NGWA Best Suggested Practice
Residential Well Cleaning
Approved by NGWA
Board of Directors:
8/12/2008
Compiled: April 2008
Purpose
As with other classes of wells, adequate residential well water production, potability, and overall
water quality is enhanced by appropriate and regular service of the well system, including cleaning of the well. Consumer confidence in residential water well systems for drinking water purposes can be enhanced by appropriate and regular service of the well system, including cleaning
of the well to assure water potability, in addition to maintaining adequate water production from
the well. For the purposes of this best suggested practice, the National Ground Water Association
defines “cleaning” as the removal from the well system of biological slime growths, natural
encrustations, and other debris.* Although it is critical to adequately clean a well before disinfecting
a well system, this document distinguishes well cleaning from well disinfection. This document
intends to assist the professional water well system contractor in understanding how to effectively
clean a residential water well system. The professional servicing residential well systems should
understand that most residential well system owners are not accustomed to performing preventive
or routine maintenance on wells. Well owners, however, can be shown routine well and pump
cleaning and maintenance procedures that can provide a more dependable as well as a higher
quality water source.
Introduction
Water well systems, like other engineered structures, can deteriorate over time, and therefore
require periodic maintenance.
It is a requirement that the well system owner should assure the system is regularly and appropriately maintained so as to maximize the quality and quantity of the system’s water, as well as the
overall long-term use of the well system. The value in servicing the water well regularly is that it
extends the life of the water well system, which in turn saves the homeowner replacement costs.
A professional water well system contractor should be engaged to:
a) Conduct annual inspection of the pumping system such as pump performance, well
seal/cap, storage tanks, controls, and water treatment equipment.
b) Appropriately clean the well if cleaning is indicated as a need from the anaerobic bacteria
sample evaluation.
c) Appropriately disinfect the well following cleaning.
*
“
Although it is
critical to adequately
clean a well before
disinfecting a
well system,
this document
distinguishes well
cleaning from
well disinfection.
For the purposes of this best suggested practice, the National Ground Water Association defines
“cleaning” as the removal from the well system of biological slime growths, natural
encrustations, and other debris.
®
Phone/ Toll-free 800 551.7379/ 614 898.7791 Fax/ 614 898.7786
Web/ www.ngwa.org and www.wellowner.org
Address/ 601 Dempsey Road/ Westerville, Ohio 43081-8978 U.S.A
While water well system contractors are not necessarily specialists in analyzing water quality,
they are a good point of contact for arranging water quality tests to check for deterioration.
The well system owner can contribute to the well system’s overall best condition by recognizing
these responsibilities:
• Always use licensed or certified water well drillers and pump installers when a well is
constructed, a pump is installed, or the system is serviced and cleaned.
• An annual well maintenance check, including recommended water quality tests, is recommended. Any source of drinking water should be checked anytime there is a change in taste,
odor, or appearance or anytime a water supply system is serviced.
• Keep hazardous chemicals such as paint, fertilizer, pesticides, and motor oil far away from
the well.
• Periodically check the well cover, seal, or well cap on top of the casing (well) to ensure it is
in good repair.
• Always maintain proper separation between the well and buildings, waste systems, or
chemical storage facilities.
• When mixing pesticides, fertilizers, or other chemicals, don’t immerse the hose inside the
tank or container. This should prevent back-siphoning of chemicals into the water supply.
• When landscaping, keep the top of the well casing at least 1 foot above the ground. Slope
the ground away from the well for proper drainage. Avoid planting shrubs or trees (both of
which send down deep roots) around wells, and keep mulch away from wells.
“
Whenever a well
shows signs of
turbidity, loss of
capacity, or taste
and odor problems,
or especially if a
bacteriological test
is total coliform
positive, it should be
checked for overall
bacterial presence
as these symptoms
usually indicate high
biological activity.
• Take care in working around the well. A damaged casing could jeopardize the sanitary
protection of the well. Homeowners should not pile snow, leaves, or other materials around
the well.
