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Previously published as Chapter 7 in the DEP publication "Ground
Water Monitoring Guidance Manual "
Introduction
Unsealed or improperly sealed wells may threaten public health and safety,
and the quality of the groundwater resources. Therefore, the proper abandonment
(decommissioning) of a well is a critical final step in its service life.
Act 610, the Water Well Drillers License Act, includes a provision for
abandonment of wells.
This legislation makes it the responsibility of a well owner to properly
seal an abandoned well according to the rules and regulations of the department.
In the absence of more stringent regulatory standards, the procedures
outlined in this section represent minimum guidelines for proper abandonment
of wells and borings. These procedures may be applicable for, but not
limited to, public and domestic water supply wells, monitoring wells,
borings or drive points drilled to collect subsurface information, test
borings for groundwater exploration, and dry wells (drains or borings
to the subsurface).
Proper well abandonment accomplishes the following: 1) eliminates the
physical hazard of the well (the hole in the ground), 2) eliminates a
pathway for migration of contamination, and 3) prevents hydrologic changes
in the aquifer system, such as the changes in hydraulic head and the mixing
of water between aquifers. The proper decommissioning method will depend
on both the reason for abandonment and the condition and construction
details of the boring or well. Effective abandonment depends on knowledge
of the well construction, geology, and the hydrogeology. The importance
of a full characterization increases as the complexity of the well construction,
site geology, and the risk of aquifer contamination increases.
Construction
information for wells drilled since 1966 may be available from the Bureau
of Topographic and Geologic Survey's (BTGS) Water Well Inventory System
database. Additional well construction data and information describing
the hydrologic characteristics of geologic formations may be available
from reports published by BTGS and the United States Geological Survey
(USGS). Site or program records also may exist. The well should be positively
identified before initiating the abandonment. Field information should
be compared with any existing information. Water levels and well depths
can be measured with a well sounder or weighted tape measure. In critical
situations, well construction details and hydrogeology can be determined
with borehole geophysics or a down hole camera. For example, a caliper
log, which is used to determine the borehole diameter, can be very helpful
in locating cavernous areas in open hole wells.
Well Preparation
If possible, the borehole must be cleared of obstructions prior to abandonment.
Obstructions such as pumps, pipes, wiring, and air lines must be pulled.
Well preparation also may involve fishing obstacles out of the borehole.
An attempt should be made to pull the casing when it will not jeopardize
the integrity of the borehole. Before the casing is pulled, the well should
be grouted to near the bottom of the casing. This will at least provide
some seal if the well collapses after the casing is pulled. The presence
of nested or telescoped casing strings complicates well abandonment. Inner
strings should be removed when possible, but only when removal will not
jeopardize the abandonment of the well. If inner strings cannot be removed
and sealing of the annular space is required, then the inner string should
be vertically split (plastic cased wells) or cut (metal-cased wells) at
intervals necessary to insure complete filling of the annular space. Damaged,
poorly constructed or dilapidated wells may need to be re-drilled in order
to apply proper abandonment techniques. Also, in situations where intermixing
of aquifers is likely, the borehole may need to be re-drilled.
Materials and Methods
Aggregate
Materials that eliminate the physical hazard and open space of the borehole,
but do not prevent the flow of water through the well bore, are categorized
as aggregate. Aggregates consist of sand, crushed stone or similar material
that is used to fill the well. Aggregates should be uncontaminated and
of consistent size to minimize bridging during placement. Aggregate is
usually not placed in wells smaller than two inches in diameter. Nominal
size of the aggregate should be no more than 1/4 of the minimum well diameter
through which it must pass during placement. Because aggregate is usually
poured from the top of the well, care must be taken to prevent bridging
by slowly pouring the aggregate and monitoring the progress with frequent
depth measurements.
Aggregates may be used in the following circumstances: 1) there is no
need to penetrate or seal fractures, joints or other openings in the interval
to be filled, 2) a watertight seal is not required in the interval to
be filled, 3) the hole is caving, 4) the interval does not penetrate a
perched or confined aquifer, and 5) the interval does not penetrate more
than one aquifer. If aggregate is used, a casing seal should be installed
(see Section 7.5.1). The use of aggregate and a casing seal must be consistent
with the future land use.
Sealants
Sealants are used in well abandonment to provide a watertight barrier
to the migration of water in the well bore, in the annular spaces or in
fractures and openings adjacent to the well bore. Sealants usually consist
of Portland cement based grouts, "bentonite" clay, or combinations
of these substances. Additives are frequently used to enhance or delay
specific properties such as viscosity, setting time, shrinkage, or strength.
