i>
*"" £!^
^ -0
-------
grouting is done by filling the annulus from the bottom upward
and 2) that as the grout cures, it gains strength and provides
support to the casing.
Several methods can be used to minimize the heat of
hydration. Adding setting-time retardants. such as bentoniteor
diatomaceous earth, to the grout mix tends to reduce peak
temperatures. Other approaches include: adding inert materials
such as silica sand to the groui; circulating cool water inside the
casing during grout curing; and increasing the water-cement
ratio of the grout mix (Kurt, 1983). However, increasing the
water-cement ratio of the grout mix results in increased shrink-
age and decreased strength upon setting and more potential to
move beyond where expected or intended before setting.
Neat cement annular seals are subject lo channeling be-
tween the casing and the seal because of temperature changes
during the curing process; swelling and shrinkage of the grout
while the mixture cures; and poor bonding between the grout
and the casing surface (Kurt and Johnson, 1982). One method
of ensuring a low-permeability grout seal in a monitoring well
is to minimize the shrinkage of the grout as it cures. Minimizing
shrinkage, lowering permeability and increasing the strength of
cured grout can be accomplished by minimizing water/cement
ratios {Kurt and Johnson, 1982). Typical vertical permeabilities
for casing/grout systems were found by Kurt and Johnson
(1982) to range from 20 to 100 x lCrs centimeters per second.
These permeabi 1 ities are higher than those determined for neat
cement grout only. This implies that the presence of casing is a
factor that increases the permeability of the system.
Methods for Evaluating Annular Seal Integrity
There are presently no fooproof field tests that can be
performed to determine if a proper annular seal has been
achieved. Of the most commonly used field tests for checking
seals in production wells, only one appears to provide basic
information on the integrity of an annular seal in a monitoring
well—geophysical logging. The accuracy of geophysical log-
ging techniques is often questioned because they are indirect
sensing techniques. The log most commonly used to cheek a
seal composed of neat cement grout is the cement bond (acous-
tic, sonic) log. A cement bond log generally indicates bonded
and non-cemented zones but cannot detect the presence of
vertical channels within the cement nor small voids in the
contact area with the casing. Cement bond logs are available for
wells with inside diameters of 2 inches or larger.
Where thermoplastic or fluorocarbon casing is installed,
there is no sound or sonic wave return recorded along the casing
as is the case with metallic pipe. As a consequence, the
information derived is even more difficult to interpret. Further,
there are no good methods available to evaluate the effective-
ness of bentonite seals. This is an area in need of further
research.
Surface Completion and Protective Measures
Two types of surface completions are common for pound-
water monitoring wells: 1) above-ground completion and 2)
flush-to-pound surfacecompletion. An above-ground comple-
tion is preferred whenever practical, but a flush-to-ground
surface may be required at some sites. The primary purposes of
either type of completion are to prevent surface runoff from
entering and infiltrating down the annulus of the well and to
protect the well from accidental damage or vandalism.
Surface Seals
Whichever type of completion is selected for a well, there
should always be a surface seal of neat cement or concrete
surrounding the well casing and filling the annular space
between the casing and the borehole at the surface. The surface
seal may bean extension of the annular seal installed above the
filter pack or it may be a separate seal emplaced on top of the
annular seal. Because the annular space near the land surface is
large and the surface material adjacent to the borehole is
disturbed by drilling activity, the surface seal will generally
extend to at least 3 feet away from the well casing at the surface;
the seal will usually taper down to the size of the borehole within
a few feet of the surface. In climates with alternating freezing
and thawing conditions, the cement surface must extend below
the frost depth to prevent potential well damage caused by frost
heaving. A suggested design for dealing with heaving condi-
tions is shown in Figure 21, If cement is mounded around the
well to help prevent surface runoff from pending and entering
around the casing, the mound should be limited in size and slope
so that access to the well is not: impaird and to avoid frost
heave damage. In some states, well installation regulations
were initially developed for water supply wells. These stan-
dards are sometimes now applied to monitoring wells, and these
may require that the cement surface seal extend to depths of 10
feet or greater to ensure sanitary protection of the well.
Above-Ground Completions
In an above-ground completion, a protective casing is
generally installed around the well casing by placing the protec-
tive casing into the cement surface seal while it is still wet and
uncured. The protective casing discourages unauthorized entry
into the well, prevents damage by contact with vehicles and
protects PVC casing from degradation caused by direct expo-
sure to sunlight. This protective casing should be cleaned
thoroughly prior to installation to ensure that it is free of any
chemicals or coatings. The protective casing should have a
large enough inside diameter to allow easy access to the well
casing and to allow easy removal of the casing cap. The
protective casing should be fitted with a locking cap and
installed so that there is at least 1 to 2 inches clearance between
the top of the in-place inner well casing cap and the bottom of
the protective casing locking cap when in the locked position.
The protective casing should be positioned and maintained in a
plumb position. The protective casing should be anchored
below frost depth into the cement surface seal and extend at
least 18 inches above the surface of the ground.
Like the inner well casing, the outer protective casing
should be vented near the top to prevent the accumulation and
entrapment of potentially explosive gases and to allow water
levels in the well to respond naturally to barometric pressure
changes. Additiomlly, the outer protective casing should have
a drain hole installed just above the top of the cement level in
the space between the protective casing and the well casing
(Figure 21). This drain allows trapped water to drain away from
the casing. This drain is particularly critical in freezing climates
where freezing of water trapped between the inner well casing
and the outer protective casing can cause the inner casing to
buckle or fail.
101
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A case-hardened steel lock is generally installed on the
locking casing cap to provide well security. However, corrosion
and jamming of the locking mechanism frequently occurs as the
lock is exposed to the elements. Lubricating the locks or the
corroded locking mechanisms is not recommended because
lubricants such as graphite, petroleum-based sprays, silicone
and others may provide the potential for sample chemical
alteration. Rather, the use of some type of protective measure to
shield the lock from the elements such as a plastic covering may
prove a better alternative.
In high-traffic areas such as parking lots, or in areas where
heavy equipment maybe working, additional protection such as
the installation of three or more "bumperguards" are suggested.
Bumperguards are brightly-painted posts of wood, steel or
some other durable material set in cement and located within 3
or 4 feet from the well.
Flush-to-Ground Surface Completions
In a flush-to-ground surface completion, a protectivestruc-
ture such as a utility vault or meter box is installed around well
casing that has been cut off below grade. The protective
structure is typically set into the cement surface seal before it
has cured. This type of completion is generally used in high-
traffic areas such as streets, parking lots and service stations
where an above-ground completion would severely disrupt
traffic patterns or in areas where it is required by municipal
easements or similar restraints. Because of the potential for
surface runoff to enter the below-grade protective structure and/
or well, this type of completion must be carefully designed and
installed. For example, the bond between the cement surface
seal and the protective structure as well as the seal between the
protective structure and removable cover must be watertight.
Use of art expanding cement that bonds tightly to the protective
structure is suggested. Installation of a flexible o-ring or gasket
at the point where the cover fits over the protective structure
usually suffices to seal the protective structure. In areas where
significant amounts of runoff occur, additional safeguards to
manage drainage may be necessary to discourage entry of
surface runoff.
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Barcelona, M.J., O.K. George and M.R. Schock, 1988.
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I985b. Sampling tubing effects on ground-water samples;
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Campbell, M.D. and J.R Lehr, 1973. Water Well Technology;
McGraw-Hill Book Company, New York, New York, 681
PP-
Campbell, M.D. and J.H. Lehr, 1975. Well cementing; Water
Well Journal, vol. 29, no. 7, pp. 39-42.
Curran, Carol M. and Mason B. Tomson, 1983. Leaching of
trace organics into water from five common plastics;
Ground Water Monitoring Review, vol. 3, no. 3,pp.68-71.
Dablow, John S. Ill, Grayson Walker and Daniel Persico,
1988. Design considerations and installation techniques
for monitoring wells cased with Teflon ® PTFE; Ground-
Water Contamination Field Methods, Collins and Johnson
editors, ASTM Publication Code Number 04-963000-38,
Philadelphia, Pennsylvania, pp. 199-205.
Driseoll, Fletcher G., 1986. Ground Water and Wells; Johnson
Division, St Paul, Minnesota, 1089 pp.
Dunbar, D., H. Tuchfeld, R. Siegel and R. Sterbentz, 1985.
Ground-water quality anomalies encountered during well
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Gross, S., 1970. Modem plastics encyclopedia; McGraw-Hill
Book Company, New York, New York, vol. 46, 1050 pp.
Hamilton, Hugh, 1985. Selection of materials in testing and
purifying water; Ultra Pure Water, January/February 1985,
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Helweg, Otto J., Verne H. Scott and Joseph C. Scalmanini,
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Marsh, J.M. and J.W. Lloyd, 1990, Details of hydrochemical
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245.
Moehrl, Kenneth E., 1964. Well grouting and well protection;
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Molz, F.J. and C.E. Kurt, 1979. Grout-induced temperature rise
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103
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Section 6
Completion of Monitoring Wells
Introduction
Once a borehole has been completed to the desired moni-
toring depth, the monitoring well must be properly installed.
Although monitoring wells can be completed in a variety of
configurations, successful completion of any monitoring well
must incorporate the following objectives:
1) the well completion must permit specific
stratigraphic zones to be sampled with complete
confidence that the sample obtained is
representative of the in-situ water quality;
2) the well completion must permit contaminants
with differing physical properties to be sampled.
For example, if the contaminant is denser or
lighter than water and therefore sinks or floats
accordingly, the well completion must allow
collection of a representative ground-water
sample;
3) the well must be constructed to prevent cross
contamination between different zones. Cross
contamination can occur if a) the intake and/or
filter pack spans more than one hydraulic unit,
b) hydraulic communication between zones occurs
along the borehole/grout interface, the casing/
grout interface, or through voids in the seal, c)
fractures intersect the wellbore, or d) if loosely
compacted soils are adjacent to the borehole;
4) the well completion should minimize any
disturbance created during the drilling process.
For example, if the well was drilled by hollow-
stem augers, the completion techniques should
eliminate the void space created by the withdrawal
of the augers; and
5) the well completion method should be cost
effective; sample integrity, of course, is of critical
importance.
To achieve these objectives, the well intake, filter pack,
and annular seal must be installed using appropriate techniques.
The following discussion addresses these techniques.
Well Completion Techniques
Well Intake Installation
In cohesive unconsolidated material or consolidated for-
mations, well intakes are installed as an integral part of the
casing sting by lowering the entire unit into the open borehole
and placing the well intake opposite the interval to be moni-
tored. Centralizing devices are typically used to center the
casing and intake in the borehole to allow uniform installation
of the filter pack material around the well intake. I f the borehole
has been drilled by a technique that creates borehole damage, it
is necessary to develop the borehole wall. When the formation
is sufficiently stable, this development should be undertaken
prior to setting the well intake. After the filter pack has been
installed, it is very difficult to clean fractures or to remove
mudcake deposits that have been formed on the borehole wall.
If the borehole was drilled with the mud rotary technique, the
borehole should be conditioned and the wallcake removed from
the borehole wall with clean water prior to the installation of the
well intake, if possible. An additional discussion on well
development is found in Section 7, entitled "Monitoring Well
Development."
In non-cohesive, unconsolidated materials when the bore-
hole is drilled by a drill-through casing advancement method,
such as a casing hammer or a cable tool technique, the well
intake should be centered inside the casing at the end of the riser
pipe and held firmly in place as the casing is pulled back. When
the well intake is being completed as a natural pack, the outside
diameter of the well intake should be between 1 and 2 inches
smaller than the outside diameter of the casing that is being
retracted. If an artificial filter pack is installed, the outside
diameter of the well intake should be at least 3 to 5 inches
smaller than the outside diameter of the casing that is being
retracted. During artificial filter pack installation, the filter
pack material must be maintained above the lower-most level
of the casing as the casing is removed. This means that the filter
pack is being emplaced continually during the time that the
casing is being pulled back and the well intake is being exposed.
This procedure minimizes the development of excessive void
space adjacent to the well intake as the casing is pulled back.
When the casing is installed through the hollow stem of a
hollow-stem auger, an artificial filter pack generally should be
emplaced because of the disparity between the outside diameter
of the auger flights and the usual 2-inch or 4-inch outside
diameter of the casing and well intake that are being installed
within the auger flights. If the augers are withdrawn and the
formation allowed to collapse around the well intake without
installing an artificial filter pack to stabilize the borehole wall,
the materials that are adjacent to the well intake maybe loose
and poorly compacted. Excessive void space adjacent to the
well intake can provide an avenue for cross contamination or
migration of contaminants. This void or loosely-compacted
zone may also interfere with the placement of proper seals.
Loosely-compacted material is difficult to adequately de-
velop from within a small diameter borehole. The surging
methods that are available generally cannot recompact the
materials adjacent to the well intake to prevent bentonite or
cement grout from migrating downward into the screened zone.
105
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Additionally, where collapse is permitted, the collapsed zone
around the well intake is highly disturbed and is no longer
stratified similar to the stratification of the natural formation.
As a consequence, there will be mixing of horizontal zones, and
the possibility exists that chemical changes can be induced by
the changes in the physical environment.
Where wells are installed in unconsolidated material by the
dual-wall reverse-circulation method, the well casing and well
intake are installed through the bit. The only option for comple-
tion with this construction method is to allow the materials to
collapse around the screen. In this instance, a greater sustained
effort is suggested in well-development procedures than is
normally required.
Filter Pack Installation
Several methods of emplacing artificial filter packs in the
annular space of a monitoring well are available, including:
1) gravity (free fall), 2) tremie pipe, 3) reverse circulation, and
4) backwashing. The last two methods involve the addition of
clean water to the filter pack material during emplacement. This
addition of fluid can cause chemical alteration of the environ-
ment adjacent to the well and pose long-term questions about
the representativeness of water samples collected from the well.
As with other phases of monitoring well construction, fluids
(clean) should only be added when no other practicable method
exists for proper filter pack emplacement. An additional discus-
sion on choosing filter pack material size can be found in the
section entitled "Artificially Filter-Packed Wells."
Placement of filter packs by gravity or free fall can be
successfully accomplished only in relatively shallow wells
where the probability of bridging or segregation of the filter
pack material is minimized. Bridging causes unfilled voids in
the filter pack and may prevent the filter pack material from
reaching the intended depth. Segregation of filter pack material
can result in a well that consistently produces sediment-laden
water samples. Segregation is a problem particularly in wells
with a shallow static water level. In this situation, the filter pack
material falls through the column of water at different rates. The
greater drag exerted on smaller particles due to their greater
surface area-to-weight ratio causes finer grains to fall at a
slower rate than coarser grains. Thus, coarser materials will
comprise the lower portion of the filter pack and finer materials
will constitute the upper part (figure 64). Segregation may not
be a problem when emplacing truly uniform filter packs where
the uniformity coefficient is less than 2.5, but placement by free
fall is not recommended in any other situation (Driscoll, 1986).
With the tremie pipe emplacement method, the filter pack
material is introduced through a rigid tube or pipe via gravity
directly into the interval adjacent to the well intake (Figure 65).
Initially, the end of the pipe is positioned at the bottom of the
well intake/borehole annulus. The filter pack material is then
poured down the tremie pipe and the tremie is raised periodi-
cally to allow the filter pack material to fill the annular space
around the well intake. The minimum diameter of a tube used
for a tremie pipe is generally 1 1/2 inches; larger-diameter pipes
are advisable for filter pack materials that are coarse-grained or
characterized by uniform it y coefficients that exceed 2.5 (Cali-
fornia Department of Health Services, 1986). When installing
a filter pack with a uniformity coefficient greater than 2.5 in
wells deeper than 250 feet, a variation of the standard tremie
Fine portion
of filter pack
Coarse portion
of filter pack
Well intake
Figure 64. Segregation of artificial filter pack materials caused
by gravity emplacement.
Sand
Casing
Well intake
«\— Tremie pipe
- Borehole wall
• Filter pack material
Figure 65. Tremie-pipe emplacement of artificial filter pack
materials.
106
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method that employs a pump to pressure feed the materials into
the annulus is suggested by the California Department of Health
Services (1986).
In the reverse circulation method, a filter pack material and
water mixture is fed into the annulus around the well intake.
Return flow of water passes into the well intake and is then
pumped to the surface through the riser pipe/casing (Figure 66).
The filter pack material should be introduced into the annulus
at moderate rate to allow for an even distribution of material
around the well intake. Care must be exercised when pulling the
outer casing so that the riser pipe is not also pulled.
Backwashing filter pack material into place is accom-
plished by allowing filter pack material with a uniformity
coefficient of 2.5 or less to fall freely through the annulus while
concurrently pumping clean fresh water down the casing,
through the well intake and back up the annulus (Figure 67).
Backwashing is a particularly effective method of filter-pack
emplacement in cohesive, non-caving geologic materials. This
method also minimizes the formation of voids that tend to occur
in tremie pipe emplacement of the filter pack.
Annular Seal Installation
The two principal materials used for annular seals are
bentonite and neat cement. Often a combination of the two
materials is used. Because the integrity of ground-water samples
depends on good seals, the proper emplacement of these seals
Funner
Filter pack
material |£
and water T~
Pump
6" Casing
(Casing pulled back during"
filter pack installation)
Riser pipe
Centralizsr —
Filter pack
Well intake
Water
Fine-grained
materials and
water
Filter pack
material
Fine-grained
materials and
water
Well intake
Figure 66. Reverse-circulation emplacement of artificial filter
pack materials.
Figure 67. Emplacement of artificial filter pack material by
backwashing.
is paramount. An additional discussion on annular seals can be
found in the section entitled "Annular Seals. "
Bentonite —
Bentonite may be emplaced as an annular seal in either of
two different forms 1) as a dry solid or 2) as a slurry. Typically
only pelletized or granular bentonite is emplaced dry; powdered
bentonite is usually mixed with water at the surface to form a
slurry and then is added to the casing/borehole annulus. Addi-
tional discussion on properties of bentonite can be found in
Chapter 5 in the section entitled "Materials Used For Annular
Seals."
Dry granular bentonite or bentonite pelletsmay be emplaced
by the gravity (free fall) method by pouring from the ground
surface. This procedure should only be used in relatively
shallow monitoring wells that are less than 30 feet deep with an
annular space of 3 inches or greater. When the gravity method
is used, the bentonite should be tamped with a tamping rod after
it has been emplaced to ensure that no bridging of the pellets or
granules has occurred. Where significant thicknesses of bento-
nite are added, tamping should be done at selected intervals
during the emplacement process. In deeper wells, particularly
where static water levels are shallow, emplacing dry bentonite
107
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via the gravity method introduces both a very high potential for
bridging and the likelihood that sloughing material from the
borehole wall will be included in the seal. If bridging occurs, the
bentonite may never reach the desired depth in the well; if
sloughing occurs, "windows" of high permeability may de-
velop as the sloughed material is incorporated into the seal.
Either situation results in an ineffective annular seal that may
allow subsequent contamination of the well.
In wells deeper than 30 feet, granular or pelletized bento-
nite can be conveyed from the surface directly to the intended
depth in the annulus by a tremie pipe. Pelletized bentonite is
sometimes difficult to work with in small-diameter tremie
pipes; a minimum of 1 1/2-inch inside diameter pipe should be
used with 1/4-inch diameter pellets to minimize bridging and
subsequent clogging of the bentonite inside the tremie pipe.
Larger-diameter tremie pipes should be used with larger-diam-
eter pellets. Where a seal of either pelletized or granular
bentonite must be placed at considerable depth beneath the
water surface, the tremie pipe can be kept dry on the inside by
keeping it under gas pressure (Riggs and Hatheway, 1986). A
dry tremie pipe has a much lower potential for bridging in the
tremie because the material does not have to fall through a
partially water-filled pipe to reach the desired depth.
Bentonite slurry can bean effective well seal only if proper
mixing, pumping, and emplacement methods are used. Bento-
nite powder is generally mixed with water in a batch mixer and
the slurry is pumped under positive pressure through a tremie
pipe down the annular space using some variety of positive
displacement pump (i.e., centrifugal, piston, diaphragm, or
moyno-type pump). All hoses, tubes, pipes, water swivels, and
other passageways through which the slurry must pass should
have a minimum inside diameter of 1/2 inch. A larger diameter
(e.g., 1-inch) tremie pipe is preferred. The tremie pipe should be
placed just above the falter pack or at the level where non-
cohesive material has collapsed into the borehole (Figure 68).
The tremie pipe should be left at this position during the
emplacement procedure so that the slurry fills the annulus
Slurry
Annular seal material
Fitterpack
Figure 68. Tremie-pipe emplacement of annular seal material
(either bentonite or neat cement slurry).
upward from the bottom. This allows the slurry to displace
ground water and any loose-formation materials in the annular
space. The tremie pipe can be raised as the slurry level rises as
long as the discharge of the pipe remains submerged at least a
foot beneath the top of the slurry. The tremie pipe can be
removed after the slurry has been emplaced to the intended level
in the annulus. The slurry should never be emplaced by free fall
down the annulus. Free fall permits the slurry to segregate thus
preventing the formation of an effective annular seal.
Bentonite emplaced as a slurry will already have been
hydrated to some degree prior to emplacement, but the ability
to form a tight seal depends on additional hydration and
saturation after emplacement. Unless the slurry is placed adja-
cent to saturated geologic materials, sufficient moisture may
not be available to maintain the hydrated state of the bentonite.
If the slurry begins to dry out, the seal may dessicate, crack, and
destroy the integrity of the seal. Therefore, bentonite seals are
not recommended in the vadose zone.
Curing or hydration of the bentonite seal material occurs
for 24 to 72 hours after emplacement. During this time, the
slurry becomes more rigid and eventually develops strength.
Well development should not be attempted until the bentonite
has completely hydrated. Because of the potential for sample
chemical alteration posed by the moderately high pH and high
cation exchange capacity of bentonite, a bentonite seal should
be placed approximately 2 to 5 feet above the top of the well
intake and separated from the filter pack by a 1-foot thick layer
of fine silica sand.
Neat Cement —
As with a bentonite slurry, a neat cement grout must be
properly mixed, pumped, and emplaced to ensure that the
annular seal will be effective. According to the United States
Environmental Protection Agency (1975), neat cement should
only be emplaced in the annulus by free fall when 1) there is
adequate clearance (i.e., at least 3 inches) between the casing
and the borehole, 2) the annulus is dry, and 3) the bottom of the
annular space to be filled is clearly visible from the surface and
not more than 30 feet deep. However, to minimize segregation
of cement even in unsaturated annular spaces, free fall of more
than 15 feet should not be attempted in monitoring wells. If a
neat cement slurry is allowed to free fall through standing water
in the annulus, the mixture tends to be diluted or bridge after it
reaches the level of standing water and before it reaches the
intended depth of emplacement. The slurry also may incorpo-
rate material that is sloughed from the borehole wall into the
seal. If the sloughed material has a high permeability y, the
resultant seal can be breached through the inclusion of the
sloughed material.
