Environmental Engineering Reference
In-Depth Information
Chapter 12
Groundwater Drawdown by Pumping
Aquifers are a valuable source of water. Groundwater is available and used in many
parts of the world for industrial and municipal purposes and for public water supply.
In several regions and urban centers the percentage of groundwater on public water
supply reaches 100%. Although the chemistry of subsurface water may be very
different, groundwater quality can fulfil highest standards nevertheless.
Groundwater is pumped from single wells or galleries of several wells. In the
vicinity of the wells the water table may decrease, depending on the type of the aquifer.
In all cases the piezometric head decreases, which is explained in more details below.
Environmental studies in connection with groundwater withdrawal are necessary
for several reasons. The maximum yield, which can be extracted on a sustainable
basis, is of high concern for the well operating agency. The drawdown of the water
table itself may also be of ecological importance, as eco-systems in the catchment
of the well can be affected. Wetlands for example are vulnerable systems, which
react quite sensitive to changes of the sub-surface or surface water table.
Water quality is another important topic for water withdrawal systems. If the
quality of pumped water is not sufficient, knowledge about the well catchment and
the flowpaths may enable counter-measures in order to avoid or reduce the migration
of polluted water towards the pumping facilities. Recharge wells may prevent such
migration if operated at an appropriate location and an appropriate recharge rate.
In this chapter we examine the change of piezometric head
h
in the vicinity of
a single pumping well. There are analytical solutions for
h
as function of distance
from the well centre
r
, which can be computed easily using MATLAB
. In all
cases other causes for groundwater flow (for example base flow) are neglected.
More complex situations are treated in the following two chapters.
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12.1 Confined Aquifer
A confined aquifer is a permeable groundwater layer between two impermeable
layers (aquitards), as shown in Fig.
12.1
. In idealized situations, which are treated in
this chapter, groundwater flows in a permeable layer, the aquifer, from all sides