Sealing mixtures should be formulated to minimize shrinkage and ensure
compatibility with the chemistry of the groundwater in the well. A grout
pump and tremie pipe are preferred for delivering grout to the bottom
of the well. This method insures the positive displacement of the water
in the well, and will minimize dilution or separation of the grout.
If aggregate is to be placed above sealant, a sufficient amount of curing
time should pass before placing the aggregate above the seal. Curing time
for grout using Type 1 cement is typically 24 - 48 hours, and 12 hours
for Type III cement. General types of sealants are defined as follows:
Neat cement grout:
Neat cement grout is generally formulated using a ratio of one 94 lb.
bag of Portland cement to no more than 6 gallons of water. This grout
is superior for sealing small openings, for penetrating any annular space
outside of the casings, and for filling voids in the surrounding rocks.
When applied under pressure, neat cement grout is strongly favored for
sealing artesian wells or those penetrating more than one aquifer. Neat
cement grout is generally preferred to concrete grout because it avoids
the problem of separation of the aggregate and the cement. Neat cement
grout can be susceptible to shrinkage and the heat of hydration can possibly
damage some plastic casing materials. Concrete grout: Concrete grout consists
of a ratio of not more than six gallons of water, one 94-lb. bag of Portland
cement, and an equal volume of sand. This grout is generally used for
filling the upper part of the well above the water bearing zone, for plugging
short sections of casings, or for filling large-diameter wells.
Concrete grout:
Makes a stronger seal than neat cement, may not significantly penetrate
seams, crevices or interstices. Grout pumps can handle sand without being
immediately damaged. Aggregate particles bigger than this may damage the
pump. If not properly emplaced, the aggregate is apt to separate from
the cement. Concrete grout should generally not be placed below the water
level in a well, unless a tremie pipe and a grout pump are used.
Grout additives:
Some bentonite (2 to 8 percent) can be added to neat cement or concrete
grout to decrease the amount of shrinkage. Other additives can be used
to alter the curing time or the permeability of the grout. For example,
calcium chloride can be used as a curing accelerator.
High-solids sodium bentonite:
This type of grout is composed of 15-20 percent solids content by weight
of sodium bentonite when mixed with water. To determine the percentage
content, the weight of bentonite is divided by the weight of the water
plus the weight of the bentonite. For example, if 75 lbs. of powdered
bentonite and 250 pounds of granular bentonite were mixed in 150 gallons
of water (at 8.34 lbs. per gallon), the percentage of high-solids bentonite
is approximately 20 percent (325/(1251+325)). High-solids bentonite must
be pumped before its viscosity is lowered. Pumping pressures higher than
those used for cement grouts are usually necessary. Hydration of the bentonite
must be delayed until it has been placed down the well. This can be done
by 1) using additives with the dry bentonite or in the water, 2) mixing
calcium bentonite (it expands less) with sodium bentonite, or 3) using
granular bentonite, which has less surface area. In addition, positive
displacement pumps such as piston, gear, and moyno (progressive cavity)
pumps must be used because pumps that shear the grout (such as centrifugal
pumps) will accelerate the congealing of the bentonite. A paddle mixer
is typically used to mix the grout. A high-solids bentonite grout is not
made from bentonite that is labeled as drilling fluid or gel.
Chip Bentonite:
Chip (coarse grade) or palletized bentonite can form adequate seals. This
type of bentonite is poured directly down the borehole. The size of the
bentonite chips also should be no more than 1/4 of the minimum well diameter
through which it must pass during placement. Because of the potential
for bridging, this material may not be suitable for deep wells or borings
where positive displacement is necessary to seal the well. When coarse
bentonite is placed above the water level, water must be added frequently
to hydrate the bentonite. Care must be taken with chip or pelletized bentonite
to not overload the interval to be sealed. Rapidly swelling bentonite
could result in incomplete hydration and a heterogeneous seal containing
lumps of dry bentonite. The level of the bentonite should be checked often
to make sure that bridging of the chips does not occur.
Bridge Seals:
A bridge seal can be used to isolate cavernous sections of a well, to
isolate two producing zones in the well, or to provide the structural
integrity necessary to support overlying materials (and thus protect underlying
aggregate or sealants from excessive compressive forces). Bridge seals
are usually constructed by installing an expandable plug made of wood,
neoprene, or a pneumatic or other mechanical packer. Additional aggregate
can be placed above the bridge.