In most situations, neat cement grout should be emplaced
by a tremie pipe. The annular space must be large enough that
a tremie pipe with a minimum inside diameter of 1 1/2 inches
can be inserted into the annulus to within a few inches of the
bottom of the space to be sealed. Grout may then either be
pumped through the tremie pipe or emplaced by gravity flow
through the tremie pipe into the annular space. The use of a
tremie pipe permits the grout to displace ground water and force
loose formation materials ahead of the grout. This positive
displacement minimizes the potential for contamination and/or
108
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dilution of the slurry and the bridging of the mixture with upper
formation material.
In pressure grouting, the cement discharges at the bottom
of the annular space and flows upward around the inner casing
until the annular space is completely filled. A side discharge
tremie may be used to lessen the possibility that grout might be
forced into the filterpack. Depending on pressure requirements,
the tremie pipe may be moved upward as the slurry is emplaced
or it may be left at the bottom of the annulus until the grouting
is completed. If the tremie pipe is not retracted while grouting,
the tremie pipe should be removed immediately afterward to
avoid the possibility y of the grout setting around the pipe. If this
occurs, the pipe may be difficult to remove and/or a channel
may develop in the grout as the pipe is removed.
In gravity emplacement, the tremie is lowered to the
bottom of the annular space and filled with cement. The tremie
pipe is slowly retracted, and the weight of the column forces the
cement into the annular space. In both gravity emplacement and
pressure grouting, the discharge end of the tremie pipe should
remain submerged at least one foot below the surface of the
grout at all times during emplacement, and the pipe should be
kept full of grout without air space. To avoid the formation of
cold joints, the grout should be emplaced in one continuous
pour before initial setting of the cement or before the mixture
loses fluidity. Curing time required for a typical Type I Portland
cement to reach maximum strength is a minimum of 40 hours.
Moehrl (1%4) recommends checking the buoyancy force
on the casing during cementing with grout. Archimedes prin-
ciple states that a body wholly or partially immersed in a fluid
is buoyed up by a force equal to the weight of the fluid displaced
by the body. Failure to recognize this fact may result in
unnoticed upward displacement of the casing during cement-
ing. This is particularly true of lighter thermoplastic well
casings. Formulas for computing buoyancy are provided by
Moehrl (1964).
Types of Well Completions
The ultimate configuration of a monitoring well is chosen
to fulfill specific objectives as stated at the beginning of this
section. Monitoring wells can be completed either as single
wells screened in either short or long intervals, single wells
screened in multiple zones or multiple wells completed at
different intervals in one borehole. The decision as to which
type of monitoring well configuration to install in a specific
location is based on cost coupled with technical considerations
and practicality of installation.
In shallow installations, it generally is more economical to
complete the monitoring wells as individual units that are in
close proximity to each other and avoid the complexity of
multiple-zone completions in a single borehole. In deeper
installations where the cost of drilling is high relative to the cost
of the materials in the well and where cost savings can be
realized in improved sampling procedures, it may be better to
install a more sophisticated multilevel sampling device. The
cost of these completions are highly variable depending on the
specific requirements of the job. Cost comparisons should be
made on a site-by-site basis. Individual well completions will
almost always be more economical at depths of less than 80
feet. A discussion of the types of monitoring well completions
is presented below.
Single-Riser/Limited-Interval Wells
The majority of monitoring wells that arc installed at the
present time are individual monitoring wells screened in a
specific zone. Well intakes are usually moderate in length,
ranging from 3 to 10 feet. These wells are individually installed
in a single borehole with a vertical riser extending from the well
intake to the surface. Because the screened interval is short,
these are the easiest wells to install and develop. A typical
example of this design is shown in Figure 21.
The intent of a well with this design is to isolate a specific
zone from which water-quality samples and/or water levels are
to be obtained. If the well intake crosses more than one zone of
permeability, the water sample that is collected will represent
the quality of the more permeable zone. If a pump is installed
just above the well intake and the well is discharged at a high
rate, the majority of the sample that is obtained will come from
the upper portion of the well intake. If the pump is lowered to
the mid-section of the well intake and pumped at a low rate, the
bulk of the sample will come from the area that is immediately
adjacent to the zone of the pump intake. At high pumping rates
in both isotropic and stratified formations, flow lines converge
toward the pump so that the sample that is obtained is most
representative of the ground water moving along the shortest
flow lines. If the well is not properly sealed above the well
intake, leakage may occur from upper zones into the well
intake.
Single-Riser/Flow-Through Wells
Flow-through wells consist of a long well intake that either
fully or nearly fully penetrates the aquifer. The well intake is
connected to an individual riser that extends to the surface.
Wells of this type are typically small in diameter and are
designed to permit water in the aquifer to flow through the well
in such a manner as to make the well "transparent" in the
ground-water flow field. An illustration of this type of well is
shown in Figure 69.
This type of well produces water samples that area com-
posite of the water quality intercepted when the well is surged,
Surface protector
Casing or risar
Water table
Ground
water
flow
direction
Well
intake
. Bottom cap
Unconsolidated
aquifer
Bottom
of aquifer
Figure 69. Diagram of a single-riser/flow-through well.
109
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bailed or pumped heavily. For example, if three or more well
volumes are evacuated prior to sampling, the sample obtained
will be a composite sample representative of the more perme-
able zones penetrated by the well intake; it will not be possible
to define the zone(s) of contribution. However, if the well is
allowed to maintain a flow-through equilibrium condition and
if a sampler is lowered carefully to the selected sampling depth,
a minimally disturbed water sample can be obtained by either
taking a grab sample or by pumping at a very low rate. This
sample will be substantially representative of the zone in the
immediate vicinity of where the sample was taken. If the
sampler is successively lowered to greater depths and the water
within the well intake is not agitated, a series of discrete samples
can be obtained that will provide a reasonably accurate profile
of the quality of the water that is available in different vertical
zones. Furthermore, if the flow-through condition is allowed to
stabilize after any prior disturbance and a downhole chemical-
profiling instrument is lowered into the well, closely-spaced
measurements of parameters such as Eh, pH, dissolved oxygen,
conductivity and temperature can be made in the borehole. This
provides a geochemical profile of conditions in the aquifer. In
specific settings, wells of this design can provide water-quality
information that is at least as reliable as either the information
obtained by multiple-zone samplers in a single well or by
information from multiple nested wells. In either application,
the described flow-through well design is lower in cost.
Nested Wells
Nested wells consist of either a series of 1) single-riser/
limited-interval wells that are closely spaced so as to provide
data from different vertical zones in close proximity to each
other or 2) multiple single-riser/limited-interval wells that are
constructed in a single borehole. Illustrations of these designs
are shown in Figures 70a and 70b. Wells of these designs are
used to provide samples from different zones of an aquifer(s) in
the same manner as individual wells.
Multiple wells are constructed in a single borehole by
drilling a 10-inch or larger diameter borehole, then setting one,
two, or three 2-inch single-riser/limited-interval wells within
the single borehole. The deepest well intake is installed first, the
filter pack emplaced, and the seal added above the filter pack.
The filter pack provides stabilization of the deepest zone. After
the seal is installed above the deepest zone, the next succeeding
(upward) well intake is installed and the individual riser ex-
tended to the surface. This next well intake is filter-packed and
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1 — ' T f
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S
Surface seal
Grout seal
Filter sand-
Grout seal-
Filter pack
Screened interval -
(a)
(b)
Figure 70. Typical nested well designs: a) series of single riser/limited interval wells in separate boreholes and b) multiple single
riser/limited interval wells In a single borehole (after Johnson, 1983).
110
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a second seal is placed above the filter pack that is emplaced
around the second well intake. If there is a long vertical interval
between successive well intakes, neat cement grout is emplaced
above the lower seal. Where vertical separation permits, a 1-
foot layer of fine silica sand should be emplaced between the
filter packs and sealants. This sand helps prevent sealant infil-
tration into the filter pack and loss of filter pack into the sealant.
This procedure is repeated at all desired monitoring intervals.
Because each riser extends to the surface and is separate from
the other risers, a good seal must be attained around each riser
as it penetrates through successive bentonite seals. A substan-
tial problem with this type of construction is leakage along the
risers as well as along the borehole wall.
The primary difficulty with multiple completions in a
single borehole is that it is difficult to be certain that the seal
placed between the screened zones does not provide a conduit
that results in interconnection between previously non-con-
nected zones within the borehole. Of particular concern is
leakage along the borehole wall and along risers where overly-
ing seals are penetrated. It is often difficult to get an effective
seal between the seal (e.g., bentonite or cement grout) and the
material of the risers.
Multiple-Level Monitoring Wells
In addition to well nests that sample at multiple levels in a
single location, a variety of single-hole, multilevel sampling
devices are available. These sampling devices range from the
simple field-fabricated, PVC multilevel sampler shown, in
Figure 71 to the buried capsule devices that are installed in a
single borehole, as shown in Figure 72. The completion of these
wells is similar to the completion of nested wells in a single
borehole. Some of these samplers have individual tubing con-
nections that extend to the surface. Samples are collected from
the tubing. With some forms of instrumentation, water levels
can also be obtained. There are, additionally, more sophisti-
cated sampling devices available, such as shown in Figure 73.
These consist of multiple-zone inflatable packers that can be
installed in a relatively small borehole. They permit the sam-
pling of formation fluids at many intervals from within a single
borehole. Disadvantages of these devices arc: 1) it is difficult,
if not impossible, to repair the device if clogging occurs, 2) it is
difficult to prevent and/or evaluate sealant and packer leakage
and 3) these installations are more expensive than single-level
monitoring wells.
Simple vacuum-lift multiple port devices can be used in
shallow wells where samples can be obtained from the indi-
vidual tubing that extends to the surface. With increasing depth,
greater sophistication is required and a variety of gas-lift
sampling devices are available commercially. Still more so-
phisticated sampling devices are available for very deep instal-
lations. These devices require durable, inflatable packer sys-
tems and downhole tools to open and close individual ports to
obtain formation pressure readings and take fluid samples.
These can be used in wells that are several thousand feet deep.
Ground
Water table
End cap
Male & female
/ couplings
' Surface
PVC pipe
Coupling
Sampling points
End cap
(a)
PVC pipe
— Screen
One-hole
rubber
stopper
(b)
Figure 71, Field-fabricated PVC multilevel sampler: a) field installation and b) cross section of sampling point (Pickens et al., 1981).
Ill
-------
Protective
casing
Screened
interval
Sampling
tube
Backfill
Packet
Pumping port coupling
Measurement port coupling
End cap
Figure 72. Multilevel capsule sampling device installation
(Johnson, 1983).
General Suggestions for Well Completions
I) Use formation samples, sample penetration logs,
drilling logs, geophysical logs, video logs and all
other pertinent information that can be obtained
relating to the well installation to make decisions
on well completion. Make every attempt to define
the stratigraphy before attempting to install well
intakes.
2) Be aware of the control that stratigraphy exerts
over flow-line configuration when the sampling
pump is and is not operating. In an isotropic
aquifer, the sample is representative of the quality
of formation water in the immediate vicinity of
the pump. In a fractured system or a stratified
aquifer, flow can be highly directional and
confined.
3) Install the well intake in the exact zone opposite
the desired monitoring depth. If the well is designed
to intercept "floaters," the well intake must extend
high enough to provide for fluctuations in the
seasonal water table. If the well is designed to
monitor "sinkers" the topography of the bottom-
most confining layer must be sufficiently defined
such that a well intake can be installed at the
topographical points where the sinkers can be
intercepted. If there is a non-aqueous phase present,
the well intake must intersect the appropriate
pathways. Vertical variations in hydraulic
conductivity must be recognized as well as
horizontal variations. In consolidated rock,
fracture zones through which migration can occur
must be intercepted. At all times, the three-
Figure 73. Multiple zone inflatable packer sampling installation
(Rehtlane and Patton, 1982).
dimensional aspect of contaminant migration must
be taken into consideration.
4) Aquifer disruption must be minimized during the
completion process. Void space should not be
unnecessarily created when pulling back casing
or augers. Non-cohesive material collapse around
the well intake should be minimized except where
natural filter pack is used.
5) The depth and diameter limitations imposed by
the type of equipment and materials used in
monitoring well construction must be considered
as an integral part of well completion. The filter
pack must be uniformly emplaced; bentonite and
cement grout must be emplaced by positive
methods so that the zones that are supposed to be
isolated are truly isolated by positive seals. The
design and installation of a monitoring well are
impacted by the constraints of cost, but the errors
resulting from a well that is improperly constructed
are much more expensive than a well that is
properly constructed. The extra time and cost of
constructing a well properly, and being as sure as
possible that the information being obtained is
reliable, is well worth the extra cost of careful
installation.
References
California Department of Health Services, 1986. The
California site mitigation decision tree manual; California
Department of Health Services, Sacramento, California,
375 pp.
Driscoll, Fletcher G., 1986. Ground water and wells; Johnson
Division, St. Paul, Minnesota, 1089 pp.
112
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Johnson, Thomas L., 1983. A comparison of well nests versus
single-well completions; Ground Water Monitoring
Review, vol. 3, no. 1, pp. 76-78.
Moehrl, Kenneth E., 1964. Well grouting and well protection;
Journal of the American Water Works Association, vol. 56,
no. 4, pp. 423-431.
Pickens, J.F., J.A. Cherry, R.M. Coupland, G.E. Gnsak, W.F.
Merritt and B.A. Risto, 1981. A multilevel device for
ground-water sampling; Ground Water Monitoring
Review, vol. 1, no. 1, pp. 48-51.
Rehtlane, Erik A. and Franklin D. Patton, 1982. Multiple port
piezometers vs. standpipe piezometers: an economic
comparison; Proceedings of the Second National
Symposium on Aquifer Restoration and Ground-Water
Monitoring, National Water Well Association,
Worthington, Ohio, pp. 287-295.
Riggs, Charles 0. and Allen W. Hatheway, 1988. Groundwater
monitoring field practice - an overview; Ground-Water
Contamination Field Methods, Collins and Johnson editors,
ASTM Publication Code Number 04-963000-38,
Philadelphia, Pennsylvania, pp. 121-136.
United States Environmental Protection Agency, 1975.
Manual of water well construction practices; United States
Environmental Protection Agency, Office of Water Supply,
EPA-570/9-75-001,156pp.
113
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Section 7
Monitoring Well Development
Introduction/Philosophy
The objective of monitoring well development is fre-
quently misconstrued to be merely a process that enhances the
flow of ground water from the formation into the well and that
minimizes the amount of sediment in the water samples col-
lected from the well. These are the proper objectives for the
development of a production well but they do not fulfill the
requirements for a monitoring well. A monitoring well should
be a "transparent", window into the aquifer from which samples
can be collected that are truly representative of the quality of
water that is moving through the formation. This objective is
difficult to attain and is unattainable in some instances. How-
ever, the objective should not be abandoned because of the
difficulty.
The interpretation of any ground-water sample collected
from a monitoring well should reflect the degree of success that
has been reached in the development of the well and the
collection of the sample. This objective is frequently overlooked
in the literature and in much of the work that has been done in
the field. Further research is required before the reliability of
samples taken from a monitoring well can" be effectively sub-
stantiated. The United States Environmental Protection Agency
(1986) in the Technical Enforcement Guidance Document
(TEGD) states that, "a recommended acceptance/rejection
value of five nephelometric turbidity units (NTU) is based on
the need to minimize biochemical activity and possible interfer-
ence with ground-water sample quality." The TEGD also out-
lines a procedure for determining the source of turbidity and
usability of the sample and well. There are instances where
minimizing turbidity and/or biochemical activity will result in
a sample that is not representative of water that is moving
through the ground. If the ground water moving through the
formation is, in fact, turbid, or if there is free product moving
through the formation, then some criteria may cause a well to be
constructed such that the actual contaminant that the well was
installed to monitor will be filtered out of the water. Therefore,
it is imperative that the design, construction and development
of a monitoring well be consistent with the objective of obtain-
ing a sample that is representative of conditions in the ground.
An evaluation of the degree of success in attaining this objective
should always be included and considered in conjunction with
the laboratory and analytical work that is the final result of the
ground-water sample-collection process.
If the ultimate objective of a monitoring well is to provide
a representative sample of water as it exists in the formation,
then the immediate objective and challenge of the development
program is to restore the area adjacent to the well to its
indigenous condition by correcting damage done to the forma-
tion during the drilling process. This damage may occur in
many forms: 1) if a vibratory method such as driving casing is
used during the drilling process, damage may be caused by
compaction of the sediment in place; 2) if a compacted sand and
gravel is drilled by a hollow-stem auger and then allowed to
collapse around the monitoring well intake, damage may be the
resultant loss of density of the natural formation; 3) if a drilling
fluid of any type is added during the drilling process, damage
may occur by the infiltration of filtrate into the formation; and
4) if mud rotary, casing driving or augering techniques are used
during drilling, damage may be caused by the formation of a
mudcake or similar deposit that is caused by the drilling
process. Other formatation damage may be related to specific
installations. Some of this damage cannot be overcome satis-
factorily by the current capability to design and develop a
monitoring well. One important factor is the loss of stratifica-
tion in the monitored zone. Most natural formations are strati-
fied; the most common stratigraphic orientation is horizontal.
The rate of water movement through different stratigraphic
horizons varies, sorption rates may differ as stratigraphy changes;
and chemical interaction between contaminants and the forma-
tion materials and ground water can vary between different
horizons. During the development process, those zones with the
highest permeability will be most affected by the development
of the well. Where a well intake crosses stratigraphic bound-
aries of varying permeability, the water that moves into and out
of the well intake will be moving almost exclusively into and
out of the high permeability zones.
Factors Affecting Monitoring Well Development
There are three primary factors that influence the develop-
ment of a monitoring well: 1) the type of geologic material, 2)
the design and completion of the well and 3) the type of drilling
technology employed it? the well construction. From these
factors it is also possible to estimate the level of effort required
during development so that the monitoring well will perform
satisfactorily.
Type of Geologic Material
The primary geologic consideration is whether or not the
monitoring well intake will be installed in consolidated rock or
unconsolidated material. If the intake is installed in consoli-
dated rock or cohesive unconsolidated material, the assumption
can often be made that the borehole is stable and was stable
during the construction of the monitoring well. In a stable
borehole, it is generally easier to: 1) install the well intake(s) at
the prescribed setting(s), 2) uniformly distribute and maintain
the proper height of a filter pack (if one was installed) above the
well intake(s), 3) place the bentonite seal(s) in the intended
115
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location and 4) emplace a secure surface seal. However, if the
well intake is opposite unconsolidated material, the borehole
may not be or may not have been stable during well installation.
Depending on the degree of borehole instability during the well
completion process the well intake, filter pack, bentonite seal
and/or surface seal may not have been installed as designed. As
a consequence, there is generally a greater degree of difficulty
expected in the development of wells that are installed in
unconsolidated formations.
The permeability of the formation also influences the ease
of development. Where permeability is greater, water moves
more easily into and out of the formation and development is
accomplished more quickly. In unconsolidated formations, the
ease or difficulty of development is less predictable because
there is considerable variation in the grain size, sorting, and
stratification of many deposits. Zones that are developed and
water samples that are collected will be more representative of
the permeable portions of a stratified aquifer and may not be
very representative of the less permeable zones.
Design and Completion of the Well
A monitoring well can be installed relatively easily at a site
where the total depth of the well will be 25 feet; the static water
level is approximately 15 feet; and the monitored interval is a
clean, well-sorted sand and gravel with a permeability that
approximates 1 x 10' centimeters per second. However, a
monitoring well is much more difficult to install at a site where
the depth of the well will be 80 feet; the well will be completed
in an aquifer beneath an aquitard; the water table in the shallow
aquifer is approximately 20 feet deep; the piezometric surface
of the semi-confined aquifer is approximately 10 feet deep; and
the monitored interval in the deeper zone is composed of fine-
-grained sand with silt. Construction of the monitoring well in
this scenario will be difficult by any technique. No matter what
construction method is used, a considerable amount of time will
be required for well completion and problems can be anticipated
during setting of the well intake, placement of the filter pack,
placement of the bentonite seal or placement of the grout.
Difficulties may also be experienced during the development
process.
Another difficult monitoring well installation is where the
well intake is placed opposite extremely fine-grained materials.
For example, extremely fine-grained materials often occur as a
series of interbedded fine sands and clays such as might be
deposited in a sequence of lake deposits. A well intake set in the
middle of these saturated deposits must be completed with an
artificial filter pack. However, because the deposits are un-
stable, it is difficult to achieve a good distribution of the filter-
pack material around the well intake during installation. Fur-
thermore, even if the filter pack installation is successful, it is
not possible to design a sufficiently fine-grained filter pack that
will prevent the intrusion of the clays that are intimately
associated with the productive fine-grained sand. As a conse-
quence, every time the well is agitated during the sampling
process, the clays are mobilized and become part or all of the
turbidity that compromises the value of the ground-water
samples. There currently is no design or development proce-
dures that are able to fully overcome this problem. The only way
to minimize the intrusion of the clays is to install an extremely
fine-grained porous filter. This falter has very limited utility
because it rapidly becomes clogged by the clays that are being
removed. After a short operational period, insufficient quanti-
ties of samples are obtained and the filter can no longer be used.
Where an artificial filter pack is installed, the filter pack
must be as thin as possible if the development procedures are to
be effective in removing fine participate material from the
interface between the filter pack and the natural formation.
Conversely, the filter pack must be thick enough to ensure that
during the process of construction, it is possible to attain good
distribution of the filter pack material around the screen. It is
generally considered that the minimum thickness of filter pack
material that can be constructed effectively is 2 inches. Two
inches is a desirable thickness in situations where there is
adequate control to ensure good filter pack distribution. If there
are doubts about the distribution, then the filter pack must be
thickened to assure that there is adequate filtration and borehole
support.
In natural filter pack installations where the natural forma-
tion is allowed to collapse around the well intake, the function
of development is twofold: 1) to remove the fine-particulate
materials that have been emplaced adjacent to the well intake
and 2) to restore the natural flow regime in the aquifer so that
water may enter the well unimpeded.
It is easier to develop monitoring wells that are larger in
diameter than it is to develop small-diameter wells. For ex-
ample, mechanical surging or bailing techniques that are effective
in large-diameter wells are much less effective when used in
wells that are less than 2 inches in diameter because equipment
to develop smaller-diameter wells has limited availability.
Further, in small-diameter wells when the depths become
excessive, it is difficult to maintain straightness and alignment
of the borehole because of the drilling techniques that are
commonly used. It may become imperative in this situation to
use centralizers on the casing and well intake that are being
installed within these boreholes or to use other methods to
center the casing or ensure straight holes.
Type of Drilling Technology
The drilling process influences not only development
procedures but also the intensity with which these procedures
must be applied. Typical problems associated with special
drilling technologies that must be anticipated and overcome are
as follows: 1) when drilling an air rotary borehole in rock
formations, fine particulate matter typically builds up on the
borehole walls and plugs fissures, pore spaces, bedding planes
and other permeable zones. This particulate matter must be
removed and openings restored by the development process; 2)
if casing has been driven or if augers have been used, the
interface between the natural formation and the casing or the
auger flights are "smeared" with fine-particulate matter that
must subsequently be removed in the development process; 3)
if a mud rotary technique is used, a mudcake builds upon the
borehole wall that must be removed during the development
process; and 4) if there have been any additives, as may be
necessary in mud rotary, cable tool or augering procedures, then
the development process must attempt to remove all of the
fluids that have infiltrated into the natural formation.
