Environmental Engineering Reference
In-Depth Information
12.2 Unconfined Aquifer
In contrast to a confined aquifer, an unconfined aquifer (also called phreatic aquifer)
is not limited by an impermeable layer from above. The upper boundary of an
unconfined aquifer is given by the groundwater table. Between the groundwater
table and the earth surface the unsaturated zone is located, where the pore space
within the porous material is filled with water and air. Within the aquifer it is only
water that flows in the pore space. The situation is schematically depicted in
Fig.
12.2
.
If measured in reference to the aquifer base, the variable
h
is the water saturated
thickness of the aquifer, which is the distance between the position of the ground-
water table and the base of the aquifer below. In contrast to the confined aquifer, in
the unconfined aquifer piezometric head
h
represents the position of the ground-
water table (Fig.
12.3
).
The following formula adapts the Thiem (
12.2
) to the situation of an unconfined
aquifer. It is derived in Sidebar 12.1 and delivers piezometric head
h
in the distance
r
from a well:
Q
pK
r
r
0
h
2
ðrÞ¼h
0
2
þ
log
(12.3)
with:
h
0
water level in well [m]
Q
pumping rate [m
3
/s]
K
aquifer hydraulic conductivity [m/s]
r
0
well radius [m]
Well
Earth Surface
pumping rate Q
Water Table
h
Groundwater
Flow
Aquifer
conductivity K
Aquitard
Fig. 12.3
Schematic cross-sectional view of a well pumping from an unconfined aquifer