Recommendations
The complexity of the abandonment procedure depends primarily on the hydrogeology,
geology, well construction, and the groundwater quality. Four principal
complicating factors have been identified; they include 1) artesian conditions,
2) multiple aquifers, 3) cavernous rocks, and 4) the threat or presence
of contamination. The recommended procedures for abandoning wells will
be more rigorous with the presence of one or more complicating factors.
The procedures may vary from a simple casing seal above aggregate to entirely
grouting a well using a tremie pipe after existing casing has been ripped
or perforated.
Casing Seal
The transition from well casing to open borehole is the most suspect zone
for migration of water. In order to minimize the movement of water (contaminated
or otherwise) from the overlying less consolidated materials to the lower
water bearing units, this zone must be sealed. Generally this can be accomplished
by filling at least the upper 10 feet of open borehole and the lower five
feet of casing with sealant. The length of open borehole sealed should
be increased if extenuating circumstances exist. Such circumstances would
include a history of bacterial contamination, saprolitic bedrock, or possibly
deep fracture zones. Water bearing zones reported in the upper 20 feet
or so of open borehole are indications of fractures and would warrant
additional sealant. Casing that is deteriorated should be sealed along
its entire length. If the casing is to be pulled the sealant used should
remain fluid for a period of time adequate for removal of the casing.
If the casing is to remain, then whenever feasible, it should be cut off
below land surface. After the casing seal discussed above achieves adequate
strength, the open casing should at a minimum, be filled with aggregate.
It is strongly suggested that a sealant be used in the upper 2 to 5 feet
of casing.
Wells in Unconfined or Semi-Confined Conditions
These are the most common type of wells in Pennsylvania. The geology may
consist of either unconsolidated or consolidated materials. When applicable,
unconfined wells in non-contaminated areas may be satisfactorily abandoned
using aggregate materials up to 10-15 feet below the ground surface. This
would apply mainly to domestic wells, and test borings or wells not covered
by existing regulations. Monitoring wells that are not covered by specific
regulatory programs and are located at sites with no known contamination,
might be abandoned in this manner. Above the aggregate, the casing seal
should be installed. A sealant may be used over the entire depth.
Wells at Contaminated Sites
An abandoned, contaminated well often mixes contaminated groundwater with
uncontaminated groundwater. Complete and uniform sealing of the well from
the bottom to the surface is required. Therefore, proper well preparation
(Section 7.3) must be done before the well is sealed with a proper sealant
(Section 7.4.2).
Wells in Cavernous Rocks
Problems can arise when filling wells that penetrate cavernous rock. Although
such wells are usually located in carbonate terrain, voids can also occur
in areas that have been deep mined. Care must be taken to insure that
aggregates and sealants are of a size and consistency to prevent their
removal by water flowing in the void. Large voids or high flow velocities
warrant placement of a bridge in competent rock over the void. Aggregate
and sealants can then be placed above the bridge.
Multiple Aquifer Wells
The main goal in sealing wells that extend into more than one aquifer
is to prevent the flow of groundwater from one aquifer to another. If
no appreciable movement of water is encountered, and there is no threat
of groundwater contamination, sealing with concrete, neat cement, grout,
or alternating layers of these materials and aggregate will prove satisfactory.
When groundwater velocities are high, the procedures for wells with artesian
flow (see the next section) are recommended. If alternating plugs (or
bridges) and aggregate layers are used, the plugs should be placed in
known nonproductive horizons or, if locations of the nonproductive horizons
are not known, at frequent intervals.
Flowing Wells
The sealing of artesian wells requires special attention. The flow of
groundwater may be sufficient to make sealing by gravity placement of
concrete, cement grout, neat cement, clay or sand impractical. In such
wells, large stone aggregate (not more than 1/4 of the diameter of the
hole), well packers (pneumatic or other), or wooden plugs will be needed
to restrict the flow and thereby permit the gravity placement of sealing
material above the zone where water is produced. If plugs are used, they
should be several times longer than the diameter of the well to prevent
tilting. Seals should be designed to withstand the maximum anticipated
hydraulic head of the artesian aquifer. Because it is very important in
wells of this type to prevent circulation between water yielding zones,
or loss of water to the surface or to the annular spacing outside of the
casing, it is recommended that pressure grouting with cement be done using
the minimum volume of water during mixing that will permit handling. In
wells in which the hydrostatic head producing flow to the surface is low,
the movement of water may be stopped by extending the well casing to an
elevation above the artesian pressure surface.