116
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Well Development
Very little research has been performed that specifically
addresses movement of fluid, with or without contaminants
present, through a stratified aquifer into monitoring wells.
Ground-water flow theory is based on the primary assumptions
of homogeneity and isotropism of the formation. In production
wells, these assumptions are acceptable because the aquifer is
stressed over a sufficient area for variations to be "averaged."
Most discussions of monitoring-well flow characteristics are
based on the acceptance of these assumptions. However, these
are not always valid assumptions for attaining the objectives of
monitoring wells.
Where it is intended to intercept a contaminant in a re-
stricted zone of a three-dimensional flow field, a monitoring
well must be installed and developed with a much greater
precision than is normal for production wells. The relative
movement of fluid in specific zones becomes significantly
more important than the gross yield. Both installation and
development must be performed with a "spot precision" that
preserves in situ conditions and permits the collection of a
representative sample.
The methods that are available for the development of
monitoring wells have been inherited from production well
development practices. These methods include: 1) surging with
a surge Mock, 2) bailing, 3) pumping, overpumping and
backwashing through the pump, 4) airlift pumping and 5) air
surging and jetting. A number of authors have written about
these available methods of development for monitoring wells.
A summary of these articles is contained in Table 36.
Based on a review of the literature and on a wide range of
actual field practices, a few generalizations about development
of monitoring wells can be made
1) using air for well development can result in
chemical alteration of the ground water both as a
result of chemical reaction with the air and as a
result of impurities introduced through the air
stream;
2) adding water to the borehole for stabilization,
surging, backwashing, flushing or any other
purpose has an unpredictable effect on ground-
water quality and at the very least causes dilution.
Even if the water added to the borehole was
originally pumped from the same formation,
chemical alteration of the ground water in the
formation can occur if the water is reinfected.
Once water has been pumped to the surface,
aeration can alter the original water quality;
3) developing the formation at the interface between
the outer perimeter of an artificial filter pack and
the inner perimeter of the borehole is extremely
difficult. Any mudcake or natural clay deposited
at this interface is very difficult to remove;
incomplete removal can have unquantifiable short-
and long-range impacts on the quality of the
sampled ground water;
4) developing a well is relatively easy when the well
intake is placed in a clean homogeneous aquifer of
relatively high permeability. It is very difficult to
develop a representative well in an aquifer that is
stratified, slowly permeable and fine-grained,
particularly where there is substantial variation
between the various stratified zones;
5) developing a larger-diameter monitoring well is
easier than developing a smaller-diameter well.
This is particularly true if the development is
accomplished by overpumping or backwashing
through the pump because suitable pumping
capacity is not commonly available for small-
diameter wells with deep static water levels.
However, a smaller-diameter well is more
"transparent" in the aquifer flow field and is
therefore more likely to yield a representative
sample,
6) collecting non-turbid sample may not be possible
because there are monitoring wells that cannot be
sufficiently developed by any available technique.
This may be the consequence of the existence of
turbid water in the formation or the inability to
design and construct a well that will yield water in
satisfactory quantity without exceeding acceptable
flow velocities in the natural formation;
7) applying many of the monitoring well-
development techniques in small-diameter (2-
inch) wells and using the design and construction
techniques discussed in the literature are easiest
in shallow monitoring situations with good
hydraulic conductivity. These techniques may be
impractical when applied to deeper or more
difficult monitoring situations.
8) Adding clean water of known quality for flushing
and/or jetting should be done only when no better
options are available. A record must be kept of the
quantities of water lost to the formation during the
flushing/jetting operation and every attempt must
be made to reestablish background levels in a
manner similar to that described in Barcelona et
al. (1985a) and/or the United States Environmental
Protection Agency (1986); and
9) dealing objectively with the conditions and
problems that exist for every installation is
essential. The problems encountered at each site
should be addressed and clearly presented in the
final report. Chemical analyses must be included
in the final report so that anyone evaluating these
analyses is able to understand the limitations of
the work.
Methods of Well Development
Monitoring well development is an attempt to remove fine
particulate matter, commonly clay and silt, from the geologic
formation near the well intake. If particulate matter is not
removed, as water moves through the formation into the well,
the water sampled will be turbid, and the viability of the water
quality analyses will be impaired. When pumping during well
development, the movement of water is unidirectional toward
the well. Therefore, there is a tendency for the particles moving
toward the well to "bridge" together or form blockages that
restrict subsequent particulate movement. These blockages
may prevent the complete development of the well capacity.
This effect potentially impacts the quality of the water dis-
charged. Development techniques should remove such bridges
117
-------
Table 36. Summary of Development Methods for Monitoring" Wells
Reference
gass (1966)
United States
Environmental
Protection
Agency (1966)
Overpumping
Works best in
clean coarse
formations and
some consolidated
rock; problems of
water disposal and
bridging
Effective develop-
ment requires flow
reversal or surges
to avoid bridges
Backwashing
Breaks up
bridging, low
cost & simple;
preferentially
develops
Indirectly indicates
method applicable;
formation water
should be used
Surge Block* Bailer
Can be effective;
size made for£2"-
well; preferential
development where
screen >5'; surge
inside screen
Applicable; forma- Applicable
tion water should
be used; in low-
yield formation,
Jetting
Consolidated
and uncon-
solidated
application;
opens fractures,
develops discrete
zones; disadvantage
is external water
needed
Airlift Pumping
Replaces air surg
ing; filter air
Air should not
be used
Air Surging
Perhaps most
widely used;
can entrain
air in form-
ation so as to
reduce per-
meability, affect
water qualify;
avoid if possible,
Air should not
be used
Barcelona et
al. ** (1963)
Staff et al.
(1901)
National
Council of the
Paper Industry
for Air and
Stream Im-
provement
(1961)
Productive wells;
surging by alternat-
ting pumping and
allowing to equili-
brate; hard to create
must be
sufficient entrance
velocities; often
use with airlift
Applicable
drawback of flow in
one direction;
smaller wells hard
to pump if water
level below suction
Suitable; periodic
removal of fines
outside water
source can be
used if analyzed
to evaluate impact
Productive walk;
use care to avoid
casing and screen
damage
Suitable; common
with cable to of;
not easily used
on other rigs
Applicable; caution
against collapse of
intake or plugging
screen with clay
Productive walk;
more common than
surge blocks but
not as effective
Suitable; use
sufficiently
heavy bailer;
advantage of
removing fines;
may be custom
made for small
diameters
Suitable
Effectiveness
depends on
geometry of
device; air
filtered; crew
may be
exposed to
contaminated
water; per-
turbed Eh in
sand and
gravel not
persistent for
more than a
few weeks
Suitable;
avoid injecting
air into intake;
chemical
interference;
air pipe never
inside screen
Methods introducing foreign materials should be
avoided (i.e., compressed air or water jets)
-------
Table 36. (Continued)
Reference
Everett (1960)
Keely and
Boateng
(1987 a and b)
Overpumping Beckwashing
Development opera-
tion must cause
flow reversal to
avoid bridging; can
alternate pump oft
and on'
Probably most Vigorous surging
desirable when action may not be
surged; second desirable due to
series of disturbance of
evacuation/ gravel pack
recovery cycles is
recommended after
resting the well for
24 hours; settlement
and loosening of
fines ocurs after
the first
development
attempt; not as
vigorous as
backwashing
Surge Block* Bailer
Suitable; periodic
bailing to remove
fines
Method quite
effective in
loosening fines but
may be inadvisable
in that filter pack
and fluids may be
displaced to degree
that damages value
as a filtering media
Jetting Airlift Pumping Air Surging
High velocity jets of
water generally
most effective; dis-
cret zones of
development
Popular but Air can become
less desirable; entrained behind
method dif- screen and
ferent from permeability
water wells;
water displaced
by short down-
ward bursts of
high-pressure injection;
important not to jet
air or water across
screen because fines
driven into screen
cause irreversible
blockage; may subsatantiafly
displace native fluids
reduce
tfl
* Schalla and Landick (1986) report on special 2- valved block
" For low hydraulic conductivity w«Iis, flush water up armulus prior to sealing; afterwards pump
-------
and encourage the movement of participate into the well.
These participate can then be removed from the well by bailer
or pump and, in most cases, the water produced will subsequently
be clear and non-turbid.
One of the major considerations in monitoring well devel-
opment is the expense. In hard-to-develop formations, it is not
unusual for the development process to take several days before
an acceptable water quality can be attained. Because develop-
ment procedures usually involve a drilling rig, crew, support
staff and a supervising geologist, the total cost of the crew in the
field often ranges in cost from $100 to $200 per hour. Thus, the
cost of development can be the most expensive portion of the
installation of a monitoring-well network. When this hourly
cost is compared to an often imperceptible rate of progress,
there is a tendency to prematurely say either, "that is good
enough" or "it can't be done. "
In most instances, monitoring wells installed in consoli-
dated formations can be developed without great difficulty.
Monitoring wells also can usually be developed rapidly and
without great difficulty in sand and gravel deposits. However,
many installations are made in thin, silty and/or clayey zones.
It is not uncommon for these zones to be difficult to develop
sufficiently for adequate samples to be coil.xted.
Where the borehole is sufficiently stable, due to installa-
tion in sound rock or stable unconsolidated materials, and
where the addition of fluids during completion and develop-
ment is permissible, it is a good practice to precondition the
borehole by flushing with clean water prior to filter pack
installation. When water is added to the well, the quality of the
water must be analyzed so that comparisons can be made with
subsequent water-quality data. Flushing of monitoring wells is
appropriate for wells drilled by any method and aids in the
removal of mud cake (mud rotary) and other finely-ground
debris (air rotary, cable tool, auger) from the borehole wall. This
process opens clogged fractures and cleans thin stratigraphic
zones that might otherwise be non-productive. Flushing can be
accomplished by isolating individual open zones in the borehole
or by exposing the entire zone. If the entire zone is exposed,
cross connection of all zones can occur.
Where it is not permissible to add fluids during completion
and development, and the borehole is stable, mechanically
scraping or scratching the borehole wall with a scraper or wire
brush, can assist in removing particulate from the borehole
wall. Dislodged particulate can be pumped or bailed from the
borehole prior to filter pack, casing and well intake installation.
Where the addition of fluid is permissible, the use of high-
-pressure jetting can be considered for screened intake develop-
ment in special applications. If jetting is used, the process
should usually be performed in such a manner that loosened
particulate are removed (e.g., bailing, pumping, flushing)
either simultaneously or alternately with the jetting. The disad-
vantages of using jetting even in "ideal conditions" are fourfold:
1) the water used in jetting is agitated, pumped, pressurized and
discharged into the formation; 2) the fine (e.g., 10-slot, 20-slot)
slotted screens of most monitoring well intakes do not permit
effective jetting, and development of the material outside the
screen may be negligible or possibly detrimental; 3) there is
minimal development of the interface between the filter pack
and the wall of the borehole (Table 36) and 4) water that is
injected forcibly replaces natural formation fluids. These are
serious limitations on the usefulness of jetting as a development
procedure.
Air development forcibly introduces air into contact with
formation fluids, initiating the potential for uncontrolled
chemical reactions. When air is introduced into permeable
formations, there is a serious potential for air entrainment
within the formation. Air entrainment not only presents poten-
tial quality problems, but also can interfere with flow into the
monitoring well. These factors limit the use of air surging for
development of monitoring wells.
After due consideration of the available procedures for
well development, it becomes evident that the four most suit-
able methods for monitoring well development are: 1) bailing,
2) surge block surging, 3) pumping/overpumping/backwashing
and 4) combinations of these three methods.
Bailing
In relatively clean, permeable formations where water
flows freely into the borehole, bailing is an effective develop-
ment technique. The bailer is allowed to fall freely through the
borehole until it strikes the surface of the water. The contact of
the bailer produces a strong outward surge of water that is
forced from the borehole through the well intake and into the
formation. This tends to breakup bridging that has developed
within the formation. As the bailer fills and is rapidly with-
drawn, the drawdown created in the borehole causes the par-
ticulate matter outside the well intake to flow through the well
intake and into the well. Subsequent bailing removes the
particulate matter from the well. To enhance the removal of
sand and other particulate matter from the well, the bailer can
be agitated by rapid short strokes near the bottom of the well.
This agitation makes it possible to bail the particulate from the
well by suspending or slurrying the particulate matter. Bailing
should be continued until the water is free from suspended
particulate matter. If the well is rapidly and repeatedly bailed
and the formation is not sufficiently conductive, the borehole
will be dewatered. When this occurs, the borehole must be
allowed to refill before bailing is resumed. Care must be taken
that the rapid removal of the bailer does not cause the external
pressure on the well casing to exceed the strength of the casing
and/or well intake thereby causing collapse of the casing and/
or well intake.
Bailing can be conducted by hand on shallow wells,
although it is difficult to continue actively bailing for more than
about an hour. Most drill rigs are equipped with an extra line that
can be used for the bailing operation. The most effective
operation is where the bail line permits a free fall in the
downward mode and a relatively quick retrieval in the upward
mode. This combination maximizes the surging action of the
bailer. The hydraulic-powered lines on many rigs used in
monitoring-well installation operate too slowly for effective
surging. Bailing is an effective development tool because it
provides the same effects as both pumping and surging with a
surge block. The most effective equipment for bailing opera-
tions is generally available on cable tool rigs.
120
-------
There area variety of dart valve, flat bottom and sand pump
bailers availa ble for the development of larger-diameter wells.
These bailers are typically fabricated from steel and are oper-
ated by using a specially designated line on the rig. For most
monitoring-well applications, small-diameter PVC or
fluoropolymer bailers arc readily available. When commercial
bailers are not available, bailers can be fabricated from readily
available materials. Bailers of appropriate diameter, length,
material and weight should be used to avoid potential breakage
of the well casing or screen. Figures 74a and 74b show a
schematic representation of typical commercially available
small-diameter bailers.
Surge Block
Surge blocks, such as are shown in Figures 75 and 76, can
be used effectively to destroy bridging and to create the agita-
tion that is necessary to develop a well. A surge block is used
alternately with either a bailer or pump so that material that has
been agitated and loosened by the surging action is removed.
The cycle of surging-pumpingftailing is repeated until satisfac-
tory development has been attained.
During the development process, the surge block can be
operated either as an integral part of the drill rods or on a
wireline. In either event, the surge block assembly must be of
sufficient weight to free-fall through the water in the borehole
and create a vigorous outward surge. The equipment that lifts
or extracts the surge block after the downward plunge must be
strong enough to pull the surge block upward relatively
rapidly. The surge block by design permits some of the fluid to
bypass on the downward stroke, either around the perimeter of
the surge block or through bypass valves.
The surge block is lowered to the top of the well intake and
then operated in a pumping action with a typical stroke of
approximately 3 feet. The surging is usually initiated at the top
of the well intake and gradually is worked downward through
the screened interval, 'he surge block is removed at regular
intervals and the fine material that has been loosened is re-
moved by bailing and/or pumping. Surging begins at the top of
the well intake so that sand or silt loosened by the initial surging
action cannot cascade down on top of the surge block and
prevent removal of the surge block from the well. Surging is
initially gentle, and the energy of the action is gradually
increased during the development process. The vigor of the
surging action is controlled by the speed, length and stroke of
the fall and speed of retraction of the surge block. B y controlling
these rates, the surging activity can range from very rigorous to
very gentle.
Surging within the well intake can result in serious difficul-
ties. Vigorous surging in a well that is designed such that
excessive sand can be produced, can result in sand-locking the
surge block. This should not occur in a properly designed
monitoring well, nor should it occur if the surge block of
appropriate diameter is properly used. As in the case of bailer
surging, if excessive force is used, it is possible to cause the
collapse of the well intake and/or the casing.
An alternative to surging within the well intake is to
perform the surging within the casing above the well intake.
This has the advantage of minimizing the risk of sand locking.
However, it also reduces the effectiveness of the surging action.
In permeable material, the procedure of surging above the well
intake is effective only for well intakes with lengths of 5 feet or
less.
If the well is properly designed, and if 1) the surge block
is initially operated with short, gentle strokes above the well
intake, 2) sand is removed periodically by alternating sand
removal with surging, 3) the energy of surging is gradually
increased at each depth of surging until no more sand is
produced from surging at that depth, and 4) the depth of surging
is incrementally increased from top to bottom of the well intake,
then surging can be conducted effectively and safely.
Where there is sufficient annular space available within the
casing, which is seldom the case with monitoring wells, it is
effective to install a low-capacity pump above the surge block.
By discharging from the well concurrent with surging, a
gradient is maintained toward the well. This set-up assists in
developing the adjacent aquifer by maintaining the movement
of particulate material toward the well.
Surging is usually most effective when performed by cable
tool-type machines. The hydraulic hoisting equipment that is
normally available on most other types of drilling equipment
does not operate with sufficient speed to provide high-energy
surging. Where properly used, the surge block in combination
with bailing or pumping may be the most effective form of
mechanical development.
Pumping/Overpumpin/Backwashing
The easiest, least-expensive and most commonly em-
ployed technique of monitoring-well development is some
form of pumping. By installing a pump in the well and starting
the pump, ground-water flow is induced toward the well. Fine-
particulate material that moves into the well is discharged by the
pump. In overpumping, the pump is operated at a capacity that
substantially exceeds the ability of the formation to deliver
water. This flow velocity into the well usually exceeds the flow
velocity that will subsequently be induced during the sampling
process. This increased velocity causes rapid and effective
migration of particulate toward the pumping well and en-
hances the development process. Proper design is needed to
avoid well collapse, especially in deep wells. Both pumping and
overpumping are easily used in the development of a well.
Where there is no backflow-prevention valve installed, the
pump can be alternately started and stopped. This starting and
stopping allows the column of water that is initially picked up
by the pump to be alternately dropped and raised up in a surging
action. Each time the water column falls back into the well, an
outward surge of water flows into the formation. This surge
tends to loosen the bridging of the fine particles so that the
upward motion of the column of water can move the particles
into and out of the well. In this manner, the well can be pumped,
overpumped and back-flushed alternately until such time as
satisfactory development has been attained.
While the preceding procedures can effectively develop a
well, and have been used for many years in the development of
production wells, pumping equipment suitable to perform these
operations may not be available that will fit into some small-
diameter monitoring wells. To be effective as a development
tool, pumps must have a pumping capability that ranges from
121
-------
Standard
Sailer of
Teflon®
Standard
Bailer of
Pvc
(a)
Bottom
Emptying
Device
Top for Variable
Capacity Point Source
Sailer of PVC
Retaining
Pin
Ball
Check
Sample
Chamber
1 Foot
Midsection
May Be Added
Here
_ Retaining
Pin
- Sell Check
(b)
Diagrams of typical bailers used in monitoring well development: a) standard type and b) "point source" bailer
(Timco Manufacturing Company, inc., 1982).
122
-------
Pressure-relief
Hole
Figure 75. Diagram of a typical surge block (Driscoll, 1986).
very low to very high or be capable of being controlled by
valving. The sampling pumps that are presently designed to fit
into small-diameter boreholes commonly do not provide the
upper range of capacities that often are needed for this type of
development. For shallow wells with water levels less than 25
feet deep, a suction-lift centrifugal pump can be used for
development in the manner prescribed. The maximum practical
suction lift attainable by this method is approximately 25 feet.
In practice, bailing or bailing and surging is combined with
pumping for the most-efficient well development. The bailing
or surging procedures are used to loosen bridges and move
material toward the well. A low-capacity sampling pump or
bailer is then used to remove turbid water from the well until the
quality is satisfactory. This procedure is actually less than
completely satisfactory, but is the best-available technology
with the equipment that is currently available.
Air lifting, without exposing the formations being devel-
oped directly to air, can be accomplished by properly imple-
mented pumping. To do this, the double pipe method of air
lifting is preferred. The bottom of the airlift should be lowered
to within no more than 10 feet of the top of the well intake, and
in no event should the air lift be used within the well intake. If
the air lift is used to surge the well, by alternating the air on and
off, there will be mixing of aerated water with the water in the
well. Therefore, if the well is to be pumped by air lifting, the
action should be one of continuous, regulated discharge. This
can be effectively accomplished only in relatively permeable
aquifers.
Where monitoring well installations are to be made in
formations that have low hydraulic conductivity, none of the
preceding well-development methods will be found to be
completely satisfactory. Barcelona et al. (1985a) recommend a
procedure that is applicable in this situation: "In this type of
geologic setting, clean water should be circulated down the well
casing, out through the well intake and gravel pack, and up the
open borehole prior to placement of the grout or seal in the
annulus. Relatively high water velocities can be maintained,
and the mudcake from the borehole wall will be broken down
effectively and removed. Flow rates should be controlled to
prevent floating the gravel pack out of the borehole. Because of
the relatively low hydraulic conductivity of geologic materials
outside the well, a negligible amount of water will penetrate the
formation being monitored. However, immediately following
the procedure, the well sealant should be installed and the well
pumped to remove as much of the water used in the develop-
ment process as possible."
All of the techniques described in this section are designed
to remove the effects of drilling from the monitored zone and,
insofar as possible, to restore the formations penetrated to
indigenous conditions. To this end, proposed development
techniques, where possible, avoid the use of introduced fluids,
including air, into the monitored zone during the development
process. This not only minimizes adverse impacts on the quality
of water samples, but also restricts development options that
would otherwise be available.
References
Barcelona, MJ., J.P. Gibb, J.A. Helfnch and E.E. Garske,
1985a. Practical guide for ground-water sampling; Illinois
State Water Survey, SWS Contract Report 374, Champaign,
Illinois, 93 pp.
Barcelona, M.J., J.P. Gibb and R. Miller, 1983. A guide to the
selection of materials for monitoring well construction and
ground-water sampling; Illinois State Water Survey, SWS
Contract Report 327, Champaign, Illinois, 78 pp.
Driscoll, Fletcher G., 1986. Ground water and wells; Johnson
Division, St. Paul, Minnesota, 1089 pp.
Everett, Lome G., 1980. Ground-water monitoring; General
Electric Company technology marketing operation,
Schenectady, New York, 440 pp.
Gass, Tyler E., 1986. Monitoring well development; Water
Well Journal, vol. 40, no. 1, pp. 52-55.
Keely, Joseph F. and Kwasi Boateng, 1987a. Monitoring well
installation, purging and sampling techniques part 1:
conceptualization Ground Water, vol. 25, no. 3, pp. 300-
313.
Keely, Joseph F. and Kwasi Boateng, 1987b. Monitoring well
installation, purging, and sampling techniques part 2: case
histories; Ground Water, vol. 25, no. 4, pp. 427-439.
National Council of the Paper Industry for Air and Stream
Improvement 1981. Ground-water quality monitoring well
construction and placement; Stream Improvement
Technical Bulletin Number 342, New York, New York,
39pp.
Scalf, M.R., J.F. McNabb, WJ. Dunlap, R.L. Cosby and J.
Fryberger, 1981. Manual of ground-water sampling
123
-------
procedures; National Water Well Association, 93 pp.
Schall, Ronald and Robert W. Landick, 1986. A new valved
and air-vented surge plunger for developing small-diameter
monitor wells; Ground Water Monitoring Review, vol. 6,
no. 2, pp. 77-80.