• Keep well records in an appropriate place that is secure and accessible. These include the
construction report or well log, as well as annual water well system maintenance and water
testing results, other reports such as for site inspections, and manuals and data on pumps,
tanks, and water treatment equipment.
• Be aware of changes in the well, the area around the well, surrounding land and properties
(for example, sinkholes or new constructions), or the quality or quantity of water the well
provides.
• If or when a residential well has come to the end of its serviceable life, have a qualified
water well contractor properly decommission the well.
Whenever a well shows signs of turbidity, loss of capacity, or taste and odor problems, or especially if a bacteriological test is total coliform positive, it should be checked for overall bacterial
presence as these symptoms usually indicate high biological activity.* High biological activity
generally means accumulation of organic matter and dictates a need for a good cleaning with
removal of accumulated debris.*
Chlorination alone might temporarily prevent gas production or other taste and odor problems,
but it leaves behind all the debris or accumulated organics which are a food source for future bacterial growth. Cleaning, followed by disinfection, returns the well closer to the original conditions.
Well cleaning must include removal of the pump with complete removal of debris from the well
bottom and should include brushing and cleaning of the well casing and well intake zones, with
removal of the pump. The contractor should also clean and flush the gravel pack and formation
surrounding the well. This flushing with proper chemicals will remove large volumes of bacteria
and their debris, thus removing many of the spaces sheltering coliforms, or in some instances,
pathogenic bacteria. Once the heavy accumulation has been removed, the disinfection process is
*
In subtropical and tropical ground water, both total coliforms and high biological activity are
typically present.
2
more effective and the well can be returned to a near new condition with cleaner water brought to
the surface.
A qualified professional ground water contractor will best clean a water well system by following the procedures listed as follows.
Understanding the Condition of the Natural Geologic Formations
at the Well
Prior to implementing strategies for restoring a water well system’s potability and production
(volume of water or flow rate), it is critical to understand the condition of the well and borehole,
as well as the natural geologic formation immediately surrounding the well borehole. Start by
collecting available well records, including the well construction log or record, as available.
The drawdown caused by the well operation brings air (oxygen) into the well and encourages
vadose zone water to move more quickly toward the well. This introduction of oxygen encourages
growth of aerobic bacteria (bacteria requiring oxygen to grow) and oxidation of metals such as
iron, especially in the upper reaches of the well. This is a normal operating condition of a well.
Aerobic bacteria tend to cause clogging by the production of large amounts of slime and the
entrapment of oxides of iron and manganese as well as other minerals. An anaerobic action is
encouraged within the well as the water in the lower levels becomes depleted of oxygen as chemical
reactions and bacterial activity use up the oxygen present. Lower levels (especially sumps) may
have no water flow and therefore no way to replace the oxygen in these areas. In addition, these
areas tend to accumulate materials brought into the well as well as the organic debris of bacterial
activity in the upper zones. As bacterial populations cycle in the oxygenated zones, the debris
sinks to the lower levels, there becoming a food source for bacteria able to live in the anoxic
(without oxygen) environment, the anaerobes. These bacteria, natural to the aquifer, which inhabit
the wellbore and the surrounding formation are responsible for the hydrogen sulfide (rotten egg
odor), methane, fishy taste, and odors. These bacteria are also responsible for low pH resulting in
iron taste and color to the water.
The condition of the well may deteriorate as populations increase and byproducts of bacterial
activity, corrosion, and minerals accumulate. In addition, accumulation of heavy biofouling activity from bacterial growth gives excellent areas for coliform to reside and proliferate, especially in
the anaerobic zone.
Economics of Well Cleaning for the Residential Customer
As with many decisions for infrastructure systems, there are certain decisions that the homeowner,
in cooperation with the professional water well system contractor, must consider when striving to
achieve a level of water quality and water quantity from the residential well system. Money is a
factor; in general, residential customers do not budget for water supply maintenance. However,
proper maintenance offers a positive payoff in extended pump and water treatment system life and
reduced impacts on plumbing fixtures and appliances. Quality of life is enhanced as people enjoy
high-quality water. In areas where ground water quality is challenging, regular maintenance may
be the only strategy that keeps rural water wells viable as a water source option.
