Fundamentals of Groundwater Hydrogeology - Introduction to Phytoremediation of Contaminated Groundwater - page 107

Fig. 4.13 The difference in how groundwater is stored in a water-table
aquifer and in a confined aquifer under conditions of a unit decrease in
head and the relative difference in the volume of groundwater released
under this unit decrease in head is indicated by the black shaded area in
the containers between the two wells.

Fig. 4.12 Transmissivity, T of a generalized water-table aquifer is
simply the hydraulic conductivity, K , times the thickness, b , thickness,
which can vary over time. The transmissivity of a confined aquifer is
the constant thickness, b , times the hydraulic conductivity, K , and,
therefore, is unchanging.

faucet is opened, water flows, and when the faucet is closed,
flow stops but water remains stored in the pipes. The amount
of water stored in these pipes is dependent on the diameter of
the pipes and the total length of all of the pipes. An analo-
gous scenario occurs in confined aquifers. Groundwater can
flow in response to a head gradient just as for a water-table
aquifer, but groundwater also can be stored. Groundwater is
released from storage during a release of pressure which
causes the stored water to expand and the aquifer material
to compress under the increased pressure from overburden
following the removal of groundwater (Fig. 4.13 ). This is
similar to the way dissolved gases are released from
pressurized, carbonated beverages; the can is opened, caus-
ing the pressure to decrease and the gas to expand and be
released from solution.

experiment, in which steady-state conditions were used to
induce flow. However, groundwater also can be yielded
from aquifer sediments to wells by other processes. When
a 1-ft groundwater-level decrease occurs in a water-table
aquifer, the groundwater yielded from sediment pore spaces
is by gravity, and the water removed from the sediments in
the 1-ft zone is replaced by air. In a confined aquifer, how-
ever, groundwater is under pressure greater than the atmo-
sphere. A similar 1-ft decrease in the potentiometric surface
can be measured when groundwater is removed, but the
sediments do not become dewatered. This is because con-
fined aquifers not only transmit water but also store ground-
water over time. Much like the relation between water flow
and water storage in the water pipes in a house, when a