Timco Manufacturing Company, Inc., 1982. Geotechnical
Products; product literature, Prairie Du Sac, Wisconsin, 24
pp.
United States Environmental protection Agency, 1986.
RCRA ground-water monitoring technical enforcement
guidance document; Office of Waste Programs
Enforcement, Office of Solid Waste and Emergency
Response," OSWER-9950.1, United States Environmental
Protection Agency, 317 pp.
Water Ports {0.25" O.D,)
fft- Polypropylene Tube (0,375' O.D.)
W- Stainless Steel Cable (0.063* O.D.
Ferrule
Stainless Stael Hex Nut (0.63")
* Viton Discs (0.05" Thick. 2.1 O.D. &
0.68 I.D.)
SCH 80 PVC Pipe (1.90" O.D.)
Top Fitting
NPT Threading
Stainless Steel Coupling
(1.325' O.D.)
Stainless Steel Pipe
(1.067* O.D. i
SCH 80 PVC Pipe (1,90" O.D.)
Bottom Fitting
Stainless Steel Hex Nut (0,63")
Slainlass St*d Tube (0.375" O.D.)
Swage Block
Figure 76. Diagram of a specialized monitoring well surge block (Schalla and Landick, 1966).
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Section 8
Monitoring Well Network Management Considerations
Well Documentation
Records are an integral part of any monitoring system.
Comprehensive records should be kept that document data
collection at a specific site. These data include boring records,
geophysical data, aquifer analysis data, ground-water sampling
results and abandonment documentation. Armed with as much
data as possible for the site, an effective management strategy
for the monitoring well network can be instituted.
Excellent records of monitoring wells must be kept for any
management strategy to be effective. Documentation of moni-
toring well construction and testing must frequently be pro-
vided as part of a regulatory program. Many states require
drillers to file a well log to document well installation and
location. Currently, some states have adopted or are adopting
regulations with unique reporting requirements specifically for
monitoring wells. At the state and federal level, guidance
documents have been developed that address reporting require-
ments. Tables 37, 38 and 39 illustrate some of the items that
various states have implemented to address monitoring well
Recordkeeping. Table 40 shows the recommendations of the
United States Environmental Protection Agency (1986). An
additional discussion on field documentation can be found in
the section entitled "Recordkeeping. "
The most critical factor in evaluating or reviewing data
from a monitoring well is location. If a monitoring well cannot
be physically located in the field and/or on a map in relationship
to other wells, only limited interpretation of the data is possible.
All monitoring wells should be properly located and referenced
to a datum. The degree of accuracy for vertical and horizontal
control for monitoring well location should be established and
held constant for all monitoring wells. In many cases, a licensed
surveyor should be contracted to perform the survey of the
wells. With few exceptions, vertical elevations should be refer-
enced to mean sea level and be accurate to 0.01 foot (Brownlee,
1985). Because elevations are surveyed during various stages
of well/boring installation, careful records must be kept as to
where the elevation is established. For example, if ground
elevation is determined during the drilling process, no perma-
nent elevation point usually can be established because the
ground is disturbed during the drilling process. A temporary pin
can be established close to the well location for use in later more
accurate measurements, but the completed well must be
resurveyed to maintain the desired accuracy of elevation. Each
completed well should have a standard surveyed reference
point. Because the top of the casing is not always level,
frequently the highest point on the casing is used. Brownlee
(1985) suggests that the standard reference point should be
consistent such that the north (or other) side of all monitoring
wells is the referenced point. Regardless of what point is
chosen, the surveyor should be advised before the survey is
conducted and the reference point clearly marked at each well.
If paint is used to mark the casing, the paint must not be allowed
on the inside of the casing. If spray paint is used, the aerosols can
coat the inside of the casing and may cause spurious water-
quality results in subsequent samples. An alternative way to
mark the casing is to notch the casing so that a permanent
reference point is designated. The United States Environmental
Protection Agency (1986) recommends that reference marks be
placed on both the casing and grout apron.
Well locations should clearly be marked in the field. Each
well should have a unique number that is clearly visible on the
well or protective casing. To ensure good documentation, the
well number may be descriptive of the method used to install the
well. For example, a well designated as C-l could represent the
first cored hole, or HS-3 could be a hollow-stem auger hole. If
multilevel sampling tubes are being used, each tube should be
clearly marked with the appropriate depth interval.
Well locations should be clearly marked on a map. The
map should also include roads, buildings, other wells, property
boundaries and other reference points. In general, maps illus-
trating comparable items should be the same scale. In addition
to the unique monitoring well number, general well designa-
tions may be desirable to include on the map. The Wisconsin
Department of Natural Resources (1985) suggests that PIEZ
(piezometer), OW (observation well), PVT (private well),
LYS (lysimeter) and OTHER be used to clarify the function of
the wells.
Files should be kept on each monitoring well so that any
suspected problems with the monitoring well can be evaluated
based on previous well performance. The accuracy and com-
pleteness of the records will influence the ability of the reviewer
to make decisions based on historical data.
Well Maintenance and Rehabilitation
The purpose of maintaining a monitoring well is to extend
the life of the well and to provide representative levels and
samples of the ground water surrounding the well. Maintenance
includes proper documentation of factors that can be used as
benchmarks for comparison of data at a later point. A scheduled
maintenance program should be developed before sample qual-
ity is questioned. This section is designed to assist the user in
setting up a comprehensive maintenance schedule for a moni-
toring system.
Documenting Monitoring Well Performance
A monitoring well network should be periodically evalu-
ated to determine that the wells are functioning properly. Once
complete construction and "as-built" information is on file for
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Table 37. Comprehensive Monitoring Wet) Documentation (After Wisconsin Department of Natural Resources, 1965}
Well Design:
• Length, schedule and diameter of casing
«Joint type (threaded, flush or solvent welded)
• Length, schedule and diameter of screen
• Percentage of open area in screen
« Slot size of screen
• Distance the filter pack extends above the screen
« Elevations of the top of well casing, bottom and top of protective
casing, ground surface, bottom of borehole, bottom of well
screen, and top and bottom of seal(s)
• Well location by coordinates or grid systems (example township
and range)
* Well location on plan sheet showing the coordinate system, scale,
a north arrow and a key
Materials:
• Casing and screen
• Filter pack (including grain size analysis)
• Seal and physical form
• Slurry or grout mix (percent cement, percent bentonite powder,
percent water)
Installation:
« Drilling method
* Drilling fluid (if applicable)
• Source of water (if applicable) and analysis of water
» Time period between the addition of backfill and construction of
well protection
Development:
• Date, time, elevation of water level prior to and after development
* Method used for development
• Time spent developing a well
* Volume of water removed
• Volume of water added (if applicable), source of water added
chemical analyses of water added
* Clarity of water before and after development
* Amount of sediment present at the bottom of the wet!
» pH, specific conductance and temperature readings
Soils Information;
• Soil sample test results
• Driller's observation or photocopied drillers log
MUcfltlaneou*:
• Water levels and dates
• Weil yield
* Any changes made in well construction, casing elevation, etc.
Table 38. Additional Monitoring Well Documentation (After Nebraska Department of Environmental Control, 1984)
Well identification number
Formation samples (depth and method of collection)
Water samples (depth, method of collection, and results)
Filter pack (depth, thickness, grain size analysis, placement method, supplier)
Date of all work
Name, address of consultant, drilling company and stratigraphic Jog preparer(s)
Description and results of pump or stabilization test if performed
Methods used to decontaminate drilling equipment and well construction malaria!
Table 39. As. Built Construction Diagram information (After Connecticut Environmental Protection Agency, 1983)
• Top of ground surface
* Protective grouting and grading at ground surface
• Well casing length and depth
« Screen length and depth
* Location and extent of gravel pack
• Location and extent of bentonite seal
• Water table
* Earth materials stratigraphy throughout boring
* For rock wells, show details of bedrock seal
* For rock wells, indicate depths of water-bearing fractures, faults or fissures and approximate yield
each well, the well should be periodically re-evaluated to check
for potential problems. The following checks can be used as a
"first alert" for potential problems:
1) The depth of the well should be recorded every
time a water sample is collected or a water-level
reading taken. These depths should be reviewed
at least annually to document whether or not the
well is filling with sediment;
2) If turbid samples are collected from a well,
redevelopment of the existing well should be
considered or a new well should be installed if
necessary (Barcelona et al, 1985a);
3) Hydraulic conductivity tests should be performed
every 5 years or when significant sediment has
accumulated;
4) Slug or pump tests should be performed every 5
years. Redevelopment is necessary if the tests
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Table 40. Field Boring Log Information (United States Environmental Protection Agency, 1988)
General:
• Project name
• Date started and finished*
• Geologist's name*
• Driller's name*
* Sheet number
Information Columns:
.Depth"
.Sample location/number*
. Blow counts and advance rate
Narrative Description:
« Geologic observations:
* soil/rock type*
• color and stain*
• gross petrology"
* friability
• moisture content*
« degree of weathering*
* presence of carbonate*
• Drilling Observations:
• loss of circulation
• advance rates*
• rig chatter
• water levels*
• amount of air used, air pressure
« drilling difficulties*
. Other Remarks:
. equipment failures
.possible contamination"
.deviations from drilling plan"
. weather
• Hole location; map and elevation*
• Rig type
• Bit size/auger size*
' Petroloflic lithologic classification scheme
used (Wentwortri unified soil classification system)
• Percent sample recovery*
> Narrative description*
• Depth to saturation*
.fractures*
.solution cavities*
.bedding*
. discontinuities* - e.g., foliation
.water-bearing zones*
.formational strike and dip"
. fossils
. changes in drilling method or equipment*
.readings from detective equipment, if any*
.amount of water yield or loss during drilling
at different depths*
.dapositional structures*
.organic content*
.odor*
.suspected contaminant*
> amounts and types of any liquids used*
• running sands*
• caving/hole stability*
'Indicates items that the owner/operator should record at a minimum.
show that the performance of the well is
deteriorating;
5) Piezometric surface maps should be plotted and
reviewed at least annually; and
6) High and low water-level data for each well
should be examined at least every 2 years to
assure that well locations (horizontally and
vertically) remain acceptable. If the water level
falls below the top of the well intake, the quality
of the water samples collected can be altered.
Where serious problems are indicated with a well(s),
geophysical logs may be helpful in diagnosing maintenance
needs. Caliper logs provide information on diameter that may
be used to evaluate physical changes in the borehole or casing.
Gamma logs can be used to evaluate lithologic changes and can
be applied to ascertain whether or not well intakes are properly
placed. Spontaneous potential logs can locate zones of low
permeability where siltation may originate. Resistivity logs
identify permeable and/or porous zones to identify formation
boundaries. Television and photographic surveys can pinpoint
casing problems and well intake failure and/or blockage. When
used in combination, geophysical logs may save time and
money in identifying problem areas. An additional discussion
of the applicability and limitations of geophysical logging tools
can be found in the section entitled "Borehole Geophysical
Tools and Downhole Cameras."
Factors Contributing to Well Maintenance Needs
The maintenance requirements of a well are influenced by
the design of the well and the characteristics of the monitored
zones. Water quality, transmissivity, permeability, storage ca-
pacity, boundary conditions, stratification, sorting and fractur-
ing all can influence the need for and method(s) of well
maintenance. Table 41 lists major aquifer types by ground-
water regions and indicates the most prevalent problems with
operation of the wells in this type of rock or unconsolidated
deposit. Problems with monitoring wells are typically caused
by poor well design, improper installation, incomplete develop-
ment, borehole instability and chemical, physical and/or bio-
logical incrustation. A brief description of the major factors
leading to well maintenance are discussed below.
Design —
A well is improperly designed if hydrogeologic conditions,
water quality or well intake design are not compatible with the
purpose and use of the monitoring well. For example, if water
is withdrawn during the sampling process and the well screen is
plugged, the hydrostatic pressure on the outside of the casing
may be great enough to cause collapse of the well intake if the
strength of the material was not sufficient for the application.
This is particularly true if the well intake material was chemi-
cally incompatible with the ground water and was weakened
due to chemical reactions. Another example is where the
operational life of the monitoring well exceeds the design life.
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If a well was installed for short-term water level measurements
and the well ultimately is used for long-term sample collection,
problems with material comparability may occur. Additionally,
if the well intake openings are improperly sized and/or if the
filter pack is incorrectly designed or installed, siltation and
turbid water samples can result.
Installation —
If productive zones are not accurately identified during the
well drilling process, well intakes can be improperly located or
zones can be improperly sealed. Incorrect installation proce-
dures and/or difficulties may also cause dislocation of well
intakes and/or seals. Improperly connected or corroded casing
can separate at joints or collapse and cause interaquifer con-
tamination. Improperly mixed grout can form inadequate seals.
If casing centralizers are not used, grout distribution may be
inadequate. If the casing is corroded or the bentonite seal not
properly placed, grout may contaminate the water samples.
Drilling mud filtrate may not have been completely removed
during the development process. The surface seal could have
been deteriorated or could have been constructed improperly,
and surface water may infiltrate along the casing/borehole
annulus. The intake filter pack must be properly installed.
Development —
Drilling mud, natural fines or chemicals used during drill-
ing must be removed during the development process. If these
constituents are not removed, water-sample quality may be
compromised. Chemicals can also cause screen corrosion,
shale hydration or plugging of the well intake. In general, the
use of chemicals is not recommended and any water added
during the development process must be thoroughly tested.
Borehole stability —
Unstable boreholes contribute to casing failure, grout fail-
ure or screen failure. Borehole instability can be caused by
factors such as improper well intake placement, excessive
entrance velocity or shale hydration.
Incrustation —
There are four types of incrustation that reduce well pro-
duction: 1) chemical, 2) physical, 3) biological or 4) a combi-
nation of the other three processes. Chemical incrustation may
be caused by carbonates, oxides, hydroxides or sulfate deposi-
tions on or within the intake. Physical plugging of the wells is
caused by sediments plugging the intake and surrounding
formation. Biological incrustation is caused by bacteria growing
in the formation adjacent to the well intake or within the well.
The bacterial growth rate depends on the quantities of nutrients
available. The velocity at which the nutrients travel partially
controls nutrient availability. Examples of common bacteria
found in reducing conditions in wells include sulphur-splitting
and hydrocarbon-forming bacteria iron-fixing bacteria occur
in oxidizing conditions. Some biological contamination may
originate from the ground surface and be introduced into the
borehole during drilling. Nutrients for the organisms may also
be provided by some drilling fluids, additives or detergents.
Incrustation problems are most commonly caused by a
combination of chemical-physical, physical-biological or a
combination of chemical-physical-biological incrustations.
Particulate moving through the well intake may be cemented
by chemical/biological masses.
Downhole Maintenance
Many wells accumulate sediment at the bottom. Sand and
silt may penetrate the screen if the well is improperly developed
or screen openings improperly sized. Rocks dropped by rock
and bong technologists (Stewart, 1970), insects or waterlogged
twigs can also enter the well through casing from the surface.
Sediment can also be formed by precipitates caused by constitu-
ents within the water reacting with oxygen at the water surface
(National Council of the Paper Industry for Air and Stream
Improvement 1982).
If sediment build-up occurs, the sediment should be re-
moved. A sediment layer at the bottom of the well encourages
bacterial activity that can influence sample quality. In wells that
are less than 25 feet deep, sediment can be removed by a
centrifugal pump, and an intake hose can be used to "vacuum"
the bottom of a well. In wells deeper than 25 feet, a hose with
afoot valve can be used as a vacuum device to remove sediment.
In some situations, bailers can also be used to remove sediment.
Sediment should be removed before purging and sampling to
eliminate sample turbidity and associated questions about sample
validity.
More traditional maintenance/rehabilitation techniques
used to restore yields of water supply wells include chemical
and mechanical methods that are often combined for optimum
effectiveness. Three categories of chemicals are used in tradi-
tional well rehabilitation: 1) acids, 2) biocides and 3) surfac-
tant. The main objectives of chemical treatment are: 1) to
dissolve the incrustants deposited on the well intake or in the
surrounding formation, 2) to kill the bacteria in the well or
surrounding formation and 3) to disperse clay and fine materials
to allow removal. Table 42 lists typical chemicals and applica-
tions in the water supply industry. Chemicals have very limited
application in the rehabilitation of monitoring wells because the
chemicals cause severe changes in the environment of the wells.
These changes may last for a long time or may be permanent.
Before redevelopment with chemicals is considered, the nega-
tive aspects of chemical alteration in an existing well with a long
period of record must be evaluated against negative aspects of
replacing the old well with a new well that may have new
problems and no history. If chemical rehabilitation is at-
tempted, parameters such as Eh, pH, temperature and conduc-
tivity should be measured. These measurements can serve as
values for comparison of water quality before and after well
maintenance.
Mechanical rehabilitation includes: overpumping, surg-
ing, jetting and air development. These processes are the same
as those used in well development and are described in greater
detail in the section entitled "Methods of Well Development."
Development with air is not recommended because the intro-
duction of air can change the chemical environment in the well.
Any type of rehabilitation for incrustation can be supplemented
by use of a wire brush or mechanical scraper with bailing or
pumping to remove the loose particles from the well.
Exterior Well Maintenance
Maintenance must also be performed on the exposed parts
of the well. Any well casing; well cap, protective casing,
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Table 41. Regional Well Maintenance Problems (Gass et al., 1980)
Ground Water Regions
Most Prevalent
Aquifer Types
• Most Prevalent Well Problems
1, Western Mountain Ranges
2. Alluvial Basins
3. Columbia Lava Plateau
4. Colorado Plateau,
Wyoming Basin
5. High Plains
6. Unglaciated Central Region
7, Glaciated Central Region
8. Unglaciated Appalachians
9. Glaciated Appalachians
10. Atlantic and Gulf Coast Plain
Alluvial
Sandstone
Limestone
Alluvial
Basaltic lavas
Alluvial
interbedded sandstone
and shale
Alluvial
interbedded sandstone,
limestone, shale
Alluvial
Sandstone
Limestone
Alluvial
Sandstone
Metamorphic
Limestone
Alluvial
Alluvial
Consolidated sedimentary
Alluvial and semiconsolidated
Consolidated sedimentary
Silt, clay, sand intrusion, iron; scale deposition; biological fouling.
Fissure plugging; casing failure; sand production.
Fissure plugging by clay and silt; mineralization of fissures.
Clay, silt, sand intrusion; scale deposition; iron; biological fouling;
limited recharge; casing failure.
Fissure and vesicle plugging by clay and silt; some scale deposition.
Clay, silt, sand intrusion; iron; manganese; biological fouling.
Low initial yields; plugging of aquifer during construction by
drilling muds and fines (clay and silt) natural to formations; fissure
plugging; limited recharge; casing failure.
Clay, silt, sand intrusion; scale deposition; iron; biological fouling;
limited recharge.
Low initial yield; plugging of voids and fissures; poor development
and construction; limited recharge.
Clay, silt, sand intrusion; scale deposition; iron; biological fouling.
Fissure plugging by clay and silt; casing failure; corrosion: salt water
intrusion; sand production.
Fissure plugging by clay, silt, carbonate scale; saltwater intrusion.
Clay, silt, sand intrusion; scale deposition; iron; biological fouling.
Fissure plugging; sand intrusion; casing failure.
Low initial yield; fissure plugging by silt and day; mineraliztion of fissures.
Predominantly cavernous production: fissure plugging by day and silt;
mineralization of fissures.
Clay, silt, fine sand intrusion; iron; scale; biological fouling.
Clay, silt, sand intrusion; scale deposition; biological fouling; iron.
Fissure plugging; mineralization; low to medium initial yield.
Clay, silt, sand intrusion; mineralization of screens; biological fouling.
Mechanical and chemical fissure plugging; biological fouling; incrustation
of well intake structure.
.Excluding pumps and declining water table.
Table 42. Chemicals Used for Well Maintenance (Gass et cl., 1980)
Chemical Name Formula Application
Concentration
Acids and biocides
inhibitors
Hydrochloric acid
Sulfamic acid
Hydroxyacetic acid
Chlorine
Diethyithiourea
DOW A-73
Hydrated ferric sulfate
Aldec 97
Polyrad 110A
HCI
NH,SO,H
C.HA
CI2
(C2H5)NCSN (C2H6)
Fe2(S04).,. 2-3H20
Carbonate scale, oxides, hydroxides
Carbonate scale, oxides, hydroxides
Biocide, chelating agent, weak scale
removal agent
Biocide, sterilization, very weak acid
Metal protection
Metal protection
For stainless steel
With sulfamic acid
Metal protection
15%; 2-3 times zone volume
15%; 2-3 times zone
50-500 ppm
0.2%
0.01%
1%
2%
.375%
volume
Chelating agents
Wetting agents
Surfactant
Citric acid
Phosphoric acid
Rochelle salt
Hydroxyacetic acid
Plutonic F-68
Plutonic L-62
DOW F-33
C6H,O7 Keeps metal ions in solution
H3PO. Keeps metal ions in solution
NaOOC (CHOH)2 COOK Keeps metal ions in solution
C2H40., Keeps metal ions in solution
Renders a surface non-repellent to a
wetting liquid
Renders a surface non-repellent to a
wetting liquid
Sodium Tripolyphosphate
Sodium Hexametaphosphate
Lowers surface tension of water thereby
increasing its cleaning power
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sampling tubing, bumper guard and/or surface seal should be
periodically inspected to ensure that monitoring well sample
quality will not be adversely affected. Suggested routine in-
spection and maintenance options should be considered:
1) Exposed well casing should be inspected. Well
casing should be of good structural integrity and
free of any cracks or corrosion;
2) The well cap should be removed to inspect for
spider webs, molds, fungi or other evidence of
problems that may affect the representativeness
of water samples. If no organisms and/or associated
evidence are found, the upper portion of the
casing should be cleaned with a long-handled
brush or other similar tool. The cleaning should
be scheduled after sample collection, and the well
should be completely purged after cleaning
(National Council of the Paper Industry for Air
and Stream Improvement, 1982);
3) When metal casing is used as protective casing
and a threaded cap is used, the casing should be
inspected for corrosion along the threads.
Corrosion can be reduced by lightly lubricating or
applying teflon tape to the threads to prevent
seizing. Corrosion of the casing can be reduced by
painting. If lubricants and/or paint are used, the
lubricants and/or paint should be prevented from
entering the well;
4) Where multilevel sampling tubes are used, the
tubes should rechecked for blockages and labeling
so that samples are collected from the intended
zones;
5) Where exterior bumper guards are used, the
bumperguards should be inspected for mechanical
soundness and periodically painted to retain
visibility; and
6) Surface seals should be inspected for settling and
cracking. When settling occurs, surface water can
collect around the casing. If cracking occurs or if
there is an improper seal, the water may migrate
into the well. Well seal integrity can best be
evaluated after a heavy rain or by adding water
around the outside of the casing. If the seal is
damaged, the seal should be replaced.
