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 well screen

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عدد المساهمات : 103
تاريخ التسجيل : 08/09/2010
العمر : 30

مُساهمةموضوع: well screen   20th سبتمبر 2010, 2:11 pm

Well Screen
The goal of a properly completed monitoring well is to provide low turbidity
water that is representative of ground-water quality in the vicinity of the well.
Monitoring wells completed in rock often do not require screens, though wells
completed in unconsolidated sediments do require screens.
Screen Length
The selection of screen length usually depends on the objective of the well.
Piezometers, for example, are generally completed using short screens (2
feet or less), as are wells where only a discrete flow path, such as thin gravel
interbedded with clays, is monitored. To avoid dilution, well screens should
be kept to the minimum length appropriate for intercepting a contaminant
plume, especially in a high-yielding aquifer. The screen length should
generally not exceed 10 feet. If construction of a water table well is the
objective, either for defining flow gradient or detecting the presence of floating
non-aqueous phase liquid (NAPL), then a longer screen spanning the water
table is acceptable, to account for NAPL's or seasonal water table
fluctuations. The RP should not use screen lengths that create a conduit for
contaminant transport across hydraulically separated geologic units.
Screen Slot Size
Well screen slot size should be selected to retain at least 90% of the filter pack
material (discussed below) in artificially filter packed wells, or a minimum of 50%
of the formation material in naturally packed wells, unless the RP 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. Cal 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 casing as a substitute for screens should not be accepted
under any conditions.
2.5.2 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
Monitoring Well Design and Construction
27
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; 6) the natural formation is shales or coals that will act as a constant
source of turbidity to ground-water samples; and 7) 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 well sorted and relatively
coarse-grained.
Filter pack material should be chemically inert. The best filter packs are made
from industrial grade quartz (Barcelona, 1985a). Any other type of sand
should be analyzed for cation exchange capacity and volatile organic
compounds (VOCs) to determine whether it will interact with analytes of
concern in the ground water. Commercially available pea gravel may be
acceptable for use in gravel aquifers; however, because the filter pack should
be chemically inert, the pea gravel itself should not be chemically active or
coated with a chemically active metal oxide. Filters constructed from fabric
should not be allowed, as they tend to plug and may be chemically reactive.
Aller et al. (1989) provide the following summary of methods for selecting the
size of filter pack materials:
"Although design techniques vary, all use the filter pack ratio to
establish size differential between the formation materials and filter
pack materials. Generally this ratio refers to either the average (50
percent retained) grain size of the formation material or the 70 percent
retained size of the formation material. For example, Walker (1974)
and Barcelona et al. (1985a [1985b in this document]) recommend
using a uniform filter pack grain size that is 3 to 5 times the 50
percent retained size of the formation materials. Driscoll (1986)
recommends a more conservative approach by suggesting that for
fine-grained formations, the 50 percent retained size of the finest
formation sample be multiplied by a factor of 2 to exclude the
entrance of fine silts, sands, and clays into the monitoring well. The
United States Environmental Protection Agency (1975) recommends
that filter pack grain size be selected by multiplying the 70 percent
retained grain size of the formation materials by a factor between 4
and 6. A factor of 4 is used if the formation is fine and uniform; a
factor of 6 is used if the formation is coarser and non-uniform. In
both cases, the uniformity coefficient of the filter pack materials
Monitoring Well Design and Construction
28
should not exceed 2.5 and the gradation of the filter material should
form a smooth and gradual size distribution when plotted. The actual
filter pack used should fall within the area defined by these two
curves. According to Williams (1981), in uniform formation materials,
either approach to filter pack material sizing will provide similar
results; however in coarse, poorly sorted formation materials, the
average grain size method may be misleading and should be used
with discretion."
Filter pack material should be installed in a manner that prevents bridging and
particle-size segregation. Filter pack material installed below the water table
should generally be tremied into the annular space. Allowing filter pack
material to fall by gravity (free fall) into the annular space is only appropriate
when wells are relatively shallow, when the filter pack has a uniform grain size,
and when the filter pack material can be poured continuously into the well
without stopping.
At least two inches of filter pack material should be installed between the well
screen and the borehole wall. The filter pack should extend at least two feet
above the top of the well screen. In deep wells the filter pack may not
compress when initially installed, consequently, when the annular and surface
seals are placed on the filter pack, the filter pack compresses sufficiently to
allow grout into, or very close to, the screen. Consequently, filter packs may
need to be installed as high as five feet above the screened interval in
monitoring wells that are deep (i.e., greater than 200 feet). The precise
volume of filter pack material required should be calculated and recorded
before placement, and the actual volume used should be determined and
recorded during well construction. Any discrepancy between the calculated
volume and the actual volume requires an explanation.
Prior to installing the annular seal, a one- to two-foot layer of chemically inert
fine sand may be placed over the filter pack to prevent the intrusion of annular
or surface sealants into the filter pack. The entire length of the annular space
that is filled with filter pack material or sand is effectively the monitored zone.
Therefore, if the filter pack or sand extends from the screened zone into an
overlying zone, a conduit for the possible transport of contaminants is
created between the two zones.
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well screen
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