Environmental Engineering Reference
In-Depth Information
(a)
(b)
Figure 5.14.
Installation of a well casing. (a) Casing being lowered into bore hole; (b) casing segments being joined.
Source
: Browns
Drilling (2005).
screens constructed of PVC, threaded joints are
generally specified since the use of glues that
contain organic solvents is discouraged. Informa-
tion on the compatibility of various screen and
casing materials with common contaminants have
been reported by Driscoll (1986), Parker et al.
(1990), and Ranney and Parker (1997). These data
indicate that for monitoring organics, stainless
steel is the material of choice and PTFE should be
avoided. For monitoring inorganics, PVC is the
preferred material, and as a compromise material
for monitoring both organics and inorganics, PVC
appears to be the best and also is the lowest cost.
In cases where PVC is the material of choice, PVC
manufactured specifically for well casing should
be used, and it should carry the designation
NSF
wc
, which indicates that the casing conforms to
national Sanitation Foundation Standard 14 for
potable water supply (national Sanitation Foun-
dation, 1988). PVC should not be used if organic
compounds are present as nAPLs.
Screen.
The openings in well screens are generally in
the form of rectangular openings called slots, and
a typical PVC well screen is shown in Figure 5.15.
Well screens are available in a variety of opening
sizes, generally in the range of 0.2-6.4 mm (0.008-
0.25 in). A screen with an opening of 0.25 mm
(0.010 in) is referred to as a “10-slot” screen, and
screens are commonly specified by their slot size;
for example, 20-slot screen has an opening size of
0.50 mm (0.020 in). The most commonly used
screen sizes are 10- and 20-slot screens. The screen
opening should be selected to retain 90% of the
gravel pack.
To detect the presence of nonaqueous phase con-
taminants in the saturated zone, well screens
Figure 5.15.
Well screen.
Source
: Browns Drilling (2005).
should be positioned to intersect either the top
(for LnAPLs) or bottom (for DnAPLs) of the
water-bearing stratum. In monitoring the water
table for the presence of LnAPLs, the screen must
be long enough to intersect the water table over
the range of annual fluctuations. In most applica-
tions, the minimum length of screen for a water
table monitoring well is 3 m (10 ft), with 1.5 m
(5 ft) above and 1.5 m (5 ft) below the average
water table elevation. If the water table has more
than a 1.5-m (5-ft) fluctuation, a longer screen is
needed. To accurately measure the concentration
of dissolved contaminants in the groundwater,
shorter screens with lengths less than 3 m (10 ft)
are generally preferred, since contaminants pre-
sent over a limited depth interval may be overdi-
luted by longer screens. If the purpose of a
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