Comparative Costs of Maintenance
Evaluating the cost of rehabilitating a well versus abandon-
ing and redrilling the well is an important consideration. Factors
that should be evaluated are the construction quality of the
well, the accuracy of the well-intake placement and the preci-
sion of the documentation of the well. Capital costs of a new
well should also be considered. The actual "cost" of rehabilita-
tion is hard to calculate. Different rehabilitation programs may
be similar in technique and price but may produce very different
results. In some situations, different treatment techniques may
be necessary to effectively treat adjacent wells. Sometimes
techniques that once improved a well may only have a short-
term benefit or may no longer be effective. However, the cost
of not maintaining or rehabilitating a monitoring well maybe
very high. The money spent through the years on man-hours for
sample collection and laboratory sample analyses may be
wasted by the collection of unrepresentative data. Proper main-
tenance and rehabilitation in the long run is a good investment.
If rehabilitation is not successful, abandonment of the well
should be considered.
Well Abandonment
Introduction
Unplugged or improperly plugged abandoned wells pose a
serious threat to ground water. These wells serve as a pathway
for surface pollutants to infiltrate into the subsurface and
present an opportunity for various qualities of water to mix.
Currently, many sites are being monitored for low concentra-
tions of contaminants. As detection limits are lowered, it
becomes more important to have confidence in the monitoring
system. An improperly installed or maintained monitoring
network can produce anomalous sample results. Proper aban-
donment is crucial to the dependability of the remaining or new
installations.
The objectives of an abandonment procedure are to: 1)
eliminate physical hazards; 2) prevent ground-water contami-
nation, 3) conserve aquifer yield and hydrostatic head and 4)
prevent intermixing of subsurface water (United States Envi-
ronmental Protection Agency, 1975; American Water Works
Association, 1984). The purpose of sealing an abandoned well
is to prevent any further disturbance to the pre-existing
hydrogeologic conditions that exist within the subsurface. The
plug should prevent vertical movement within the borehole and
confine the water to the original zone of occurrence.
Many states have regulations specifying the approved
procedures for abandonment of water supply wells. Some states
require prior notification of abandonment actions and extensive
documentation of the actual abandonment procedures. How-
ever, few states have specific requirements for abandonment of
monitoring wells.
Well Abandonment Considerations
Selection of the appropriate method for abandonment is
based on the information that has been compiled for each well.
Factors that are considered include 1) casing material, 2)
casing condition, 3) diameter of the casing, 4) quality of the
original seal, 5) depth of the well, 6) well plumbness, 7)
hydrogeologic setting and 8) the level of contamination and the
zone or zones where contamination occurs. The type of casing
and associated tensile strength limit the pressure that can be
applied when pulling the casing or acting as a guide when
overdrilling. For example, PVC casing may break off below
grade during pulling. The condition of any type of casing also
may prohibit pulling. The diameter of the casing may limit the
technique that is selected. For example, hollow-stem augers
may not be effective for overdrilling large-diameter wells
because of the high torque required to turn large-diameter
augers. The quality of the original annular seal may also be a
determining factor. For example, if a poor seal was constructed,
then pulling the casing may be accomplished with minimum
effort. The depth of the well may limit the technique applied.
The plumbness of a well may influence technique by making
overdrilling or casing pulling more difficult. The hydrogeology
of the site may also influence the technique selected. For
example, hollow-stem augers may be used for overdrilling in
unconsolidated deposits but not in rock formations. The avail-
ability of a rig type and site conditions may also be determining
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factors. The level of contamination and zone in which contami-
nation occurs may modify the choice of technique. If no cross-
contamination can occur between various zones and contami-
nation cannot enter from the surface, grouting the well from
bottom to top without removing the casing maybe sufficient.
Well Abandonment Procedures
Well abandonment procedures involve filling the well with
grout. The well may be filled completely or seals placed in
appropriate zones and the well only partially filled with grout.
Completely filling the well minimizes the possibility of bore-
hole collapse and shifting of seals. The material used to fill the
well can be either carefully selected natural material with a
permeability that approximates the permeability of the natural
formation or a grout mixture with a lower permeability. If more
than one zone is present in the well, then either intermediate
seals must be used with natural materials or the well must be
grouted. Monitoring wells are most commonly abandoned by
completely filling the well with a grout mixture.
Wells can be abandoned either by removing the casing or
by leaving all or part of the casing in place and cutting the casing
off below ground level. Because the primary purpose of well
abandonment is to eliminate vertical fluid migration along the
borehole, the preferred method of abandonment involves cas-
ing removal. If the casing is removed and the borehole is
unstable, grout must be simultaneously emplaced as the casing
is removed in order to prevent borehole collapse and an inad-
equate seal. When the casing is removed, the borehole can be
sealed completely and them is less concern about channeling in
the annular space or inadequate casing/grout seals. However, if
the casing is left in place, the casing should be perforated and
completely pressure-grouted to reduce' the possibility of annu-
lar channeling. Perforating small-diameter casings in situ is
difficult, if not impossible.
Many different materials can be used to fill the borehole.
Bentonite, other clays, sand, gravel, concrete and neat cement
all may have application in certain abandonment situations.
Appendix C contains recommendations for well abandonment
that are provided by the American Water Works Association
(1984). These guidelines address the use of different materials
for falling the borehole indifferent situations. Regardless of the
type of material or combination of materials used for monitor-
ing well abandonment, the sealant must be free of contaminants
and must minimize chemical alteration of the natural ground-
water quality. For example, neat cement should not be used in
areas where the pH of the ground water is acidic. The ground
water will attack the cement and reduce the effectiveness of the
seal; the neat cement also raises the pH and alters ground-water
chemistry.
Procedures for Removing Casing —
If the well was not originally grouted, the casing maybe
pulled by hydraulic jacks or by "bumping" the casing with a rig.
A vibration hammer also may be used to speed up the task.
Casing cutters can be used to separate the drive shoe from the
bottom of the casing (Driscoll, 1986). If the well intake was
installed by telescoping, the intake may be removed by
sandlocking (United States Environmental Protection Agency,
1975).
A properly sized pulling pipe must, be used to successfully
implement the sandlocking technique. Burlap strips, 2 to 4
inches wide, and approximately 3 feet long are tied to the
pulling pipe. The pipe is lowered into the borehole to penetrate
approximately 2/3 of the length of the well intake. The upper
portion of the well intake above the burlap is slowly filled with
clean angular sand by washing the sand into the well, The
pulling pipe is then slowly lifted to create a locking effect.
Constant pressure is applied and increased until the well intake
begins to move. In some instances, jarring the pipe may assist
in well intake removal, but in some cases this action may result
in loss of the sand lock. As the well intake is extracted from the
well, the sand packing and pipe are removed. Many contractors
have developed variations of this sandlocking technique for
specific situations. For example, slots can be cut in the pulling
pipe at the level adjacent to the top of the well intake to allow
excess sand to exit through the pulling pipe. These slots prevent
the well intake from being overfilled and sandlocking the entire
drill sting. Slots can also be cut in the pipe just above the burlap
so that sand can be backwashes or bailed from the inside pipe
if the connection should need to be broken. Right and left-hand
couplings located between the drill pipe and pulling pipe may
be installed to disconnect the drill string if it becomes locked.
Well intakes that are 2 to 6 inches in diameter can be removed
by latch-type tools. For example, an elliptical plate cut in half
with a hinge may be used. The plate folds as it is placed in the
well and unfolds when lifted. If the well intake has a sump, the
tool can be locked under the sump; if there is no sump, the tool
can be locked under the well intake (Driscoll, 1986).
Another technique that may be used in conjunction with
sandlocking involves filling the borehole with a clay-based
drilling fluid through the pulling pipe while pulling the well
intake and casing from the bottom. The fluid prevents the
borehole from collapsing. The level of the fluid is observed to
determine if the borehole is collapsing. Fluid rises if collapse is
occurring. If fluid is falling, it is an indication that fluid is
infiltrating into the surrounding formation. In this technique,
the borehole is grouted from the bottom to the surface.
Overdrilling can also be used to remove casing from the
borehole. In overdrilling, a large-diameter hollow-stem auger
is used to drill around the casing. A large-diameter auger is used
because a larger auger is less likely to veer off the during during
drilling. The hollow stem should beat least 2 inches larger than
the casing that is being removed. For example, a 3 1/4-inch
inside-diameter auger should not be used to overdrill a 2-inch
diameter casing. The augers are used to drill to the full depth of
the previous boring. If possible, the casing should be pulled in
a "long" string, or in long increments. If the casing sticks or
breaks, jetting should be used to force water down the casing
and out the well intake. If this technique fails, the augers can be
removed one section at a time and the casing can be cut off in
the same incremental lengths. After all casing has been re-
moved, the hollow-stem augers are reinserted and rotated to the
bottom of the borehole. All the debris from the auger interior
should be cleaned out, the augers extracted and the borehole
filled with grout by using a tremie pipe (Wisconsin Department
of Natural Resources, 1985). The technique of overdrilling is
not limited to hollow-stem augers. Overdrilling can also be
accomplished by direct rotary techniques using air, foam or
mud.
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Limiting factors in overdrilling are the diameter of the well
and the hydrogeology of the surrounding formation. When
overdrilling, an attempt should be made to remove all annular
sealant so a good seal can be obtained between the borehole wall
and the grout. The plumbness of the original installation is a! so
very important if the well was not installed plumb, then
overdrilling may be difficult.
A variation of overdrilling was used by Perrazo et al.
(1984) to remove 4-inch PVC casing from monitoring wells.
First, the well was filled with a thick bentonite slurry to prevent
the PVC cuttings from settling in the borehole. The auger was
regularly filled with slurry to keep the casing full and to form a
mudcake on the wall. This mudcake served as a temporary seal
until a permanent seal was installed. A hollow-stem auger was
used with a 5 to 10-foot section of NW rod welded onto the lead
auger for use as a guide in drilling out the PVC casing. The auger
was rotated, and the casing was cut and spiraled to the surface.
A 2-inch diameter roller bit was threaded onto a drill rod and
advanced to ensure the bottom area would be sealed to the
original depth. The grout mixture was pumped down the drill
stem and out the roller bit, displacing the bentonite slurry and
water to the surface. In wells where there was not sufficient
pressure to displace the bentonite slurry and standing water, the
roller bit and drill stem were removed, a pressure cap was
threaded onto the top auger flight and grout was pumped
through the cap until increasing pressure forced the grout to
displace the bentonite slurry and water. The augers were then
removed and the grout was alternately "topped off as each
flight was removed.
Another technique involves jetting casing out of the well
with water. If the casing sticks or breaks off, a small-diameter
fish tail-type bit is connected to an A-rod to drill out the
thermoplastic casing. The drilling fluid flushes the cuttings to
the surface. After the borehole is cleaned, a tremie pipe is used
to emplace grout from the bottom to the surface (Wisconsin
Department of Natural Resources, 1985).
Procedures for Abandonment Without
Casing Removal —
If the casing is in poor condition, the interval adjacent to the
water-bearing zones can be ripped or perforated with casing
rippers, and then the casing is filled and pressure grouted
(United States Environmental Protection Agency, 1975; Driscoll,
1986). A concern when using this method is the accurate
placement and effectiveness of the cuts (Perazzo et al., 1984).
Casing may begun-perforated by using a device that fires steel
projectiles through the casing and into the formation. A jet-
perforating device may be used that is similar to the gun-
perforator except that a pre-shaped charge of high explosives is
used to bum holes through the casing (Ingersoll-Rand, 1985).
The top portion of the casing is then pulled so that a watertight
plug in the upper 15 to 20 feet can be attained. This step may be
omitted where the annular space was originally carefully grouted
(Driscoll, 1986).
Using Plugs —
Three types of bridge plugs can be used to isolate hydraulic
zones. These include: 1) permanent bridge seals, 2) intermedi-
ate seals and 3) seals at the uppermost aquifer. The permanent
bridge seal is the most deeply located seal that is used to form
a "bridge" upon which fill material can be placed. Permanent
bridge seals prevent cross-contamination between lower and
upper water-bearing zones. Permanent seals are comprised of
cement. Temporary bridges of neoprene plastic or other elas-
tomers can provide support for a permanent bridge during
installation (United States Environmental protection Agency,
1975).
Intermediate seals are located between water-bearing zones
to prevent intermixing of different-quality water. Intermediate
seals are comprised of cement, sand/cement or concrete mixes
and are placed adjacent to impermeable zones. The remaining
permeable zones are filled with clean disinfected sand, gravel
or other material (United States Environmental Protection
Agency, 1975).
The seal at the uppermost aquifer is located directly above
the uppermost productive zone. The purpose is to seal out
surface water. An uppermost aquifer seal is typically comprised
of cement, sand/cement or concrete. In artesian conditions, this
seal prevents water from flowing to the surface or to shallower
formations (United States Environmental Protection Agency,
1975). This plugging technique is generally used to isolate
usable and non-usable zones and has been used extensively in
the oil and gas industry.
If artesian conditions are encountered, several techniques
can be used to abandon the well. To effectively plug an artesian
well, flow must be stopped and the water level lowered during
seal emplacement. The water level can be lowered by: 1)
drawing down the well by pumping nearby wells, 2) placing
fluids of high specific gravity in the borehole or 3) elevating the
casing high enough to stop the flow (Driscoll, 1986). If the rate
of flow is high, neat cement or sand/cement grout can be piped
under pressure, or a packer can be located at the bottom of the
confining formation above the production zone (United States
Environmental Protection Agency, 1975). Fast-setting cement
can sometimes be used in sealing artesian wells (Herndon and
Smith, 1984).
Grouting Procedures for Plugging
All materials used for grouting should be clean and stable;
water used should be free from oil and other contaminants
(Driscoll, 1986). Grout should be applied in one continuous
grouting procedure from bottom to top to prevent segregation,
dilution and bridging of the sealant. The end of the tremie pipe
should always remain immersed in the slurry of grout through-
out the emplacement procedure. Recommendations for grout
proportions and emplacement procedures are discussed in the
section entitled "Annular Seals."
Many states permit or recommend a cement/bentonite
mixture. The bentonite possesses swelling characteristics that
make it an excellent plugging material (Van Eck, 1978). The
grout mixture used should be compatible with soil and water
chemistry. For example, a salt-saturated cement should be used
for cementing in a salt-saturated area. The cement/bentonite
mixture should not extend through the vadose zone to the land
surface or be used in areas of low soil moisture because cracking
and channeling due to dessication can allow surface water to
infiltrate along the casing (Driscoll, 1986). To ensure that the
borehole was properly grouted, records should be kept of the
132
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calculated volume of the borehole and the volume of grout that
was used; any discrepancy should be explained.
A concrete cap should be placed on the top of a cement/
bentonite plug. The concrete cap should be marked with apiece
of metal or iron pipe and then covered by soil. The metal allows
for easy location of the well in the future by a metal detector or
magnetometer.
Clean-up, Documentation and Notification
After abandonment is accomplished, proper site clean-up
should be performed. For example, any pits should be back-
filled and the area should be left clean (Fairchild and Canter,
1984). Proper and accurate documentation of all procedures
and materials used should be recorded. If regulations require
that abandonment of wells be reported, information should be
provided on the required forms and in compliance with the state
regulations. Table 43 shows information that is typically recorded
on a well abandonment form. The location of abandoned wells
should be plotted on a map and referenced to section lines, lot
lines, nearby roads and buildings as well as any outstanding
geological features (Aller, 1984).
Table 43. Welll Abandonment Data (After Wisconsin
Department of Natural Resources, 1985)
Name of property owner
Address of owner/property
Well location (street, section number, township and range)
Type of well installation method and date (drilled, driven,
bored, dug), purpose of well (OW, PIEZ, LYS)
Depth of well
Diameter of well
Depth of casing
Depth to rock
Depth to water
Formation type
Material overlying rock (clay, sand, gravel, etc.)
Materials and quantities used to fill well in specific zones,
detailing in which formations and method used
Casing removed or left in place
Firm completing work
Signature of person doing work
Address of firm
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137
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Appendix A
Drilling and Constructing Monitoring Wells With
Hollow-Stem Augers
[This report was produced as a part of this cooperative agreement
and was published by Hackett (1987 and 1988).]
Introduction
Since the 1950's, hollow-stem augers have been used
extensively by engineers and exploration drillers as a practical
method of drilling a borehole for soil investigations and other
Geotechnical work. The widespread use and availability of
hollow-stem augers for Geotechnical investigations has re-
sulted in the adaptation of this method to drilling and installing
ground-water monitoring wells. To date, hollow-stem augers
represent the most widely used drilling method among ground-
water professionals involved in constructing monitoring wells
(McCray, 1986). Riggs and Hatheway (1988) estimate that
more than 90 percent of all monitoring wells installed in
unconsolidated materials in North America are constructed by
using hollow-stem augers.
The drilling procedures used when constructing monitor-
ing wells with hollow-stem augers, however, are neither stan-
dardized nor thoroughly documented in the published litera-
ture. Lack of standardization is partially due to variable
hydrogeologic conditions which significantly influence hol-
low-stem auger drilling techniques and monitoring well con-
struction practices. Many of these construction practices evolved
in response to site-specific drilling problems which are unique
to hollow-stem augers.
This report presents an objective discussion of hollow-
stem auger drilling and monitoring well construction practices.
The drilling equipment will be reviewed, and the advantages
and limitations of the method for drilling and installing moni-
toring wells will be presented.
Auger Equipment
The equipment used for hollow-stem auger drilling in-
cludes either a mechanically or hydraulically powered drill rig
which simultaneously rotates and axially advances a hollow-
stem auger column. Auger drills are typically mounted on a
self-contained vehicle that permits rapid mobilization of the
auger drill from borehole to borehole. Trucks are frequently
used as the transport vehicle; however, auger drills may also be
mounted on all-terrain vehicles, crawler tractors or tracked
carriers (Mobile'Drilling Company, 1983). These drilling rigs
often have multi-purpose auger-core-rotary drills which have
been designed for Geotechnical work. Multi purpose rigs may
have: 1) adequate power to rotate, advance and retract hollow-
stem augers; 2) adequate drilling fluid pumping and tool hoisting
capability for rotary drilling; and 3) adequate rotary velocity,
spindle stability and spindle feed control for core drilling
(Riggs, 1986).
The continuously open axial stem of the hollow-stem auger
column enables the borehole to be drilled while the auger
column simultaneously serves as a temporary casing to prevent
possible collapse of the borehole wall. Figure 1 shows the
typical components of a hollow-stem auger column. The lead
end of the auger column is fitted with an auger head (i.e., cutter
head) that contains replaceable teeth or blades which breakup
formation materials during drilling. The cuttings are carried
upward by the flights which are welded onto the hollow stem.
A pilot assembly, which is commonly comprised of a solid
center plug and pilot bit (i.e., center head), is inserted within the
hollow center of the auger head (Figure 1). The purpose of the
center plug is to prevent formation materials from entering the
Drive Cap
Rod to Cap
Adapter
• Auger Connector
Hollow Stem
Auger Section
Center Rod
Center Plug
Pilot Assembly
Components
Pilot Bit
L .,— Auger Connector
^T
[T ^ Auger Head
\\^^ Replaceable
r\5 Carbide Insert
Auger Tooth
Figure 1. Typical components of a hollow-stem auger column
(after Central Mine Equipment Company, 1987).
141
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hollow stem of the lead auger, and the pilot bit assists in
advancing the auger column during drilling, A center rod,
which is attached to the pilot assembly, passes through the
hollow axis of the auger column. Once the borehole is advanced
to a desired depth for either sampling the formation or installing
the monitoring well, the center rod is used to remove the pilot
assembly. After a sample of the formation has been collected,
the center rod is used to reinsert the pilot assembly into the
auger head prior to continued drilling. The top of the center rod
is attached to a drive cap (Figure 1). The drive cap is used to
connect the auger column to the spindle of the drill rig. This
"double adapter" drive cap ensures that the center rod and pilot
assembly rotate along with the auger column.
The auger column is comprised of a series of individual
hollow auger sections which are typically 5 feet in length.
These individual 5-foot auger sections are joined together by
either slip-fit keyed box and pin connections, slip-fit box and
pin connections or threaded connections (Figure 2). The major-
ity of hollow-stem augers have keyed, box and pin connections
for transfer of drilling torque through the coupling and for easy
coupling and uncoupling of the auger sections (Riggs, 1987).
Box and pin connection of the connections use an auger bolt to
prevent the individual auger sections from slipping apart when
the auger column is axially retracted from a borehole (Figures
2a and 2b). Where contaminants area concern at the drilling
site, an o-ring may be used on the pin end of the connection to
minimize the possible inflow of contaminants through the joint.
Joints with o-rings will leak as the o-rings become worn and it
is difficult to assess the degree of wear at each joint in the auger
column when drilling. Augers with watertight threaded connec-
tions are available; however, these threaded connections
commonly are used with commercial lubricants which may
contain hydrocarbon or metallic based compounds. When
threaded hollow-stem augers are used for the installation of
water-quality monitoring wells, the manufacturer recommends
that no lubricants be used on the threads (H.E. Davis, Vice
President Mobile Drilling Pacific Division, personal communi-
cation, 1987). When lubricants are used on the hollow-stem
auger threads, a nonreactive lubricant, such as a fluorinated
based grease, may be used to avoid introducing potential
contaminants that may affect the ground-water samples col-
lected from the completed well.
The dimensions of hollow-stem auger sections and the
corresponding auger head used with each lead auger section are
not standardized between the various auger manufacturers. A
typical range of hollow-stem auger sizes with slip-fit, box and
pin connections is shown in Table 1, and the range of hollow-
stem auger sizes with threaded connections is shown in Table
2. Hollow-stem auger diameters are typically referenced by the
inside versus the outside (i.e., flighting) diameter. All refer-
ences made to the diameter of the hollow-stem auger in this
report will refer to the inside diameter, unless stated otherwise.
Tables 1 and 2 also list the cutting diameter of the auger heads
which are mounted on the lead augers. Common diameters of
hollow-stem augers used for monitoring well construction
range from 3 1/4 to 8 1/4 inches for slip-fit, box and pin
connected augers and 3 3/8 to 6 inches for threaded augers.
The hollow axis of the auger column facilitates the collec-
tion of samples of unconsolidated formations, particularly in
unsaturated cohesive materials. Two types of standard sam-
Key Way
Auger Bolt
• O-Ring
a. Keyed, Box and Pin Connection
Auger Bolt
Pin End
b. Box and Pin Connection
c. Threaded Connection
Figure 2. Three common methods for connecting hollow-stem
auger sections.
142
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Table 1. Typical Hollow-Stem Auger Sizes with Slip-Fit, Box and Pin Connections (from Central Mine Equipment Company, 1987)
Hollow-Stem
Inside Diameter (In.)
Flighting Diameter
(in.)'
Auger Head
Cutting Diameter (in.)
2114
2314
3114
33/4
4114
6114
8114
55/8
6118
65/8
7118
7518
95/8
11 5/8
61/4
6314
7114
7314
81/4
10114
12112
.NOTE: Auger flighting diameters should be considered minimum manufacturing dimensions.
Table 2. Hollow-Stem Auger Size with Threaded Connections (from Mobile Drilling Company, 1982)
Hollow-Stem
Inside Diameter (in.)
Flighting Diameter
(in.)"
Auger Head
Cutting Diameter (In.)
