from wells was described as originating from
water bearing strata, as described in an early topic on geol-
ogy by Hitchcock (1840). Moreover, the term aquifer often
is relative; in humid areas, for example, fine-grained silts or
fractured rock are considered to be low yielding but may be
considered an excellent source of water in more arid areas.
Regardless, the groundwater in aquifers can be classified
under two major conditions with respect to Bernoulli's
Law and geologic conditions—unconfined and confined
aquifers.

4.4.1 Unconfined Aquifers

In unconfined water-table conditions, the uppermost surface
of the saturated zone not under capillary tension represents
the special case of Eq. 4.8 where water is at atmospheric
pressure. The pressure head in Eq. 4.8 is zero; thus, the total
head is simply the elevation head of the water above a
common datum (Fig. 4.8 ).
Unconfined aquifers are often near land surface, espe-
cially in low-lying areas. Because the water-table surface is
exposed to the atmosphere and infiltrating water, the posi-
tion of the water table can rise and fall. The water table is
essentially a free surface that can react to a stress by moving
upward during recharge and downward during discharge
(Fig. 4.8 ).
Because the water-table surface is often near or at the
land surface in the case of some surface-water features, it has
long been believed that the water-table surface follows the
topography of the area. This is partially true in that water
flow in unconfined aquifers is in response to differences in
elevation head and gravity, and this situation can be initiated
by higher topographic elevations. However, topography can
be an overestimate of the shape of the water-table profile.
For example, in the steep topographic gradients common to

Fig. 4.7 The relation between total head, H or elevation head, Z and
pressure head, p ,or c , for laboratory settings such as those used by
Darcy ( top ) and a monitoring well in the field ( bottom ).

Fig. 4.8 The unsaturated zone,
capillary zone and fringe, and
water table. The distinction is
made between a capillary zone
and fringe such that the capillary
zone represents where the voids
may be completely filled with
water but held under tension
( saturated tension ) and does not,
therefore, flow freely to a well.
The capillary fringe is where
water is under tension but does
not completely saturate the voids
( unsaturated tension ).