Science of Water : Concepts and Applications

All about Water 37
DEEP WELLS
Deep wells are the usual source of groundwater for municipalities. Deep wells tap thick and extensive
aquifers that are not subject to rapid fl uctuations in water (piezometric surface—the height to which
water will rise in a tube penetrating a confi ned aquifer) level and that provide a large and uniform
yield. Deep wells typically yield water of a more constant quality than shallow wells, although the
quality is not necessarily better. Deep wells are constructed by a variety of techniques; we discuss
two of these techniques (jetting and drilling) below.
Jetted Wells
Jetted well construction commonly employs a jetting pipe with a cutting tool. This type of well cannot
be constructed in clay, hardpan, or where boulders are present. Jetted wells are not acceptable
as a public water supply.
Drilled Wells
Drilled wells are usually the only type of well allowed for use in most public water supply systems.
Several different methods of drilling are available; all of which are capable of drilling wells of
extreme depth and diameter. Drilled wells are constructed using a drilling rig that creates a hole
into which the casing is placed. Screens are installed at one or more levels when water-bearing
formations are encountered.
COMPONENTS OF A WELL
The components that make up a well system include the well itself, the building and the pump, and
the related piping system. In this section, we focus on the components that make up the well itself.
Many of these components are shown in Figure 2.6.
WELL CASING
A well is a hole in the ground called the borehole. The hole is protected from collapse by placing a
casing inside it. The well casing prevents the walls of the hole from collapsing and prevents contaminants
(either surface or subsurface) from entering the water source. The casing also provides a column
of stored water and housing for the pump mechanisms and pipes. Well casings constructed of steel or
plastic material are acceptable. The well casing must extend a minimum of 12 in. above grade.
GROUT
To protect the aquifer from contamination, the casing is sealed to the borehole near the surface
and near the bottom where it passes into the impermeable layer with grout. This sealing process
keeps the well from being polluted by surface water and seals out water from water-bearing strata
that have undesirable water quality. Sealing also protects the casing from external corrosion and
restrains unstable soil and rock formations.
Grout consists of near cement that is pumped into the annular space (it is completed within 48 h
of well construction). It is pumped under continuous pressure starting at the bottom and progressing
upward in one continuous operation.
WELL PAD
The well pad provides a ground seal around the casing. The pad is constructed of reinforced concrete
6 ft × 6 ft (6 in. thick) with the wellhead located in the middle. The well pad prevents contaminants
from collecting around the well and seeping down into the ground along the casing.