21/2
3318
4
6114
8114
81/2
11
8
9
11
13114
" NOTE: Auger flighting diameters should be considered minimum manufacturing dimensions.
piers which are used with hollow-stem augers are split barrel
and thin-walled tube samplers.
Split-barrel samplers are typically driven 18 to 24 inches
beyond the auger head into the formation by a hammer drop
system. The split-barrel sampler is used to collect a represen-
tative sample of the formation and to measure the resistance of
the formation to penetration by the sampler. The samples are
used for field identification of formation characteristics and
may also be used for laboratory testing. Thin-walled tube
samplers may be advanced a variable length beyond the auger
head either by pushing or driving the sampler into the format-
ion. These samplers are designed to recover relatively un-
disturbed samples of the formation which are commonly used
for laboratory testing. Standard practices for using split-barrel
samplers and thin-wall tube samplers are established under
ASTM Standards Dl586-84 and Dl587-83, respectively. The
ability of hollow-stem augers to accommodate these samplers,
and thus to permit the collection of undisturbed samples of the
formation, is often cited as a major advantage of the hollow-
stem auger method of drilling (Minning, 1982; Richter and
Collecting, 1983; Gass, 1984).
In addition to these standard samplers, continuous sam-
pling tube systems are commercially available which permit the
collection of unconsolidated formation samples as the auger
column is rotated and axially advanced (Mobile Drilling Com-
pany, 1983; Central Mine Equipment Company, 1987). Con-
tinuous sampling tube systems typically use a 5-foot barrel
sampler which is inserted through the auger head. The barrel
sampler replaces the traditional pilot assembly during drilling;
however, the sampler does not rotate with the augers. The open
end of the sampler extends a short but adjustable distance
beyond the auger head, and this arrangement allows sampling
to occur simultaneously with the advancement of the auger
column. After the auger column has advanced a distance up to
5 feet, the loaded sampler is retracted from the auger column.
The loaded sampler is either immediately emptied and rein-
serted through the auger head or exchanged for another empty
sampler. Multi-purpose drill rigs that are capable of core
drilling can also use core barrels for coring either unconsoli-
dated material or rock.
Borehole Drilling
There are several aspects of advancing a borehole with
hollow-stem augers that are important considerations for ground-
water monitoring. For clarity and continuity, the topic of
drilling a borehole with hollow-stem augers will be presented
under three subheadings: 1) general drilling considerations; 2)
drilling with hollow-stem augers in the unsaturated and satu-
rated zones; and 3) potential vertical movement of contami-
nants within the borehole.
General Drilling Considerations
When drilling with hollow-stem augers, the borehole is
drilled by simultaneously rotating and axially advancing the
auger column into unconsolidated materials or soft, poorly
consolidated formations. The cutting teeth on the auger head
break up the formation materials, and the rotating auger flights
convey the cuttings upward to the surface. In unconsolidated
materials, hollow-stem auger drilling can be relatively fast, and
several hundred feet of borehole advancement per day is
possible (Keely and Boateng, 1987a). Drilling may be much
slower, however, in dense unconsoldiated materials and in
coarse materials comprised primarily of cobbles. A major
limitation of the drilling method is that the augers cannot be
used to drill through consolidated rock. In unconsolidated
deposits with boulders, the boulders may also cause refusal of
the auger column. According to Keely and Boateng (1978a),
this problem may be overcome in sediments with cobbles by
removing the pilot assembly from the auger head and replacing
the assembly with a small tri-cone bit. It is then possible to drill
through the larger cobbles by limited rotary drilling, without the
use of drilling fluids.
The depths to which a borehole may be advanced with a
hollow-stem auger depend on the site hydrogeology (i.e., den-
143
-------
sity of the materials penetrated and depth to water) and on the
available power at the spindle of the drill rig. Riggs and
Hatheway (1988) state that, as a general rule, the typical maxi-
mum drilling depth, in feet, with 3 1/4-inch to 4 1/4-inch
diameter hollow-stem augers, is equivalent to the available
horsepower at the drill spindle, multiplied by a factor of 1.5.
This general rule on maximum drilling depths may be influ-
enced by the types of formations being drilled. Hollow-stem
augers have been used to advance boreholes to depths greater
than 300 feet; however, more common depths of borehole
advancement are 75 to 150 feet (Riggs and Hatheway, 1988).
The United States Environmental Protection Agency (1986)
generally recognizes 150 feet as the maximum drilling depth
capability of hollow-stem augers in unconsolidated materials.
One significant advantage of using hollow-stem augers for
ground-water monitoring applications is that the drilling method
generally does not require the circulation of drilling fluid in the
borehole (Scalf et al, 1981; Richter and Colletme, 1983). By
eliminating or minimizing the use of drilling fluids, hollow-
stem auger drilling may alleviate concerns regarding the poten-
tial impact that these fluids may have on the quality of ground-
water samples collected from a completed monitoring well.
Without the use of drilling fluids, the drill cuttings may also be
more easily controlled. This is particularly important where the
cuttings are contaminated and must be contained for protection
of the drilling crew and for disposal. In addition, subsurface
contaminants encountered during the drilling process are not
continuously circulated throughout the borehole via a drilling
fluid.
The potential for formation damage from the augers (i.e.,
the reduction of the hydraulic conductivity of the materials
adjacent to the borehole) varies with the type of materials being
drilled. In homogeous sands and gravels, hollow-stem auger
drilling may cause minimal damage to the formation. Where
finer-grained deposits occur, however, smearing of silts and
clays along the borehole wall is common. Keely and Boateng
(1987a) indicate that interstratified clays and silts can be
smeared into coarser sand and gravel deposits and can thereby
alter the contribution of ground-water flow from various strata
to the completed monitoring well. Smearing of silts and clays
along the borehole wall may also be aggravated by certain
drilling practices that are designed to ream the borehole to
prevent binding of the auger column (Keely and Boateng,
1987a). These reaming techniques, which may be used after
each few feet of borehole advancement, include either rotating
the auger column in a stationary position or rotating the auger
column while the column is alternately retracted and advanced
over a short distance in the borehole.
The diameter of the borehole drilled by hollow-stem au-
gers is influenced by the outside diameter of the auger head and
auger flighting, the type of formation material being drilled and
the rotation of the augers. As shown in Tables 1 and 2, the
cutting diameter of the auger head is slightly larger than the
corresponding outside diameter of the flighting on the hollow-
stem auger. The cutting diameter of the auger head will there-
fore initially determine the diameter of the borehole. However,
as the cuttings are conveyed up the flights during drilling, the
diameter of the borehole may also be influenced by the packing
of the cuttings on the borehole wall. Cuttings from cohesive
formation materials with silts and clays may easily compact
along the borehole wall, whereas noncohesive sands and
gravels may not. Where cuttings are readily compacted on the
sidewalls, the borehole diameter may reflect the outside diam-
eter of the auger flights as opposed to the cutting diameter of the
auger head. In noncohesive materials, the borehole diameter
may be enlarged due to caving of the side walls. In addition,
reaming techniques used to prevent binding of the auger column
in the borehole often serve to enlarge the diameter of the
borehole beyond the outside diameter of the the auger flights.
The diameter of the borehole may also be influenced by the
eccentric rotation of the augers which do not always rotate
about a vertical axis. As a result of these factors, the borehole
diameter may be variable over the length of the borehole.
Drilling with Hollow-Stem Augers in the
Unsaturated and Saturated Zones
The drilling practices used to advance a borehole with
hollow-stem augers in saturated materials and unsaturated
materials are usually the same when drilling in finer-grained
deposits or compacted sands and gravels. However, certain
lossely compacted saturated sands, known as "heaving sands"
or "sandblows," may pose a particular drilling difficulty
(Minning, 1982; Perry and Hart, 1985; Keely and Boateng,
1987a). Heaving sands can necessitate changes in basic drilling
equipment and changes in drilling practices. The following
discussion focuses first on the drilling procedures used to
advance a borehole through the unsaturated zone. These
procedures are then contrasted with the drilling techniques used
to advance the auger column into saturated heaving sands.
Unsaturated Zones —
When drilling in the unsaturated zone, the hollow-stem
auger column is typically comprised of the components shown
in Figure 1. A pilot assembly, center rod and drive cap
commonly are used, and the borehole is advanced without the
use of a drilling fluid. When the borehole has been advanced to
a desired sampling depth, the drive cap is detached from the
auger column, and the center rod and pilot assembly are
removed from the hollow axis of the auger column (Figures 3 a
and 3b). A split barrel sampler or thin-walled tube sampler,
attached to a sampling rod, is then lowered through the axis of
the hollow-stem column. The sampler is advanced beyond the
auger head either by driving or pressing the sampler into the
formation materials (Figure 3c). The loaded sampler and sam-
pling rod are removed from the auger column, and the pilot
assembly and center rod are reinserted prior to continued
drilling. When formation samples are required at frequent
intervals during borehole advancement, the sequential removal
and reinsertion of the pilot assembly and center rod can be time
consuming. In order to minimize the time required to collect
undisturbed formation samples, continuous sampling tube sys-
tems can be used to replace the traditional pilot assembly.
Continuous samplers enable the collection of formation samples
simultaneously with the advancement of the borehole (Figure
4). Driscoll (1986) states that the pilot assembly and center rod
may be omitted when drilling through some dense formation
materials because these cohesive materials usually form only a
limited 2 to 4-inch thick blockage of material inside the hollow
center of the auger head. Drilling with an open auger head in
the unsaturated zone, however, is not a common practice and is
not recommended where detailed samples of the formation are
required.
144
-------
^fo®p*W§y|
;.:O".'
Wi^i?$£$&§'$£t
L "!K ,Cv **.'o AJ* .O ' V1, • e*'^-* -..5-r "~Vft- • s
jjg'Auger
Jy Column ^
£& Column ?
p^»-\. j-
..,?
'-
Split Barref;
or Thin
,Walled Tube
^Sampler .•£
Open Axis ol
Auger
Column with
Pilot
Assembly
kand Center j
Removed
Wf^il^fS
•b'o.V^T.-.O.'o - i-V'-^J^VS-^-o • :?'a "
'•'Sampling'
b-Rod
i
-'
§^€»«I
^>^§
^•o« a "i^,-. O-^.-;->T*«J
^tfi^i^^c
n:oyb;t.?.- OioSvoSSUn'
iO
00^0.:^:v•A^o>.i•\>.o:•.Vo•or
Figure 3. Sequential steps showing borehole advancement with pilot assembly and collection of a formation sample
(after Riggs, 1983).
Heaving Sands —
The drilling techniques used to advance the auger column
within heaving sands may vary greatly from those techniques
used when drilling in unsaturated materials. The problem may
occur when the borehole is advanced to a desired depth without
the use of drilling fluids for the purpose of either sampling the
formation or installing a monitoring well. As the pilot assembly
is retracted, the hydrostatic pressure within the saturated sand
forces water and loose sediments to rise inside the hollow center
of the auger column (Figure 5). Keely and Boateng (1987a)
report that these sediments can rise several tens of feet inside
the lower auger sections. The resulting "plug" of sediment
inside the hollow auger column can interfere with the collec-
tion of formation samples, the installation of the monitoring
well or even additional drilling.
The difficulties with heaving sands may be overcome by
maintaining a positive pressure head within the auger column.
A positive pressure head can be created by adding a sufficient
amount of clean water or other drilling fluid inside the hollow
stem. Clean water (i.e., water which does not contain analytes
of concern to a monitoring program) is usually preferred as the
drilling fluid in order to minimize potential interference with
samples collected from the completed well. The head of clean
water inside the auger column must exceed the hydrostatic
pressure within the sand formation to limit the rise of loose
sediments inside the hollow-stem. Where the saturated sand
formation is unconfined, the water level inside the auger col-
umn is maintained above the elevation of the water table. Where
the saturated sand formation is confined, the water level inside
the auger column is maintained above the potentiometric sur-
face of the formation. If the potentiometric surface of the
formation rises above the ground elevation, however, the heav-
ing sand problem may be very difficult to counteract and may
represent a limitation to the use of the drilling method.
There are several drilling techniques used to maintain a
positive pressure head of clean water within the auger column.
One technique involves injecting clean water through the auger
column during drilling. This method usually entails removal of
the pilot assembly, center rod and drive cap. A special coupling
or adapter is used to connect the auger column to the spindle of
145
-------
Auger Drilling
Auger Column
Barrel Sampler
— Non-rotating
Sampling Rod
Auger Head
Figure 4. Diagram of continuous sampling tube system (after
Central Mine Equipment Company, 1987).
the drilling rig. Clean water is then injected either through the
hollow-center coupling or through the open spindle of the drill
rig as the auger column is advanced (Figure 6). Large diameter,
side-feed water swivels are also available and can be installed
between the drive cap and the hex shank which connects the
auger column to the spindle of the drill rig. Clean water is
injected through the water swivel and into the auger column as
the augers are advanced.
Another drilling technique used to overcome heaving
sands is to first advance the auger column by using a
"nonretrievable" knock-out plate. The knock-out plate is wedged
inside the auger head and replaces the traditional pilot assembly
and center rod (Figure 7a). A major disadvantage of this
drilling technique is that the knock-out plate cannot be alter-
nately removed and reinserted from the auger column to permit
the collection of formation samples as the auger column is
advanced. Once the auger column is advanced to a desired
depth, the column is filled to a sufficient height with clean
water. A ramrod commonly is used to strike and remove the
knock-out plate from the auger head (Figure 7b). The head of
clean water in the auger column must exceed the hydrostatic
pressure in the sand formation to prevent loose sediments from
rising inside the auger column once the knock-out plate is
removed. The nonretrievable knock-out plate should be con-
structed of inert materials when drilling a borehole for the
installation of a water-quality monitoring well. This will mini-
mize concerns over the permanent presence of the knock-out
plate in the bottom of the borehole and the potential effect the
plate may have on ground-water samples collected from the
completed well.
Reverse flight augers represent another unique center plug
design which has had measured success in overcoming prob-
lems with heaving sands (C. Harris, John Mathes and Associ-
ates, personal communication, 1987). The flighting on the
center plug and center rod rotates in an opposite direction from
the flighting on the auger column (Figure 8). As the auger
column advances through the heaving sands, the sand deposits
arc pushed outward from the auger head by the reverse flighting
on the center plug. A sufficient head of clean water is main-
tained inside the auger column to counteract further the hy-
drostatic pressure in the heaving sand formation. Once drilling
is completed, the reverse flight center plug is slowly retracted
from the auger column so that movement of sand into the
hollow stem is not induced.
Although the use of clean water as drilling fluid is
recognized by the United States Environmental Protection
Agency as a proper drilling technique to avoid heaving sand
problems (United States Environmental protection Agency,
1986), the use of any drilling fluid maybe undesirable or pro-
hibited at some ground-water monitoring sites. In these in-
stances, the problem may be overcome by using commercial or
fabricated devices that allow formation water to enter the auger
column, but exclude formation sands. Perry and Hart (1985)
detail the fabrication of two separate devices that allow only
formation water to enter the hollow-stem augers when drilling
in heaving sands. Neither one of these two devices permit the
collection of formation samples as the auger column is ad-
vanced through the heaving sands. The first device consists of
a slotted coupling attached to a knock-out plate (Figure 9). As
the auger column advances below the water table, formation
water enters the auger column through the slotted coupling
(Figure lOa). When the auger column is advanced to the
desired depth, a ramrod is used to dislodge the knock-out plate
with slotted coupling from the auger head (Figure lOb). Perry
and Hart (1985) report that the slotted coupling generally is
successful in counteracting heaving sand problems. However,
where clays and silts are encountered during drilling, the
openings in the slotted coupling may clog and restrict format-
ion water from entering the auger column. To overcome this
plugging problem, Perry and Hart (1985) fabricated a second
device to be used when the slotted coupling became plugged.
The second device is actually a screened well swab (Figure 11).
The swab is connected to a ramrod and is lowered through the
auger column once the column is advanced to the desired
depth. The ramrod is used to strike and remove the knock-out
plate from the auger head (Figure 12). The screened well swab
filters the sand and allows only formation water to enter the
auger column (Perry and Hart, 1985). Once the water level rises
inside the auger column to a height that offsets the hydrostatic
pressure in the formation, the screened well swab is slowly
removed so that movement of sand into the hollow stem is not
induced.
Commercial devices that permit only formation water to
enter the auger column during drilling are also available. These
devices include a variety of patented designs, including
nonwatertight flexible center plugs. These devices replace the
146
-------
:•:;• Pilot Assembly
;':•'•... Being Retracted
Pilot
Assembly
a. Borehole Advanced into Saturated
Sand with Auger Column
Containing Pilot Assembly
Figure 5. Diagram showing heaving sand with hollow-stem auger drilling.
b. Movement of Loose Sands inlo the
Hollow Center of Auger as the Pilot
Assembly is Removed
traditional pilot assembly in the auger head. Some flexible
center plugs are seated, inside the auger head by means of a
specially manufactured groove in the hollow stem. These
flexible center plugs allow split-barrel samplers and thin-
walled tube samplers to pass through the center plug so that
samples of the water bearing sands can recollected (Figure 13).
The flexible center plug, however, cannot be retracted from the
auger head and therefore severely restricts the ability to install
a monitoring well through the auger column. The monitoring
well intake and casing can be inserted through the flexible
center plug, but the plug eliminates the installation of filter pack
and annular sealant (i.e., bentonite pellets) by free fall through
the working space between the well casing and auger column.
Potential Vertical Movement of Contaminants
Within the Borehole
The potential for contaminants to move vertically within
the borehole during drilling is an important consideration when
selecting a drilling method for ground-water monitoring. Ver-
tical mixing of contaminants from different levels within a
single borehole may be a problem with several different drilling
methods, including hollow-stem augers. As the auger column
advances through deposits which contain solid, liquid or gas-
phase contaminants, there may be a potential for these con-
taminants to move either up or down within the borehole.
Where vertical movement of contaminants occurs within the
borehole, the cross contamination may be a significant source
of sampling bias (Gillham et al, 1983).
Vertical movement of contaminants within the borehole
may occur when contaminants from an overlying stratum are
carried downward as residual material on the augers. The
potential for small amounts of contaminated material to adhere
to the auger head and lead auger is greatest in cohesive clayey
deposits (Gillham et al., 1983). Contaminants may also adhere
to split-barrel samplers and thin-walled tube samplers. If these
sampling devices are not adequately cleaned between usage at
successive sampling depths, contaminants from an overlying
stratum may be introduced in a lower stratum via the sampling
device. Where reaming techniques have enlarged the borehole
beyond the outside diameter of the auger flights, contaminants
147
-------
Auger
Drill Rig
Water Swivel
Clean Water
Spindle Adapter Assembly Used for
Injecting Fluids Inside Auger Column
Figure 6. Injecting clean water through open drill spindle to
counteract heaving sand (after Central Mine
Equipment Company, 1987).
from an overlying stratum may slough, fall down the annular
space and come in contact with a lower stratum (Keely and
Boateng, 1987a). Even small amounts of contaminants that
move downward in the borehole, particularly to the depth at
which the intake of the monitoring well is to be located, may
cause anomalous sampling results when analyzing samples for
contaminants at very low concentrations. According to Gillharm
et al. (1983), this potential for sampling bias is greatest at
monitoring sites where shallow geological formations contain
absorbed or immiscible-phase contaminants.
Contaminants may also move upward within a borehole
during hollow-stem auger drilling. As the auger column is
advanced through a stratum containing contaminants, the con-
taminants may be carried upward along with the cuttings.
Contaminated material from a lower stratum may therefore be
brought into contact with an uncontaminated overlying stratum
(Keely and Boateng, 1987a). Cohesive materials within the
contaminated cuttings may 'smear and pack the contaminants on
the sidewalk Where contaminants are displaced and smeared
on the sidewall at the intended monitoring depth, these contamin-
ants may serve as a persistent source of sampling bias.
Vertical movement of dissolved-phase contaminants within
a borehole may also occur where two or more saturated zones
with different heads are penetrated by the auger column. When
the water level in a contaminated, overlying saturated zone is
higher than the potentiometric surface of an underlying
uncontaminated zone, downward leakage of contaminated water
within the borehole may occur. This downward movement of
water may occur even if the augers are continually rotated in an
attempt to maintain the upward movement of cuttings (Gillham
et al., 1983). Conversely, the upward leakage of contaminants
in the borehole may occur where the potentiomernc surface of
an underlying contaminated zone is higher than the water level
in an overlying saturated zone.
The vertical movement of contaminants within the bore-
hole drilled with hollow-stem augers is not well documented in
the published literature. Lack of documentation is partially due
to the difficulty of diagnosing the problem in the field. The
determination that an aquifer was contaminated prior to dril-
ling, during drilling or after installation of the monitoring well
may not easily be made. Keel y and Boateng (1987b), however,
recount a case history in which apparent vertical movement of
contaminants in the borehole occurred either during hollow-stem
auger drilling antd/or after installation of the monitoring well.
This case study involves a site at which a heavily contaminated,
unconfined clayey silt aquifer, containing hard-chrome plating
wastes, is underlain by a permeable, confined sand and gravel
aquifer. Water samples collected from monitoring wells de-
veloped in the lower aquifer showed anomalous concentrations
for chromium. Although vertical ground-water gradientsat the
site were generally downward, the areal distribution and con-
centrations of chromium in the lower aquifer were not indica-
tive of long-term leakage through the aquitard. Based on their
investigation of the site, Keely and Boateng (1987b) conclude
that the localized pattern of chromium values in the lower
aquifer resulted from either vertical movement of contaminants
in the borehole or vertical movement of contaminants through
faulty seals along the casing of the monitoring wells. The
authors hypothesize that the vertical movement of the contamin-
ants in the borehole may have occurred when contaminated
solids from the upper aquifer fell down the annular space during
hollow-stem auger drilling.
The potential for cross contamination during drilling may
be reduced if contamination is known or suspected at a site.
Where a shallow contaminated zone must be penetrated to
monitor ground-water quality at greater depths, a large-diam-
eter surface casing may be used to seal off the upper contami-
nated zone before deeper drilling is attempted. Conventional
hollow-stem auger drilling alone, however, may not always be
adequate for installation of a larger diameter surface casing.
Depending on the hydrogeological conditions at the site, a
"hybrid" drilling method may be necessary in which conven-
tional hollow-stem auger drilling is combined with a casing
driving technique that advances the surface casing as the
borehole is advanced. Driving techniques used to advance and
install surface casing may include conventional cable tool
drilling, rotary drilling with casing hammer or a drop hammer
system on an auger drill rig.
