Water and the Hydrogeology of Watersheds - Urban Watersheds: Geology, Contamination, and Sustainable Development - page 95

Water from the aquifer converges on the well from all directions and the hydraulic gra-
dient gets steeper near the well. For these reasons, the three-dimensional shape of water
withdrawal from a well is termed a cone of depression . The cones of depression shown
in Figure 3.34 result from a pumping well withdrawing water from unconfined (a) and
confined aquifers (b).
Transmissivity (T) is a measure of the amount of water that can be transmitted horizon-
tally through a unit width by the full saturated thickness of the aquifer under a hydraulic
gradient of 1. Taking into account the definition of hydraulic conductivity, transmissivity
(T) is actually equal to the hydraulic conductivity (K) multiplied by the thickness of the
aquifer (b) (Equation 3.7).

Lateral radius
of influence
Lateral area
of influence

Unsaturated zone

Unconfined
sand and
gravel
aquifer

Water table

Saturated zone

Water surface
while pumping

Cone of depression
Pumping water level

Confining
layer
bedrock

(a)

Land surface

Pumping well

Lateral extent of cone

Potentiometric surface

Drawdown

Cone of depression Unsaturated zone

Confining unit

Confined aquifer

Confining unit

(b)

FIGURE 3.34
(a) Cone of depression from a pumping well in an unconfined aquifer (IEPA 2010). (b) Cone of depression from
a pumping well in a confined aquifer. (Modified from Heath, R.C., Basic Ground-Water Hydrology , United States
Geological Survey, Water Supply Paper 2220, United States Government Printing Office, Alexandria, VA, 1983.)