to those for the plastic casing with which the screen is used.
2.3
FILTER PACK
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NOTE: The use of artificial filter-pack
construction is recommended in formations where the
screen slot opening, selected on the basis of a
naturally developed well, is smaller than 0.25 mm
(0.010 inch) (No. 10 slot). Artificial filter-pack
material should also be specified when fine uniform
material, or extensively laminated, non-homogenous
formations are encountered. Guidance for designing
and selecting the screen slot size, and filter pack
gradation may be found in EPA's "Handbook of
Suggested Practices for the Design and Installation
of Ground-Water Monitoring Wells", (EPA document
number 600/4-89/034); or "Groundwater and Wells", by
Fletcher G. Driscoll, published by Johnson Well
Screens, 1986. If the well is not a filter pack
type, this paragraph should be deleted in its
entirety, along with other references to filter pack
wells throughout the specification. Sterilizing the
filter material before being placed is not required
for extraction and injection wells installed at
hazardous, toxic, and radioactive waste (HTRW)
sites.When requiring sterilization, the strength of
the sterilizing agent should be stated, and how much
is required per cubic yard of filter pack.
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Filter pack material shall be a product of a commercial sand and gravel
supplier, shall be properly sized and graded for the surrounding soil
encountered, and shall be composed of clean, round, hard, waterworn
siliceous material, free of flat or elongated pieces, organic matter, or
other foreign matter. The filter material shall be of a size which will
allow the maximum flow of water into the well and prevent the infiltration
of sand and silt. The gradation of the filter material shall be such that
the uniformity coefficient is not more than 2.5. The filter material shall
be [thoroughly sterilized with chlorine or hypochlorite immediately before
being placed] [placed as directed].
2.4
BENTONITE SEAL
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NOTE: Before installing an annular seal, the state
regulatory agency should be consulted. The state, or
local municipality, where the well is being
installed, may have specific requirements for
sanitary, and/or wellhead protection.
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The bentonite seal, intended to keep grout from entering the filter pack,
shall consist of hydrated granular, or pelletized, sodium montmorillonite
furnished in sacks or buckets from a commercial source and shall be free of
impurities which adversely impact the water quality. If the bentonite seal
is located above any borehole fluid levels, a layer of fine sand shall be
SECTION 02521
Page 19
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