Conventional hollow-stem auger drilling may be used to
set protective surface casing where the shallow geological
formations are comprised of cohesive materials. In this situa-
tion, a large-diameter borehole maybe advanced by the auger
column to a depth below the known contamination (Figure
14a). The auger column is then fully retracted from the borehole
at sites where the borehole will remain open due to the cohesive-
ness of the formation (Figure 14 b). A large-diameter surface
148
-------
Clean Wale/*Level
Within Auger Column"
a, Borehole Advanced into Saturated
Sand with Auger Column Containing
Nonretrievable Knock-Out Plate
b. Clean Water. Added to Auger
Column Along with Removal of
Knock-Out Plate by Ramrod
Figure 7. Use of a nonretrievable knock-out plate and auger column filled with clean water to avoid a heaving sand problem.
casing is then set and grouted into place. After grouting the
large-diameter surface casing into place a hollow-stem auger
column of smaller outside diamteter is used to advance the
borehole to the desired depth for installation of the monitoring
well (Figure 14c). Typical dimensions for augers used in this
scenario might be an 8 1/4-inch diameter hollow-stem auger
with an auger head cutting diameter of 12 1/2 inches to
advance the borehole below the contaminated zone. A nominal
10-inch diameter surface casing would commonly be installed
within the 12 1/2-inch diameter borehole. Four-and-one-
quarter-inch diameter augers with an eight-and-one-quarter-
inch auger head cutting diameter might then be used to continue
drilling after the surface casing is set.
When the shallow geological formations are comprised of
noncohesive materials and the borehole will not stand open, a
hybrid drilling technique can be used in which the surface
casing is advanced simultaneously with the auger column.
According to Keely and Boateng (1987a), this alternate drilling
technique is used to advance the auger column a few feet at a
time and then to drive the surface casing to the new borehole
depth. The auger column is telescoped inside the surface casing
as the casing is driven outside the augers (Figure 15). Five-foot
lengths of casing typically are used with this technique, and the
casing is driven either by using the same conventional 140-
pound drop hammer that is used to advance split-barrel samplers
or a heavier 300-pound drop hammer. The sequential steps of
augering and casing advancement continue until the surface
casing extends below the depth of known contamination. Once
the surface casing is set, a smaller diameter hollow-stem auger
column can be used to advance the borehole to the desired depth
for monitoring well installation.
Monitoring Well Installation
Monitoring wells may be constructed for water-quality
sampling, water-level measurement or both. The intended
purpose of the well influences the design components of a
monitoring well. The following discussion will focus on tech-
niques used to install water-quality monitoring wells which
consist of a well casing and intake, filter pack and annular seal.
The methods used to construct water-quality monitoring
wells with hollow-stem augers depend primarily on site
hydrogeology. In particular, the cohesiveness of the formation
149
-------
Auger Column—
Filled with Clean
Water as
Borehole is
Advanced
Reverse Flight
Auger and
Center Rod
Water Level
4
7
Saturated Sand
Formation
f
- Reverse Flight
Auger and
Center Rod
Slowly Retracted
from Auger
: Column
-'Auger Column
Filled with Clean
Water as Reverse
Flight Auger is
Retracted
a. Reverse Flight Auger Pushes
Cuttings Outwardly While Head
of Clean Water is Maintained
Inside Auger Column
b. Reverse Flight Auger Slowly
Being Retracted from Auger
Column
Figure 8. Use of a reverse flight auger to avoid a heaving sand problem (after Central Mine Equipment Company, 1987).
Nipple
Lock Nut
• Knock-Out Plate
• Slotted Coupling
Plug
Figure 9. Diagram of a slotted coupling
(after Perry and Hart, 1985).
materials penetrated by the auger column may influence the
well construction practices used. If the formation materials are
cohesive enough so that the borehole remains open, the entire
auger column may be retracted from the borehole prior to the
installation of the monitoring well casing and intake, filter pack
and annular seal. However, even in cohesive formation mate-
rials, drillers may refrain from the practice of fully retracting the
auger column from a completed borehole to avoid unexpected
caving of the borehole. The string of well casing and attached
intake may be centered in the open borehole by using casing
centralizers. The filter pack and annular sealant can then be
emplaced through the working annular space between the
borehole and well casing.
When the auger column penetrates noncohesive materials
and the borehole will not remain open, the auger column is used
as a temporary casing during well construction to prevent the
150
-------
Ramrod
Knock-Out Plate '•%&&£
with Slotted
Auger Column
Filled with
Formation Watei
Column as
Borehole is
Advanced
'"""Ramrod
a. Borehole Advanced into
Saturated Sand with Auger
Column Containing Nonretrievable
Knock-Out Plate with
Slotted Coupling
b. Knock-Out Plate with Slotted
Coupling Removed from Auger
Head by Ramrod
Figure 10. Use of a nonretrievable knock-out plate with a slotted coupling to avoid a heaving sand problem
(after Perry and Hart, 1985).
Supporting Pipe
Pipe Flange
-t- Brass Screen
Pipe Flange
Inside Diameter of
Hollow-Stem Auger
C - — Ball Valve
Nipple
Figure 11. Diagram of a screened well swab
(after Perry and Hart, 1985).
possible collapse of the borehole wall. When the auger column
is used as a temporary casing during well construction, the
hollow axis of the auger column facilitates the installation of the
monitoring well casing and intake, filter pack and annular
sealant. However, the practices that are used to emplace these
well construction materials through the working space inside
the hollow-stem augers are not standardized among contrac-
tors. Lack of standardization has resulted in concerns about the
proper emplacement of the filter pack and annular seal in the
monitoring well. To address these concerns, the topic of
monitoring well construction through hollow-stem augers is
presented in three separate discussions 1) well casing diameter
versus inside diameter of the hollow-stem auger 2) installation
of the filter pack; and 3) installation of the annular seal.
Well Casing Diameter Versus Inside Diameter of
the Hollow-Stem Auger
Once the borehole has been advanced to the desired depth
for installation of the monitoring well, the pilot assembly and
center rod (if used) are removed, and the depth of the borehole
is measured. A measuring rod or weighted measuring tape is
lowered through the hollow axis of the auger column. This
depth measurement is compared to the total length of the auger
151
-------
Ramrod with
Screened Welt
Swati Attached
Waier Level
Water Level
Rising Inside
Auger Column
After Removal of
Knock-Out Plate
Screened Weil
Swab, Attached io
Ramrod, Used to
Filler Out Sand and
Permit Formation
Water to Enter
Auger Column
Knock-out Plate
with Clogged
Slotted Coupling
Removed from
Auger Heed by
Ramrod
Figure 12. Use of a screened well swab to avoid a heaving and problem (after Perry end Hart, 1985).
column in the borehole to determine whether loose sediments
have risen inside the hollow stem. Provided that the hollow
stem is clear of sediment, a sting of well casing with attached
intake is lowered inside the auger column. Threaded, flush-joint
casing and intake are commonly used to provide a string of
casing with a uniform outside and inside diameter.
Although the well casing and intake may be centered
inside the auger column, many contractors place the well casing
and intake toward one side of the inner hollow-stem wall
(Figure 17). The eccentric placement of the casing and intake
within the hollow-stem auger is designed to create a maximum
amount of working space (shown by the distance "A" in Figure
17) between the outer wall of the casing and the inner wall of
the auger. This working space is used to convey and emplace the
filter pack and the annular sealant through the auger column.
Table 3 lists the maximum working space (A) that is available
between various diameters of threaded, flush-joint casing and
hollow-stem augers, if the casing is set toward one side of the
inner hollow-stem wall.
The selection of an appropriate sized hollow-stem auger
for drilling and monitoring-well construction should take into
account the nominal diameter of the well casing to be installed
and the working space needed to properly convey and emplace
the filter pack and annular sealant. The smallest hollow-stem
augers typically used for installing 2-inch nominal diameter
casing are 3 1/4-inch diameter augers; the smallest hollow-stem
augers typically used for installing 4-inch nominal diameter
casing are 6 1/4-inch diameter augers (Riggs and Hatheway,
1988). Table 3 shows, however, that the maximum working
space available between a 2-inch nominal diameter casing and
a 3 1/4-inch diameter hollow-stem auger is less than 1 inch (i.e.,
0.875 inch). This small working space can make the proper
emplacement of the filter pack and annular seal very difficult,
if not impossible. Too small a working space can either restrict
the use of equipment (i.e., tremie pipe) that maybe necessary
for the placement of the filter pack and annular seal or inhibit
the ability to properly measure the actual emplacement of these
materials in the borehole. A small working space can also
increase the possibility of bridging problems when attempting
to convey the filter pack and annular sealant between the
hollow-stem auger and well casing. Bridging occurs when the
filter pack or annular seal material spans or arches across the
152
-------
Table 3. Maximum Working Space Available Between Various Diameters of Threaded, Flush-Joint Casing and Hollow-Stem Augers
Nominal
Diameter
of Casing
(in.)
2
3
4
5
6
OuUide
Diameter
of Casing
*(in.)
2.375
3.500
4.500
5.563
6.625
Working Space "A" (see Figure 17) for
Various Inside Diameter Hollow-Stem
Augers ** (in.)
31/4 33/4 41/4 61/4
0.875 1.375 1.875 3.875
0.250 0.750 2.750
1.750
0.687
81/4
5.815
4.750
3.750
2.687
1.625
Based on ASTM Standards D-1785 and F-480
' inside diameters of hoiiow-stem augers taken from Table 1.
Flexible Center "?:|
Plug Permitting "-:
Collection of
Water-Bearing
Sands, but
Preventing
Heaving Sands
from Entering j
Hollow Stem M
Saturated Sand
Figure 13. Flexible center plug in an auger head used to overcome heaving sands and permit sampling of formation materials
(after Diedrich Drilling Equipment, 1986).
153
-------
Shallow
Contaminant
Zone
"^ Large-Diameter
— Auger Used to -
-x" Advance
_ Borehole in ~
_ -. Cohesive
— ' Materials
~_ Open Borehole •
< Shallow
Contaminant
Zone
_ "Protective
Surface
_ Set Below
_T~_ Contaminant
Zone
a. Large- Diameter Borehole —
I Advanced Below Known Depth
- of Contamination •
• Auger Column Retracted from-
Borehole Which Remains Open
Qy8 to 0ohesiwa Materials • —
Grouted Annular
±_ . Space
Small-Diameterl.
Auger Used to — • —
Advance ~H~.'J
Borehole to a I— ". "
Deeper Depth _-—_ __
for Installation of— , .
Monitoring Well • • —
c. Surface Casing Installed Below
Known Depth of Contamination
with Drilling Continued Using
Smaller Diameter Auger
Figure 14. Sequence showing the installation of protective surface casing through a shallow contaminated zone in a cohesive
space between the inner diameter of the auger and the outer
diameter of the casing. The bridge of filter pack or annular seal
material forms a barrier which blocks the downward movement
of additional material through the working space. As a result,
gaps or large unfilled voids may occur around the well intake or
well casing due to the nonuniform placement of the filter pack
or annular seal. Bridged material can lock the casing due to the
nonuniform placement of the filter pack or annular seal. Bridged
material can lock the casing and auger together and result in the
well casing being retracted from the borehole along with the
augers. Most contractors prefer to use 4 1/4-inch diameter
augers to install 2-inch nominal diameter casing, and 8 1/4-inch
diameter augers to install 4-inch nominal diameter casing to
create an adequate working space that facilitates the proper
emplacement of the filter pack and annular seal (C. Harris, John
Mathes and Associates, personal communication, 1987). Ac-
cording to United States Environmental Protection Agency
(1986), the inner diameter of the auger should be 3 to 5 inches
greater than the outer diameter of the well casing for effective
placement of the filter pack and annular sealant. Based on the
United Sates Environmental Protection Agency guideline for
effective working space, 6 1/4-inch diameter hollow-stem
augers would be the recommended minimum size auger for
installing a 2-inch nominal diameter casing. In addition, the
maximum diameter of a well which could be installed through
the hollow axis of the larger diameter augers, which are com-
monly available at this time, would be limited to 4 inches or less.
Installation of the Filter Pack
After the well casing and intake are inserted through the
hollow axis of the auger column, the next phase of monitoring
well construction commonly involves the installation of a filter
pack. The filter pack is a specially sized and graded, rounded,
clean silica sand which is emplaced in the annular space
between the well intake and borehole wall (Figure 16).
The primary purpose of the filter pack is to filter out finer-
sized particles from the formation materials adjacent to the well
intake. The filter pack also stabilizes the formation materials
and thereby minimizes settlement of materials above the well
intake. The appropriate grain size for the filter pack is usually
selected based on a sieve analysis of the formation material
adjacent to the well intake. The filter pack is usually a uniform,
well-sorted coarse to medium sand (i.e., 5.0 mm to 0.40 mm).
However, graded filter packs may be used in a monitoring well
which has an intake installed in a fine-grained formation. The
graded filter pack may filter and stabilize silt and clay-sized
Formation particles more effectively. The completion of a
154
-------
Drop Hammer
used to Drive
Casing
-'
i-^i&Borehale vtow:
Driven Flosfi
with Borehole
Wall in Non-
Cohesive
Materials '•
. Auger Column Advances
Borehole Slightly Beyond
Casing
b. Driving the Casing to the
New Borehole Depth
Figure 15. Sequence showing the installation of protective surface casing through a shallow contaminated zone in a noncohesive
formation (after Keely and Boateng, 1987a).
monitoring well with a properly sized, graded and emplaced
filter pack minimizes the extent to which the monitoring well
will produce water samples with suspended sediments.
The filter pack typically extends from the bottom of the
well intake to a point above the top of the intake (Figure 16).
The filter pack is extended above the top of the well intake to
allow for any settlement of the filter pack that may occur during
well development and to provide an adequate distance between
the well intake and the annular seal. As a general rule, the length
of the filter pack is 10 percent greater than the length of the
intake to compensate for settlement. United States Environ-
mental Protection Agency (1986) recommends that the filter
pack extend from the bottom of the well intake to a maximum
height of 2 feet above the top of the intake, with the maximum
height specified to ensure discrete sample horizons.
The thickness of the filter pack between the well intake and
borehole wall generally will not be uniform because the well
casing and intake usually are not centered in the hollow axis of
the auger column. The filter pack, however, should be at least
thick enough to completely surround the well intake. Tables 1
and 2 show that the cutting diameter of the auger head ranges
from 4 to 7 1/4 inches larger than the inside diameter of the
hollow-stem auger. When the well casing and intake are posi-
tioned toward one side of the inner hollow-stem wall (Figure
17), the annular space between the well intake and borehole
wall may be as small as 2 to 3 5/8 inches. This annular space
may still be adequate to preclude bridging and irregular em-
placement of the filter pack however, there is marginal
tolerance for borehole sloughing or installation error. The
proper installation of a falter pack with hollow-stem augers can
be difficult if there is an inadequate working space between the
casing and the auger column through which the filter pack is
conveyed (Minning, 1982; Richter and Collentine, 1983; Gass,
1984; schmidt 1986 Keely and Boateng, 1987b).
155
-------
Locking Casing Cap
Vent Hole
Protective Casing—^-
Qround Surface
Inner Casing Cap
— Lock
Draintole
Surface Seal
Filter Pack
Completion Depth -
r "J Water Table
r.t
Borehole
Well Intake
Plug
Figure 16. Typical design components of a ground-water
monitoring well.
The volume of filter pack required to fill the annular space
between the well intake and borehole wall should be predeter-
mined prior to the emplacement of the filter pack. In order to
determine the volume of filter pack needed, three design criteria
should be known. These three criteria include 1) the design
length of the fiter pack; 2) the diameter of the borehole; and 3)
the outside diameter of the well intake and casing. This infor-
mation is used to calculate both the volume of the borehole and
the volume of the well intake and casing over the intended
length of the filter pack. Once both volumes are calculated, the
volume of the well intake and casing is subtracted from the
volume of the borehole to determine the volume of filter pack
needed to fill the annular space between the well intake and
borehole wall. For example, Figure 18 illustrates a 2-inch
nominal diameter well casing and intake inserted through the
hollow axis of a 4 1/4-inch diameter hollow-stem auger. Based
on the cutting diameter of the auger head, the diameter of the
borehole is shown as 8 1/4 inches and the length of the well
intake is 10 feet. The design length of the filter pack is 12 feet
to ensure that the filter pack extends 2 feet above the top of the
intake. The volume of the borehole over the 12 foot design
length of the filter pack will be 4.36 cubic feet. Using 2.375
inches as the outside diameter of the well intake and casing, the
volume of the intake and casing over the 12-foot design length
of the filter pack will be 0.38 cubic feet. By subtracting 0.38
cubic feet from 4.36 cubic feet, the volume of filter pack needed
to fill the annular space is determined to be 3.98 or approxi-
mately 4 cubic feet.
Once the theoretical volume of filter pack is calculated, this
volume is divided by the design length of the filter pack to
determine the amount of the material which should be needed
to fill the annulus for each lineal foot that the auger column is
retracted. Referring again to the example illustrated in Figure
18,4 cubic feet divided by 12 feet would equal approximately
one-third cubic foot per foot. Therefore, for each foot that the
auger column is retracted, one-third cubic foot of filter pack
should be needed to fill the annular space between the well
intake and borehole wall.
The methods which are used to convey the filter pack
through the working space in the auger column and to emplace
this material in the annular space between the well intake and
borehole wall depend on: 1) the cohesiveness of the formation
materials; 2) the height of a standing water column in the
working space between the casing and augers; and 3) the grain-
size and uniformity coefficient of the filter pack.
In cohesive formation materials in which the borehole
stands open, the filter pack commonly is emplaced by axially
retracting the auger column from the borehole in short incre-
ments and pouring the filter pack down the working space
between the casing and auger column. Prior to filter pack em-
placement, a measuring rod or weighted measuring tape is
lowered to the bottom of the borehole through the working
space between the well casing and auger column (Figure 19a)
so that the total depth of the borehole can be measured and
recorded. The auger column is initially retracted 1 or 2 feet
from the borehole (Figure 19b). A measured portion of the
precalculated volume of the filter pack is slowly poured down
the working space between the well casing and auger column
(Figure 19c). The filter pack is typically poured at a point
diametrically opposite from the measuring rod or weighted
measuring tape. As the filter pack is being poured, the measur-
ing device is alternately raised and lowered to "feel" and
measure the actual placement of the filter pack. If a weighted
measuring tape is used as the measuring device, the tape is kept
in constant motion to minimize potential binding and loss of
the weighted tape as the filter pack is being poured. Continuous
measurements of the depth to the top of the emplaced filter
pack are usually made as the filter pack is slowly poured down
the working space in order to avoid allowing the emplaced filter
pack to rise up between the well intake/casing and the inside of
the hollow-stem auger. If the filter pack is permitted to rise up
between the casing and auger, the filter pack may lock the
casing and auger together and result in the casing being re-
tracted from the borehole along with the augers. Once the filter
pack is emplaced to the bottom of the auger column, the augers
are retracted another 1 to 2 feet and a second measured portion
of the filter pack is added. These steps are repeated until the
required length of filter pack is emplaced. By knowing the
theoretical amount of filter pack needed to fill the annular space
between the well intake and borehole wall for each increment
in which the auger column is retracted, the emplacement of the
filter pack may be closely monitored. Calculations of the "filter
pack needed" versus "filter pack used" should be made and
recorded for each increment that the auger column is retracted.
Any discrepancies should be explained.
Placement of filter pack by free fall through the working
space between well casing and auger column can present the
potential for bridging or segregation of the filter pack material.
As described earlier, bridging can result in unfilled voids within
the filter pack or in the failure of the filter pack materials to be
properly conveyed through the working space between the well
casing and auger column. Bridging problems, however, may be
minimized by: 1) an adequately sized working space between
the well casing and auger column; 2) slowly adding the filter
156
-------
Threaded, Flush-
Joint Casing
and Intake
Maximum
Working Space
Hollow-Stern
Auger
Inside Diameter of
Hollow-Stem Auger
-A-IL-
Outside D»ameter
of Casing
b. Cross-Sectional View
Figure 17. Plan and cross-sectional views showing the maximum working apace (A) between the well casing and the hollow-Stern
auger.
pack in small amounts; and 3) carefully raising and lowering
the measuring rod or weighted measuring tape while the filter
pack is being added.
Segregation of graded filter pack material during free fall
through the working space between the well casing and auger
column may still occur, especially where the static water level
between the casing and augers is shallow. As the sand-sized
particles fall through the standing column of water, a greater
drag is exerted on the smaller sand-sized particles due to the
higher surface area-to-weight ratio. As a result, coarser par-
ticles fall more quickly through the column of water and reach
the annular space between the well intake and borehole wall
first. The coarser parrticles may therefore comprise the bottom
portion of the filter pack, and the smaller-sized particles may
comprise the upper portion of each segment of filter pack
emplaced. Driscoll (1986) states that segregation may not be
a significant problem when emplacing uniform grain size, well-
sorted filter packs with a uniformity coefficient of 2.5 or less.
However, graded filter packs are more susceptible to segrega-
tion problems, and this could result in the well consistently
producing water samples with suspended sediment.
Potential bridging problems or segregation of graded filter
packs may be minimized by using a tremie pipe to convey and
emplace the filter pack. The use of a tremie pipe may be
particularly important where the static water level between the
well casing and auger column is shallow. Schmidt (1986) has
suggested that at depths greater than 50 feet, a tremie pipe
should be used to convey and emplace filter pack through
hollow-stem augers. A tremie pipe is a hollow, thin-walled,
rigid tube or pipe which is commonly fabricated by connecting
individual lengths of threaded, flush-joint pipe. The tremie pipe
should have a sufficient diameter to allow passage of the filter
pack through the pipe. The inside diameter of a tremie pipe used
for filter pack emplacement is typically 1 1/2 inches or greater
to minimize potential bridging problems inside the tremie.
Emplacement of the filter pack begins by lowering a
measuring rod or weighted measuring tape to the bottom of the
borehole, as previously described in the free fall method of
filter pack emplacement. The auger column commonly is
retracted 1 to 2 feet, and the tremie pipe is lowered to the bottom
of the borehole through the working space between the well
casing and auger column (Figure 20a). A measured portion of
the precalculated volume of filter pack is slowly poured down
the tremie and the tremie is slowly raised as the filter pack
discharges from the bottom of the pipe, tilling the annular
space between the well intake and borehole wall (Figure 20b).
Once the filter pack is emplaced to the bottom of the auger
157
-------
2-Inch Nominal Diameter -
Well Casing and Intake
4 1/4+inch Diameter
Hollow-Stem Auger
Design Length
of Filter Pack
12 Feet
T
Length of
Well Intake
10 Feet
1
Borehole
' Diameter "
8 1/4 inches
Figure 18. Illustration for the sample calculation of a filter pack
as described in the text.
column, the augers are retracted another 1 to 2 feet and a second
measured portion of the filter pack is added through the tremie
pipe. This alternating sequence of auger column retraction
followed by addtional filter pack emplacement is continued
until the required length of filter pack is installed. Similar to the
free fall method of filter pack emplacement, careful measure-
ments usually are taken and recorded for each increment of
filter pack which is added and emplaced.
During filter pack emplacement, whether by free fall or
tremie methods, the auger column may be refracted from the
borehole in one of two ways (C. Harris, John Mathes and
Associates, personal communication, 1987). One method of
retracting the augers is to use the drive cap to connect the auger
column to the drill head. The drill head then pulls back the auger
column from the borehole. This technique, however, com-
monly requires the measuring rod, weighted measuring tape or
tremie pipe (if used) to be removed from the working space
between the wall casing and auger column each time the auger
column is retracted. A second method of retracting the augers
is to hook a winch line onto the outside of the open top of the
auger column. The winch line is then used to pull the augers
back. The use of a winch line to pull the auger column from the
borehole enables the measuring rod, weighted measuring tape
or tremie pipe to remain in the working space between the well
casing and auger column as the augers are retracted. This latter
auger retraction technique may provide greater continuity be-
tween measurements taken during each increment of filter
pack emplacement. Retracting the auger column with the
winch line can also permit the option of adding filter pack
while the auger column is simultaneously withdrawn from the
borehole. Bridging problems, which lock the well casing and
augers together and cause the casing to pull out of the borehole
along with the augers, may also be more readily detected when
the auger column is retracted by using a winch line. The use of
a winch line, however, may pull the auger column off center. If
the auger column is pulled off center, them maybe an increased
potential for the casing to become wedged within the augers.