Wells with Complicating Factors at Contaminated
Sites
Wells with one or more of the above complicating factors that are to be
abandoned in areas with contaminated groundwater or in areas where the
groundwater is at a high risk for future contamination, require the most
rigorous abandonment procedures. In general, the entire length of these
wells should be sealed. When the threat of contamination has been established,
the elimination of a potential flow path is critical. For example, a contaminated
well in a karst terrain must be carefully sealed to avoid worsening the
situation. In general, the entire lengths of these wells should be sealed.
In some situations, a bridge seal may have to be installed, and casing
may have to be perforated. In each case, a prudent method should be selected
that will eliminate all potential vertical flow paths.
Monitoring Wells
Monitoring wells should be abandoned in accordance with the rules and
regulations of the program under which they were installed and operated.
Monitoring wells which do not fall under the jurisdiction of a regulatory
program, or fall under a program that has no rules or regulations for
abandonment, should be abandoned under the following guidelines. Monitoring
wells that were installed and continue to function as designed, can usually
be abandoned in place. Exceptions would include wells whose design precludes
complete and effective placement of sealant and wells in locations subject
to future disturbance that could compromise the abandonment. In such instances
all tubing, screens, casings, aggregate, backfilling, and sealant should
be cleaned from the boring and the hole should be completely filled with
an appropriate sealant.
Monitoring wells that are abandoned in place should be completely filled
with sealant. Screened intervals can be backfilled with inert aggregate
if sealant will alter the groundwater chemistry and thereby jeopardize
ongoing monitoring at the facility. Intervals between screens, and between
the last screen and the surface, must be filled with sealant. Generally,
sealant must be emplaced from the bottom of the interval being sealed.
Protective casings, riser pipes, tubing, and other appurtenances at the
surface which could not be removed should be cut off below grade after
the sealant has properly set. When the abandonment will be completed below
the finished grade, the area of the boring should be covered with a layer
of bentonite, grout, concrete, or other sealant before backfilling to
grade.
Existing Regulations and Standards
The Water Well Drillers License Act requires that the owner or consultant
who is to abandon the well notify the department of the intent to decommission
a well at least 10 days before the well is sealed or filled. Individual
department bureaus may have specific regulations or guidelines.
The Bureau of Oil and Gas Management regulates the plugging of oil and
gas wells. Plugging provisions for oil and gas wells in coal and non-coal
areas are established in § 210 and § 211 of Act 223, and §
78.91 - 78.97 of Chapter 78. These sections describe methods that would
stop any vertical flow of fluids or gas within the well bore. Alternate
methods of plugging also are allowed if they would afford the same level
of protection. Alternate methods must be approved before the plugging
is initiated.
The Bureau of Mining and Reclamation regulates the abandonment of borings
and wells associated with the mining of coal. Coal exploration holes must
be abandoned according to the § 87.93 for surface mining of bituminous
coal, § 88.83 for anthracite coal mining, § 89.54 for deep mining
of bituminous coal, and § 90.93, coal refuse disposal. The Bureau
of Water Supply and Community Health uses the AWWA Standard A 100-90 for
abandonment of public water supply wells. This standard is referenced
in Section 3.3.5.11 of Part II of the Public Water Supply Manual.
Reporting
All abandoned wells shall be reported to BTGS, along with any bureau that
requires a report, on forms required by BTGS (and any other forms). If
available, the original driller's log should be included along with the
details of the well abandonment procedure. A photograph should be taken
of the site, and a reference map should be made to locate the abandoned
well. It also may be appropriate to survey the exact location of the well.
This is especially important for wells associated with contaminated sites.
References
AMERICAN WATER WORKS ASSOCIATION, 1990, Abandonment of Test Holes, partially
completed wells and completed wells: AWWA Standard for Water Wells, A100-90,
pp. 25-26.
DRISCOLL, F.G., 1986, Groundwater and Wells, 2nd ed., Johnson Filtration
Systems, Inc., St. Paul, Minnesota 55112, 1089 pp.
NYE, J.D., September 1987, Abandoned Wells - How One State Deals with
Them, Water Well Journal, pp. 41-46
RENZ, M.E., May 1989, In Situ Decommissioning of Ground Water Monitoring
Wells, Water Well Journal, pp. 58-60.
U.S. ENVIRONMENTAL PROTECTION AGENCY, 1975, Manual of Water Well Construction
Practices, Office of Water Supply, EPA-570/9-75001.
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