The age of the well can be a significant factor. The passage of time, coupled with changes in
natural water quality and the need for maintenance, can impact the decisions to be reached as to
how much money the homeowner should invest to achieve a desired level of water quality and
water quantity.
Certain cleaning options have the potential to damage the mechanical structures of an existing
well. For instance, if the well’s water intake areas, or the well casing, have weakened over time,
3
“
These bacteria,
natural to the aquifer,
which inhabit the
wellbore and the
surrounding formation are responsible
for the hydrogen
sulfide (rotten egg
odor), methane,
fishy taste, and odors.
These bacteria are
also responsible for
low pH resulting in
iron taste and color
to the water.
they may be damaged or destroyed by more aggressive cleaning practices. In these instances, the
well owner and the contractor may decide to proceed directly to new well construction, or prepare
for that option if cleaning causes irreparable damage.
There are a number of means available to a professional water well system contractor cleaning
water wells. These are arranged into two categories: mechanical processes and chemical processes.
Either the mechanical or the chemical processes are most effective when used in concert.
Mechanical Processes Inventory for Residential Well Cleaning
Airburst:
(Frazier Technologies Inc.) A specialized method of cleaning employing an air or
gas (typically nitrogen)-actuated seismic tool to loosen both hardened deposits and
biofouling as part of a cleaning process.
Airlift:
Lifts fluid in a well to the land surface using compressed air, often associated with
airlift development or an airlift test.1
Bailer:
A cylindrical device suspended from a wire rope or cable, which is used to
remove sediment or other material from a well or open borehole. Bailers may
be equipped with various types of valves or trap doors at their base to allow
sediment to enter the bailer and be retained within it while the bailer is pulled to
the land surface. Types of bailers include dart valve bailers, flapper valve bailers,
suction bailers, and scows.2
Carbon dioxide: Either in its solid dry ice form or as part of a specialized process using liquid
and carbon dioxide gas, is used as part of a well cleaning process that employs
chemical, thermal, and mechanical activity to loosen deposits.
Casing scraper: May be used to remove deposits on the casing. Probably more universally
thought of as a tight-fitting swab or surge block with a brush.
Develop the well: To surge, bail, pump, swab, airlift, jet, or otherwise conduct operations at a well
that will increase the well’s specific capacity by removing fine sediment and
drilling fluids, and create a well-packed and graded filter pack envelope around
the well.2
Hooded or shrouded pumps: Pipe, larger in diameter than the well pump, is inserted over the
pump to increase suction in given areas to improve the removal of debris.
Hydraulic fracturing (hydrofrac): Stimulate (improve) well production by increasing the secondary porosity and permeability of the producing formation. During a hydraulic
fracturing operation, a fluid, typically clean water for residential wells, is introduced into the borehole under high hydraulic pressure such that the fracture
gradient of the formation material is exceeded, and the rock formation yields
more water.
Injected water: Any injected water must be from a potable source (not pond water) and free
from undesirable microorganisms and debris. All injections and cleaning
generates dirty water that must be properly contained, treated, or hauled off
for disposal.
4
Jetting:
Developing a well by applying high-velocity horizontal streams (“jets”) of water
to the well screen. Although used in the past, compressed air jets are discouraged due to the encouragement of increased aerobic bacterial activity. Jet development can be conducted using the application of chemical additives, and with
simultaneous pumping or airlifting.2
Pump-and-surge*: Well development process that involves alternately pumping and surging a
well. This is accomplished by pumping the well at sequentially increasing
pumping rates to allow the water in the pump column to backflow into the well
to provide a surging action that helps to flush out fine-grained sediment and
augment the well development process. To accommodate the pump-and-surge
development process, the pump equipment is installed without a foot valve
(jet pumping system) or an in-line check valve (submersible pumping system).