When the formation materials adjacent to the well intake
are noncohesive and the borehole will not remain open as the
auger column is retracted, the method for installing the filter
pack may require the use of clean water (C. Harris, John Mathes
and Associates, personal communication, 1987). Similar to the
other methods of filter pack emplacement, a measuring rod or
weighted measuring tape is first lowered to the bottom of the
borehole through the working space between the well casing
and auger column. Clean water is then added to the working
space between the casing and augers to maintain a positive
pressure head in the auger column. As the auger column is
slowly retracted using a winch line, a measured portion of the
precalculated volume of filter pack is poured down the working
space between the well casing and auger column. The head of
clean water in the working space between the casing and augers
usually holds the borehole open while the filter pack material is
emplaced in the annular space between the well intake and
borehole wall. This procedure of slowly retracting the auger
column with a winch line while filter pack material is poured
through a positive pressure head of clean water in the working
space continues until the required length of filter pack is
installed. Once again, measurements of the emplaced filter
pack usually are taken and recorded along with calculations of
"filter pack needed' versus "filter pack used."
If the formation materials adjacent to the well intake are
noncohesive and comprised of coarse-grained sediments, an
artificial filter pack may not have to be installed. The natural
coarse-grained sediments from the formation may instead be
allowed to collapse around the well intake (with appropriately
sized openings) as the auger column is refracted from the
borehole. This procedure initially involves retracting the auger
column 1 to 2 feet. A measuring rod or weighted measuring tape
is then lowered through the working space between the auger
column and casing to verify the collapse of formation material
around the well intake and to measure the depth to the top of
"caved" materials. Once the formation materials collapse
around the well intake and fill the borehole beneath the auger
column, the augers are retracted another 1 to 2 feet. This
alternating sequence of refracting the auger column and verify-
ing the collapse of formation materials by measuring the depth
to the top of the caved materials continues until the coarse-
158
-------
Weighted
Measuring Tape
Well Casing
-C1
Hollow-Stem
Auger
Weighted
Measuring Tape
Well Casing
/"""
Plan View
Weighted
Measuring Tape
Cross-Sectional view
a. Placement of Weighted
Measuring Tape
Weighted
Measuring Tape
C ^^
Hottow-Siem
Auger -^^^, Fj,ter Pack
Plan View PtMmg
Auger Column
Retracted Weighted-"'
1 to 2 Feet Measuring Tape
from Borehole
b. Auger Column Retracted
Filler Rack
Cross -Sectional View
c. Filter Pack Free-Fails Through
Working Space Between Casing
and Auger
Figure 19. Free fall method of filter pack emplacement with a hollow-stem auger.
grained sediments extend to a desired height above the top of the
well intake. The finer-grained fraction of the collapsed forma-
tion materials is later removed from the area adjacent to the well
intake during well development.
Installation of the Annular Seal
Once the well intake, well casing and filter pack are
installed through the hollow axis of the auger column, the final
phase of monitoring well construction typically involves the
installation of an annular seal. The annular seal is constructed
by emplacing a stable, low permeability material in the annular
space between the well casing and borehole wall (Figure 16).
The sealant is commonly bentonite, expanding neat cement or
a cement-bentonite mixture. The annular seal typically extends
from the top of the filter pack to the bottom of the surface seal.
The annular seal provides: 1) protection against the movement
of surface water or near-surface contaminants down the casing-
borehole annulus; 2) isolation of discrete sampling zones; and
3) prevention of the vertical movement of water in the casing-
borehole annulus and the cross-contamination of strata. An
effective annular seal requires that the casing-borehole annulus
be completely filled with a sealant and that the physical integ-
rity of the seal be maintained throughout the life of the monitor-
ing well. The sealant should ideally be chemically nonreactive
to minimize any potential impact the sealant may have on the
quality of ground-water samples collected from the completed
monitoring well.
Although bentonite and cement are the two most widely
used annular sealants for monitoring wells, these materials have
the potential for affecting the quality of ground-water samples.
Bentonite has a high cation exchange capacity and may have an
appreciable impact on the chemistry of the collected ground-
water samples, particularly when the bentonite seal is in close
proximity to the well intake (Gibb, 1987). Hydrated cement is
highly alkaline and may cause persistent, elevated pH values in
ground-water samples when the cement seal is near or adjacent
to the well intake (Dunbar et al, 1985). Raising the pH of the
ground water may further alter the volubility and presence of
other constituents in the ground-water samples.
An adequate distance between the well intake and the
annular sealant is typically provided when the filter pack is
extended 2 feet above the top of the well intake. Bentonite
pellets are commonly emplaced on top of the filter pack in the
saturated zone (United States Environmental Protection
Agency, 1986). Water in the saturated zone hydrates and
expands the bentonite pellets thereby forming a seal in the
casing-borehole annulus above the filter pack. The use of
bentonite pellets direct] y on top of the filter pack generally is
preferred because the pellet-form of bentonite may minimize
159
-------
Weighted
Measuring Tape
C
Plan—View
Weighted
Measuring Tape
C
Auger Column *«
Retracted
1 to 2 Feet j*
from Borehole jfif
Well Casing
- - C1
Tremie Pipe
Tremie Pipe
Positioned to
Bottom of
Borehole
- — C'
Weighted
Measuring Tape
^r
Tremie Pipe
Slowly Raised as
Filter Pack is
Poured
mm
Cross -Sectional View
a. Weighted Measurng Tape and
Tremie Ptpe in Retracted
Auger Column
Filter Pack
Material Poured
Down Tremie
Filter Pack
b. Filter Pack Poured Through Bottom-
Discharge Tremie Pipe
Figure 20. Tremie method of filter pack emplacement with a hollow-stem auger.
the threat of the bentonite infiltrating the filter pack. United
States Environmental Protection Agency (1986) recommends
that there be a minimum 2-foot, height of bentonite pellets in
the casing-borehole annulus above the filter pack. The bento-
nite pellets, however, should not extend above t.hc saturated
zone.
Bentonite pellets are emplaced through the hollow-stem
augers by free fall of the pellets through the working space
between the well casing and auger column. Prior to emplacing
the bentonite pellets, the theoretical volume of bentonite pellets
needed to fill the annular space between the well casing and
borehole wall over the intended length of the seal is determined
(see section on Installation of the Filter Pack for a discussion on
how to calculate the theoretical volume of material needed). A
measuring rod or weighted measuring tape is lowered to the top
of the filter pack through the working space between the casing
and augers. A depth measurement is taken and recorded. The
auger column is then retracted 1 or 2 feet from the borehole and
a measured portion of the precalculated volume of bentonite
pellets is slowly poured down the working space between the
well casing and auger column. In some instances, the bentonite
pellets may be individually dropped, rather than poured, down
this working space. The bentonite pellets free fall through the
working space between the casing and augers and fill the
annular space between the well casing and borehole wall
immediately above the filter pack. As the bentonite pellets are
being added, the measuring rod or weighted measuring tape is
slowly raised and lowered to lightly tamp the pellets in place
and to measure the depth of emplacement of the bentonite
pellets. Once the bentonite pellets are emplaced to the bottom
of the auger column, the augers are again retracted 1 or 2 feet
from the borehole and more bentonite pellets are added. This
procedure continues until the bentonite pellets are installed to
the required height above the filter pack. Actual depth measure-
ments of the emplaced pellets are recorded and compared with
the calculations for the volume of "bentonite pellets needed"
versus "bentonite pellets used."
The free fall of bentonite pellets through the working space
between the well casing and auger column provides the op-
portunity for bridging problems to occur. Bridging problems
are likely to occur particularly when the static water level in the
working space is shallow and the well is relatively deep. As
bentonite pellets fall through a column of standing water, the
bentonite on the outer surface of the pellet starts to hydrate and
the pellet surface expands and becomes sticky. Individual
bentonite pellets may begin sticking to the inside wall of the
160
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auger column or to the outer surface of the well casing after
having fallen only a few feet through a column of water between
the casing and augers. Bentonite pellets may also stick together
and bridge the working space between the casing and augers.
As a result, the pellets may not reach the intended depth for
proper annular seal emplacement. The bentonite pellets will
continue to expand as the bentonite fully hydrates. An expand-
ing bridge of bentonite pellets in the working space may
eventually lock the well casing and auger column together
causing the casing to pull back out of the borehole as the auger
column is retracted.
Careful installation techniques can minimize the bridging
of bentonite pellets in the working space between the casing and
augers. These techniques include: 1) adequately sizing the
working space between the well casing and auger column; 2)
slowly adding individual bentonite pellets through the working
space; and 3) frequently raising and lowering the measuring
device to breakup potential bridges of pellets. Driscoll (1986)
reports that freezing the bentonite pellets or cooling the pellets
with liquid nitrogen to form an icy outer coating may enable
the bentonite pellets to free fall a greater depth through standing
water before hydration of the pellets begins. The frozen
bentonite pellets should, however, be added individually in the
working space between the casing and augers to avoid clump-
ing of the frozen pellets as they contact the standing water in the
working space.
The potential problem of bentonite pellets bridging the
working space between the well casing and auger column may
be avoided by using instead a bentonite slurry, neat cement
grout or cement-bentonite mixture pumped directly into the
annular space between the well casing and borehole wall in the
saturated zone. In the unsaturated zone, neat cement grout or a
cement-bentonite mixture commonly is used as the annular
sealant. In either instance, the slurry is pumped under positive
pressure through a tremie pipe which is first lowered through
the working space between the well casing and auger column.
However, tremie emplacement of a bentonite slurry or cement-
based grout directly on top of the filter pack is not recommended
because these slurry mixtures may easily infiltrate into the
filter pack. Ramsey et al, (1982) recommend that a 1 to 2-foot
thick fine sand layer be placed on top of the filter pack prior to
emplacement of the bentonite slurry or cement grout. The fine-
sand layer minimizes the potential for the grout slurry to
infiltrate into the filter pack. If bentonite pellets are initially
emplaced on top of the filter pack, prior to the addition of a
bentonite slurry or cement-based grout the pellets serve the
same purpose as the fine sand and minimize the potential for the
infiltration of the grout slurry into the filter pack. When bento-
nite pellets are used, a suitable hydration period, as recom-
mended by the manufacturer, should be allowed prior to the
placement of the grout slurry. Failure to allow the bentonite
pellets to fully hydrate and seal the annular space above the
filter pack may result in the grout slurry infiltrating into the filter
pack.
A side-discharge tremie pipe, rather than a bottom-dis-
charge tremie pipe, should be used to emplace bentonite slurry
or cement-based grouts above the filter pack. Aside-discharge
tremie may be fabricated by plugging the bottom end of the pipe
and drilling 2 or 3 holes in the lower 1 -foot section of the tremie.
The pumped slurry will discharge laterally from the tremie and
dissipate any fluid-pumping energy against the borehole wall
and well casing. This eliminates discharging the pumped slurry
directly downward toward the filter pack and minimizes the
potential for the sealant to infiltrate into the filter pack.
Prior to emplacing a bentonite slurry or cement-based
grout via the tremie method, the theoretical volume of slurry
needed to fill the annular space between the well casing and
borehole wall over the intended length of the annular seal is
determined (see section on Installation of the Filter Pack for a
discussion on how to calculate the theoretical volume of mate-
rial needed). An additional volume of annular sealant should
be prepared and readily available at the drill site to use if a
discrepancy occurs between the volume of "annular sealant
needed" versus "annular sealant used." The installation of the
annular sealant should be completed in one continuous opera-
tion which permits the emplacement of the entire annular seal.
The procedure for emplacing a bentonite slurry or cement-
based grout with a tremie pipe begins by lowering a measuring
rod or weighted measuring tape through the working space
between the well casing and auger column. A measurement of
the depth to the top of the fine sand layer or bentonite pellet seal
above the filter pack is taken and recorded. The auger column
is commonly retracted 2 1/2 to 5 feet, and a side-discharge
tremie pipe, with a minimum 1 -inch inside diameter, is lowered
through the working space between the casing and augers. The
bottom of the tremie is positioned above the fine sand layer or
bentonite pellet seal. A measured portion of the precalculated
volume of bentonite slurry or cement-based grout is pumped
through the tremie. The grout slurry discharges from the side
of the pipe, filling the annular space between the well casing and
borehole wall. As the grout slurry is pumped through the
tremie, the measuring rod or weighted measuring tape is slowly
raised and lowered to detect and measure the depth of slurry
emplacement. Once the slurry is emplaced to the bottom of the
auger column, the augers are retracted by using a winch line, the
measuring rod or tape and tremie pipe may remain inside the
working space between the casing and augers as the augers are
pulled back from the borehole. Retracting the auger column
with the winch line may also permit the option of pumping the
grout slurry through the tremie while the auger column is
simultaneously withdrawn from the borehole. A quick-dis-
connect fitting can be used to attach the grout hose to the top of
the tremie pipe. This fitting allows the grout hose to be easily
detached from the tremie as individual 5-foot auger sections are
disconnected from the top of the auger column. By successively
retracting the auger column and pumping the bentonite slurry or
cement-based grout into the annular space between the well
casing and borehole wall, the annular sealant is emplaced from
the bottom of the annular space to the top. The tremie pipe can
be moved upward as the slurry is emplaced, or it can be left in
place at the bottom of the annulus until the annular seal is
emplaced to the required height. Measurements of the depths
of the emplaced annular seal are taken and recorded. Calcula-
tions of the theoretical volume of "annular sealant needed"
versus "annular sealant used" should also be recorded, and any
discrepancies should be explained.
Summary
Hollow-stem augers, like all drilling methods, have ad-
vantages and limitations for drilling and constructing monitor-
161
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ing wells. Advantages of using hollow-stem auger drilling
equipment include: 1) the mobility of the drilling rig; 2) the
versatility of multi-purpose rigs for auger drilling, rotary drill-
ing and core drilling; 3) the ability to emplace well casing and
intake, filter pack and annular seal material through the hollow-
stem auger, and 4) the utility of the hollow-stem auger for
collecting representative or relatively undisturbed samples of
the formation. Other advantages associated with hollow-stem
augers relate to the drilling procedure and include: 1) relatively
fast advancement of the borehole in unconsolidated deposits; 2)
minimal formation damage in sands and gravels; 3) minimal, if
any, use of drilling fluids in the borehole and 4) good control
or containment of cuttings exiting from the borehole. Limitat-
ions of the drilling procedure include: 1) the inability to drill
through hard rock or deposits with boulders; 2) smearing of the
silts and clays along the borehole wall; 3) a variable maximum
drilling depth capability, which is typically less than 150 feet
for most rigs; and 4) a variable borehole diameter.
The drilling techniques used to advance a borehole with
hollow-stem augers may vary when drilling in the unsaturated
versus the saturated zone. In the unsaturated zone, drilling
fluids are rarely, if ever, used. However, in a saturated zone in
which heaving sands occur, changes in equipment and drilling
techniques are required to provide a positive pressure head of
water within the auger column. This may require the addition
of clean water or other drilling fluid inside the augers. If a
positive pressure head of water cannot be maintained inside the
auger column when drilling in heaving sands, the heaving sands
may represent a limitation to the use of hollow-stem augers for
the installation of a monitoring well.
The vertical movement of contaminants in the borehole
may be a concern when drilling with hollow-stem augers.
When monitoring the quality of ground water below a known
contaminated zone, hollow-stem auger drilling may not be
advisable unless protective surface casing can be installed.
Depending on the site hydrogeology, conventional hollow-
stem auger drilling techniques alone may not be adequate for
the installation of the protective surface casing. A hybrid
drilling method may be needed which combines conventional
'hollow-stem auger drilling with a casing driving technique that
advances the borehole and surface casing simultaneously.
The procedure used to construct monitoring wells with
hollow-stem augers may vary significantly depending on the
hydrogeologic conditions at the drill site. In cohesive materials
where the borehole stands open, the auger column may be fully
retracted from the borehole prior to the installation of the
monitoring well. In noncohesive materials in which the bore-
hole will not remain open, the monitoring well is generally
constructed through the hollow axis of the auger column.
The procedures used to construct monitoring wells inside
the hollow-stem augers may also vary depending on specific
site conditions and the experience of the driller. The proper
emplacement of the filter pack and annular seal can be difficult
or impossible, if an inadequate working space is available
between the well casing and hollow-stem auger. An adequate
working space can be made available by using an appropri-
ately-sized diameter hollow-stem auger for the installation of
the required-size well casing and intake. The maximum diam-
eter of a monitoring well constructed through the hollow-stem
auger of the larger diameter augers now commonly available
will typically be limited to 4 inches or less. Assurance that the
filter pack and annular seal are properly emplaced is typically
limited to careful measurements taken and recorded during
construction of the monitoring well.
References
Central Mine and Equipment Company, 1987. Catalog of
product literature; St. Louis, Missouri, 12 pp.
Diedrich Drilling Equipment, 1986. Catalog of product
literature; LaPorte, Indiana, 106 pp.
Driscoll, Fletcher G., 1986. Groundwater and Wells; Johnson
Division, St. Paul, Minnesota, 1089 pp.
Dunbar, Dave, Hal Tuchfield, Randy Siegel and Rebecca
Sterbentz, 1985. Ground water quality anomalies
encountered during well construction, sampling and
analysis in the environs of a hazardous waste management
facility; Ground Water Monitoring Review, vol. 5, No. 3,
pp. 70-74.
Gass, Tyler E., 1984. Methodology for monitoring wells;
Water Well Journal, vol. 38, no. 6, pp. 30-31.
Gibb, James P., 1987. How drilling fluids and grouting
materials affect the integrity of ground water samples from
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Gillham, R.W., M.L. Robin, J.F. Barker and J.A. Cherry,
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American Petroleum Institute, Washington D.C., 206 pp.
Hackett, Glen, 1987. Drilling and constructing monitoring
wells with hollow-stem augers, part I: drilling
considerations; Ground Water Monitoring Review, vol. 7,
no. 4, pp. 51-62.
Hackett, Glen, 1988. Drilling and constructing monitoring
wells with hollow-stem augers, part II: monitoring well
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1, pp. 60-68.
Keely, Joseph F. and Kwasi Boateng, 1987a. Monitoring well
installation, purging and sampling techniques part 1:
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010
313.
Keely, Joseph F. and Kwasi Boateng, 1987b. Monitoring well
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Minning, Robert C., 1982. Monitoring well design and
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Perry, Charles A. and Robert J. Hart, 1985. Installation of
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Ramsey, Robert J.. James M, Montgomery and George E.
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162
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Second National Symposium on Aquifer Restoration and
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Richter, Henry R. and Michael G. Collentme, 1983. Will my
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Proceedings of the NWWA/U.S. EPA Conference on
Characterization and Monitoring of the Vadose
(Unsaturated) Zone, Las Vegas, Nevada, pp. 611-622.
Riggs, Charles O., 1986. Exploration for deep foundation
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Building Industry Press, Beijing, China, pp. 146-161.
Riggs, Charles O., 1987. Drilling methods and installation
technology for RCRA monitoring wells; RCRA Ground
Water Monitoring Enforcement Use of the TEGD and
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Riggs, Charles O., and Allen W. Hatheway 1988. Ground-
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Scalf, M.R., J.F. McNabb, W.J. Dunlap, R.L. Cosby and J.
Fryberger, 1981. Manual of ground-water sampling
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Schmidt, Kenneth D., 1986. Monitoring well drilling and
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Conference on Southwestern Ground Water Issues, Tempe,
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163
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Appendix B
Matrices for Selecting Appropriate Drilling Equipment
The most appropriate drilling technology for use at a
specific site can only be determined by evaluating both the
hydrogeologic setting and the objectives of the monitoring
program. The matrices presented here were developed to assist
the user in choosing an appropriate drilling technology. These
matrices address the most prevalent hydrogeologic settings
where monitoring wells are installed and encompass the drilling
technologies most often applied. The matrices have been devel-
oped to act as guidelines; however, because they are subjective,
the user is invited to make site-specific modifications. Prior to
using these matrices, the prospective user should review the
portion in Section 4 entitled "Selection of Drilling Methods for
Monitoring Well Installation."
Several general assumptions were used during develop-
ment of the matrices. These are detailed below:
1) Solid-flight auger and hollow-stem auger drilling
techniques are limited to a practical drilling depth
of 150 feet in most areas based on the equipment
generally available;
2) Formation samples collected:
a) during drilling with air rotary, air rotary with
casing hammer and dual-wall air rotary tech-
niques are assumed to be from surface dis-
charge of the circulated sample;
b) during drilling with solid-flight augers, hol-
low-stem augers, mud rotary or cable tool
techniques are assumed to be taken by stan-
dard split-spoon (ASTM Dl 586) or thin-
wall (ASTM D1587) sampling techniques to
a depth of 150 feet at 5-foot intervals;
c) below 150 feet, during mud rotary drilling
are assumed to be circulated samples taken
from the drilling mud at the surface dis-
charge; and
d) below 150 feet, during cable-tool drilling are
assumed to be taken by bailer.
If differing sampling methodologies are employed,
the ratings for reliability of samples, cost and time
need to be re-evaluated. (Wireline or piston
sampling methods are available for use with
several drilling techniques; however, these
methods were not included in the development of
the matrices);
3) Except for wells installed using driving and jetting
techniques, the borehole is considered to be no
less than 4 inches larger in diameter than the
nominal diameter of the casing and screen used to
complete the well (e.g., a minimum 6-inch
borehole is necessay for completion of a 2-inch
diameter cased well);
4) Artificial filter pack installation is assumed in all
completions except for wells installed using
driving and jetting techniques;
5) The development of ratings in the matrices is
based on the largest expressed casing diameter in
each range listed in the "General Hydrogeologic
Conditions & Well Design Requirements"
statement;
6) For purposes of the "General Hydrogeologic
Conditions & Well Design Requirements air is
not considered as a drilling fluid; and
7) In the development of the dual-wall rotary
technique ratings in the matrices, air is consider-
to be the circulation medium.
Each applicable drilling method that can be used in the
described hydrogeologic setting and with the stated specific
design requirements has been evaluated on a scale of 1 to 10
with respect to the criteria listed in the matrix. A total number
for each drilling method was computed by adding the scores for
the various criteria. The totals represent a relative indication of
the desirability of drilling methods for the specified conditions.
165
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INDEX TO MATRICES 1 THROUGH 40
Iff.1
C MM! mS mt
Matrix
Number
1
2
3
4
5
6
7
a
9
10
11
12
13
14
15
16
17
18
19
20
21
S>.
S3
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
I
"a
•a
=5
E
a
u.
Q.
o
in
I
in
*•"
JE
I
8
T~
A
.c
8-
Q
™
o
V
O
19
5
o
166