GROUNDWATER - Water Quality Engineering in Natural Systems - page 177

1986). Gravel pack gradation is designed to retain
about 70% of the aquifer matrix (recall that the
screen slots are sized to retain 90% of the gravel
pack). The annular space in the borehole above
the gravel pack is generally sealed to prevent the
movement of water down the gravel pack from the
ground surface and other parts of the aquifer.

ground surface

backfill

well casing

borehole

water table

Seal. Annular seals typically consist of bentonite
pellets or cement grout, and seals are typically
30-100 cm (1-3 ft) thick. Bentonite pellets are
poured down the well casing and expand to form
the seal; the pellets are typically available in
10-mm (0.4-in) and 13-mm (0.5 in) diameters and
swell to 10-15 times their dry size when immersed
in water. For nested wells in the same borehole,
the bentonite pellet seal is provided in the entire
length between the top of the filter pack of the
deep well and the bottom of the filter pack of the
shallow well and to a height of about 1 m (3 ft)
above the filter pack of the shallow well. The
annular space above the seal is typically backfilled
with native material extracted from the aquifer
during drilling. monitoring wells are completed at
the surface with locking caps and/or casing to
prevent tampering and a concrete surface pad to
protect the annular seal. In some cases, the annular
space from ground surface to the top of the ben-
tonite pellet seal is filled with a cement-bentonite
mix with a cement-to-bentonite ratio of 19 : 1 by
weight. This mix may change, depending on the
site conditions (Prakash, 2004).

seal

screen

gravel pack

Figure 5.16. Gravel pack.

monitoring well is to measure the potentiometric
head within the aquifer, a relatively short screen
of length 1-2 m (3-6 ft) is usually sufficient.
Gravel Pack. The annular space between the bore-
hole and well screen is typically filled with clean
sand and fine gravel, which is called the gravel
pack or filter pack . A typical gravel pack in an
unconsolidated formation is shown in Figure 5.16.
The placement of such gravel packs requires the
insertion of a temporary outer casing to keep the
borehole open while the gravel is poured down
the borehole, and then the withdrawal of the
temporary casing to complete the gravel pack
installation. Gravel packs normally extend 60-
90 cm (2-3 ft) above the top of the well screen to
allow for settlement of the gravel pack material
during well development, and the gravel pack
should be 5-8 cm (2-3 in) thick. In cases where at
least 90% of the aquifer matrix is retained by a
10-slot screen, an artificial gravel pack may not be
necessary, and a natural gravel pack will develop
when the well is pumped (Aller et al., 1989).
Gravel packs placed between the well screen and
aquifer formation are typically composed of
graded silica sand. Blasting sands and other
general-use sands should not be used as gravel
packs since they may contain materials that sorb
dissolved metals, potentially compromising the
integrity of the groundwater sample (Bedient
et al., 1999). The gravel pack material should have
an average grain size that is twice the average
grain size of the aquifer matrix and a uniformity
coefficient ( d 60 / d 10 ) between 2 and 3 (Driscoll,

Well Development, Purging, and Sampling. moni-
toring wells are typically developed using sub-
mersible pumps until the pumped water is free of
sediment. Water level, pH, specific conductivity,
and temperature are recorded during well devel-
opment. The well should be purged before each
sampling event. If the well has been installed in a
low permeability aquifer using a dry drilling
method, bailing out 3-10 casing volumes may be
sufficient to permit collection of representative
groundwater samples. If fluids have been intro-
duced during drilling, larger volumes of water
must be removed. During routine collection of
groundwater samples, removal of 3-5 casing
volumes is a generally accepted minimum purge
(Bedient et al., 1999). Alternatively, wells are
purged until measurements of water-quality
parameters, such as temperature, specific conduc-
tance, and pH have stabilized. After purging, the
water level in the well should be allowed to
recover. Water-quality samples are sometimes
collected using disposable polyethylene bailers.