Fundamentals of Groundwater Hydrogeology - Introduction to Phytoremediation of Contaminated Groundwater - page 106

installed in an aquifer that underlies less permeable
sediments will rise above the aquifer under confined
conditions (Fig. 4.11 ). The surface of the groundwater
level is not exposed to the atmosphere. If the groundwater
level rises above ground surface, such as in valleys, ground-
water will flow under its own pressure without being
pumped. Such a condition was probably first observed in
1126 AD in Europe when wells drilled in the valley
sediments in the Artois region gushed groundwater above
land surface—as a result, freely flowing wells became
associated with this region, and are called artesian wells
(De Weist 1965). All groundwater in confined conditions is
at pressures greater than atmospheric. Groundwater levels
measured in wells in confined aquifers satisfy the Bernoulli
equation and the total head measured represents the eleva-
tion head and pressure head.
Unlike the water-table surface in an unconfined aquifer
where the head is simply the elevation head, the water

surface defined by groundwater levels measured in wells in
confined aquifers is less clear. It is a potentiometric surface,
which represents the level to which groundwater will rise in
a hypothetical well installed in a confined aquifer. If a well,
or natural spring, does not penetrate a confined aquifer, the
groundwater level will not rise above the aquifer even
though it has the potential to do so.

4.4.5 Confining Units

Aquifers are considered confined when they are overlain by
less permeable geologic strata. Geologic units that do not
transmit useful quantities of groundwater to wells are called
confining beds or confining units (Fig. 4.11 ). These units
typically are composed of shale or unconsolidated silts or
clay. Ironically, some confining units consist of clay
minerals in which the porosity, n , is higher than in adjacent
sand aquifers, but because the effective porosity, n e , is lower,
little water can be transmitted to wells. An important char-
acteristic for confining units is the hydraulic conductivity of
the material in the vertical direction, rather than in the
horizontal position as is the case for aquifers.

4.5 Aquifer Properties

Unconfined and confined aquifers have many properties that
can be measured. These include aquifer transmissivity, stor-
age coefficient, and heterogeneity, which are discussed here.

4.5.1 Transmissivity

How can groundwater flow be estimated in a confined aqui-
fer? First, the hydraulic conductivity, K , of the sediments
that compose the confined aquifer is multiplied by the aqui-
fer thickness, b . The resulting term is called transmissivity,
T , such that T Kb , in units L 2 /T. Transmissivity can range

from 1,000 ft 2 /d (92 m 2 /d) in poor aquifers to greater than
100,000 ft 2 /d (9,200 m 2 /d) in excellent aquifers. The useful-
ness of transmissivity is evident after substitution in Eq. 4.4 ,
such that Q

¼

Kbwi , where b is the
thickness and w is the width of the aquifer, and finally
Q

¼ KiA , then Q ¼

¼ Twi (Fig. 4.12 ).

4.5.2 Storage Coefficient

It was once thought that aquifers transmitted water through
porous media only in the presence of a decreasing head
gradient. This may have been a result of Darcy's column

Fig. 4.11 A well installed through a confining unit into the underlying
aquifer can be used to measure the pressure of the aquifer at that point.
If the pressure is higher than land surface, a flowing well results.