Fundamentals of Groundwater Hydrogeology - Introduction to Phytoremediation of Contaminated Groundwater - page 112

the well in response to the induced hydraulic gradient toward
the well (Fig. 4.17 ). If the pumping rate, Q , in the well is
greater than the rate of groundwater flow to the well, the
groundwater level will continue to decline until the rate of
groundwater flow equals the rate of pumpage, the ground-
water level goes below the pump intake, the well goes dry, or
groundwater is released from storage. In plan view, ground-
water can converge to the well from all directions. From a
flow net analysis, this convergence and head decline at the
location of a pumped well is called the cone of depression
(Fig. 4.17 ).
The groundwater resource has been accessed by wells by
many civilizations throughout history in many areas of the
world (Bennison and Bollenbach 1947). Perhaps the earliest
wells were constructed in Israel during the Pre-Pottery Neo-
lithic period around 6,000 year BC (Galili and Nir 1993).
Some of the earliest attempts to drill deeply to access

groundwater were made by the Chinese and Egyptians
nearly 5,000 years ago (De Weist 1965). The Chinese used
a churn drill made of wood to go through solid rock; deep-
ening these wells took generations of laborers. Joseph's well
in Cairo is almost 300 ft deep, through solid rock, and
supplied groundwater to many civilizations including the
Greeks and Romans until about 300 BC . A well was used
by Eratosthenes (276-195 BC ) to provide the earliest esti-
mate of the circumference of the earth. In the United States,
one of the oldest drilled wells is located in St. Augustine,
Florida, drilled in the seventeenth century.
Unconfined and confined aquifers characterized by
differences in groundwater yields also have differences in
the cone of depression. In unconfined aquifers, pumped
groundwater is derived from the specific yield of the aquifer
sediments by gravity flow. As the groundwater level in the
aquifer decreases the transmissivity, T , decreases, because
T Kb . Moreover, the rate of the development of the cone

of depression is slow because drainage is by gravity only. In
confined aquifers, pumpage reduces the pressure surface
rather than dewatering the aquifer sediments. Pumping
causes a decrease in groundwater pressure and can produce
expansion of the remaining groundwater and compression of
the aquifer sediments. As such, the cone of depression for
confined aquifers expands more rapidly than for unconfined
conditions.

¼

4.10 Groundwater Fluctuations

Measurements of the groundwater level at a particular well
over time reflects the basic water budget of Eq. 2.2;
water inflow -water outflow ¼ D S . The groundwater-level surface

in an unconfined aquifer is not fixed in space or time; it is
exposed to atmospheric conditions. If water inflow exceeds
water outflow, such as when it rains and recharge occurs, the
groundwater level increases. If water outflow exceeds water
inflow, the groundwater level declines. If water inflow
equals water outflow, the groundwater level does not
change; however, groundwater may be removed or added
to the system but will not be measurable because the flows
occur at equal rates. On the other hand, it is possible for the
groundwater level to increase not because of an increase in
inflow but because an outflow has stopped.
For unconfined water-table conditions, the relation
between the amount of precipitation and the potential for
the groundwater level to increase by recharge can be deter-
mined by using aquifer porosity. For example, if 1 in.
(2.5 cm) of precipitation is added to an aquifer that has
50% total porosity, the groundwater level can rise 2 in.
(5.1 cm). The rate of rise will be faster if the soil is already
wet, and slower if it is dry, due to that particular soil's
hysteresis curve. One inch of precipitation across 1 acre of

Fig. 4.17 A cone of depression ( dashed line ) for a pumped well in a
water-table aquifer and confined aquifer (Modified from Heath 1983).