least a portion of the screen always contacts the water table regardless of its seasonal
fluctuations. The owner or operator should not employ well intake designs that cut across
hydraulically separated geologic units. Except in settings where DNAPLs may exist, wells
may have a bottom sump to allow sediments that enter the well to settle, preventing "silting
in" of the well. (See Section for further guidance on selecting well screen length.)
Screen Slot Size
Well screen slot size should be selected to retain from 90% to 100% of the filter pack
material (discussed below) in artificially filter packed wells, or from 50% to 100% of the
formation material in naturally packed wells, unless the owner/operator can demonstrate that
turbidity free water (<5 nephelometric turbidity units) can be obtained using a larger slot size.
Although this is a higher percentage than is usually required in a production well, the low
withdrawal rates and the infrequent use of a monitoring well necessitate the higher percentage
exclusion. EPA emphasizes that filtering a sample subsequent to its collection is not the
solution for dealing with turbidity in an improperly designed well. Furthermore, well screens
should be factory slotted. Manually slotting screens in the field should not be performed
under any circumstances.
Filter Packs/Pack Material
The annular space between the borehole wall and the screen or slotted casing should
be filled in a manner that minimizes the passage of formation materials into the well. The
driller should generally install an artificial filter pack around each well intake. As discussed
above, wells in rock often do not require screens, and thus do not require filter packs.
However, they are the exception; most wells will require filter packs and a screened length of
casing. Aller et al. (1989) provide a comprehensive discussion of the purpose and selection
of filter pack materials.
An artificial filter pack is appropriate in most geologic settings. In particular, an
artificial filter pack should be used when: 1) the natural formation is poorly sorted; 2) a long
screened interval is required and/or the intake spans highly stratified geologic materials of
widely varying grain sizes; 3) the natural formation is a uniform fine sand, silt, or clay; 4) the
natural formation is thin bedded; 5) the natural formation is poorly cemented sandstone; 6)
the natural formation is highly fractured or characterized by relatively large solution channels;
7) the natural formation is shale or coal that will act as a constant source of turbidity to
ground water samples; and 8) the diameter of the borehole is significantly greater than the
diameter of the screen (Aller et al., 1989). Using natural formation material as filter pack is
recommended only when the natural formation materials are relatively coarse grained,
permeable, and uniform in grain size (Aller et al., 1989).
Filter pack material should be chemically inert. The best filter packs are made from
industrial grade glass (quartz) sand or beads (Barcelona, 1985a). Any other type of sand
November 1992
6 40






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