Also known as backwashing development, overpumping development, and
pumping development.2
Rawhiding*:
A method of developing a well by pumping the well without a foot valve (jet
pumping system) or an in-line check valve (submersible pumping system)
installed to prevent backflow through the pump, to allow backwashing of the
well through the pump’s column pipe (see pump-and-surge).2
Recirculation*:
Using the standing water in the well and recirculating it to remove debris and
biological material in the well.
Sand pumps:
A type of bailer used to remove sediment from a borehole or well. The sediment
is kept in the bailer with a hinge flap valve and a plunger located within the barrel of the sand pump. Also termed a moran bucket and suction bailer.2
Sonar jet:
A technique for cleaning scale from a well casing by use of sonic waves that are
emitted from sequentially timed blasts in the well.
Surge block:
A plunger-like tool consisting of solid or perforated pipe sandwiched between
leather or rubber discs that is used in well development.2
Wire brush
To remove scale from a well casing by scraping it off with a steel or stiff plastic
brushing tool. The brushing tool is often constructed of partially uncoiled wire
rope or cable or a similar very stiff material.2
Chemical Processes Inventory for Residential Well Cleaning
Acidize:
*
To add or circulate acid in a well or open borehole to clean scale or open the
aquifer to improve water production characteristics. This is currently the primary
cleaning chemistry used to remove mineral deposits from the well screen, gravel
pack, and the immediate formation. Acids that are typically used to acidize wells
include citric acid, sulfamic acid, food-grade phosphoric acid, and hydrochloric
acid (HCl). The acid used should be chosen carefully in combination with the
rock formation to maximize efficiency or productivity. Acidizing limestone
Pump-and-surge, rawhiding, and recirculation are often used in place of pulling the pump and do
not allow thorough cleaning of the well and removal of the debris from the well bottom or sump
zone. Further, rawhiding, recirculation, and pump-and-surge often develop and clean the higher
zones but often pack the lower region with even more debris, encouraging poor anaerobic and
contaminant accumulation.
5
with HCl (the most effective choice for this application) to open pore spaces and
fractures is more commonly used, but it is not effective if the carbonate rock is
dolomitic or shaly. Acid is a dangerous material and the practice of acidification
can be hazardous; therefore, only people trained in these procedures with the
proper equipment should perform acid cleaning of wells. While chemical
inhibitors can and should be used to limit corrosion of the well structure,
extreme care should be used in the use of this type of chemistry. Only NSFapproved inhibitors should be used due to their often extreme toxicity.2
Acid blends:
A blend of two or more acids, typically with dispersants or surfactants, to
improve the cleaning procedure. Usually a mineral acid is used to remove or
loosen the mineral deposits while a range of organic acids used appropriately
can be used to improve the removal of iron, manganese, and biofouling. These
are often available (National Science Foundation 60 listed) in convenient packages with instructions from well equipment and material suppliers.
Hydrochloric:
A very aggressive liquid mineral acid very corrosive to stainless steel. But it is a
good solvent for iron and sulfate minerals. However, if carbonates are present it
more typically will dissolve those first. Its fumes are very acidic and often cause
severe deterioration of electrical components, as well as being hazardous to soft
tissue. If combined with chlorine in solution, chlorine gas is generated. When
HCl is used on iron sulfide clogs, dangerous levels of hydrogen sulfide may be
generated. Both chlorine and H2S are at best irritating and at worst lethal, especially in confined spaces. Limited numbers of National Science Foundation
(NSF)-listed formulations are available, and commercial or industrial products
are often highly contaminated with heavy metals. The acid is relatively ineffective against biofouling. The cleaner forms of HCl, such as NSF approved and
food grade quality, should only be used.
Phosphoric:
A strong, primarily food-grade, liquid mineral acid that is effective against metal
and mineral hydroxides and somewhat effective against biofouling. It is an
excellent iron control acid and effective against both carbonates and sulfate
minerals. If left in the formation, phosphorus-containing compounds or products
can form minerals which have been implicated in increased bacterial activity.
Dispersants should be used to prevent this formation. Phosphorus can attach to
clays and a variety of rock minerals and promote biofilm regrowth; therefore,
it is necessary to remove spent phosphoric acid from the well.
Sulfamic:
Solid granular tablet or powdered acid often used in blends that are relatively
effective against carbonates, weakly effective against sulfate minerals and iron
deposits. Sulfamic is used in blends to keep pH down during cleaning procedures.
As with all blending, this should be performed at the surface.
Muriatic:
Industrial name for hydrochloric acid, usually a lower concentration of 20% or
less acid. Often of substandard purity, industrial grade muriatic (such as sold for
masonry cleaning) can contain heavy metals. This is no longer recommended for
well cleaning use.
6
Glycolic acid (hydroxyacetic acid): An organic acid especially effective against biofouling and
carbonate clogging. NSF lists 60 products available, often as a blend. While this
acid has some bactericidal activity, it must be maintained at 5% concentration to
do so. Hydroxyacetic has been known to leave significant organic salts in the
aquifer which lead to enhanced bacterial growth. As with any of the potential
“downsides” of chemical choices, proper dosage, careful application, and
thorough post-dosage development serve to maximize benefits and minimize
disadvantages.
Acetic:
An organic acid that is the active compound in vinegar which can be as high as
90%. As with any acid, mixing and treating in closed unventilated areas is not
recommended. The high concentration form of the acid has a pungent and often
dangerous off-gassing which can lead to lung and eye damage. However, when
mixed at recommended levels, it is much safer to handle than mineral acids.
Laboratory testing has shown only limited effectiveness against biofouling at
ambient temperatures. It has been successfully used for many years for this purpose, although hydroxyacetic acid is generally preferred. More so than hydroxyacetic (glycolic), however, residual of acetic can be a source of organic carbon
for regrowth. Acetates remaining after cleaning are typically a small fraction of
the assimilable organic carbon (AOC) found in ground water if good developmental practices are used.
Citric:
A weak organic acid, usually in a granular form. It provides increased control of
iron redeposition during acid cleaning with a strong mineral acid. It has considerable organic carbon and is an excellent food source for bacterial growth. This
acid should not be used in ground water with significant calcium content as it
forms calcium citrate.
Oxalic:
A very strong organic acid in crystalline form. It is very effective in dissolving
iron and manganese deposits. Care should be used as byproducts of cleaning can
form oxalates which are known rodent poisons.
Hypochlorites:
Salts of hypochlorous acid that are powerful oxidizers. Calcium and sodium
hypochlorite are used as a disinfectant for water wells. The sodium form is
the best disinfectant due to its better solubility and penetration rate. Calcium
hypochlorite is the powdered form and is considerably less soluble in hard water
areas. Calcium hypochlorite is a strong oxidizer and as such should not be stored
damp or mixed with organic material of any type. The product can be explosive
even mixed with a simple organic detergent or motor oil.
Biodispersants: Organic polymers that are designed specifically to disperse the polysaccharide
“slime” produced by bacterial cells. They also provide control of mineral deposit
components once they have been dissolved by acid cleaner. This dispersant
action provides for a substantial increase in the washout of the debris from the
well and the surrounding formation following the cleaning procedure.
Surfactants:
Surface-active agents allow penetration and easy movement in cracks and
crevices that require cleaning by reducing the friction between the cleaning solution and the surfaces of the screen and mineral formation. Surfactants are necessary for preventing air locks or the plugging of flow pathways by air bubbles.
7
Caustics:
“
The decision to clean
the well should be
based on one or more
factors, such as poor
production, observational reports dealing
with water quality,
or field or laboratory
analysis.
Chemicals which raise pH to alkaline range (7.0+). Caustics are useful in emulsifying oils, neutralizing acids, removing bacterial slime, and are main components of certain chemistries which degrade biocides.
Sodium hydroxide: Caustic material sold industrially as “Liquid Caustic,” a 50% solution.
Potassium hydroxide: Caustic sold industrially as “Liquid Potash,” a 45% solution.
Magnesium hydroxide: Caustic material sold commercially as a 50% to 60% slurry. As a caustic
it is often used to neutralize acid cleaners. It is difficult to overtreat the product
because solutions of the material will not exceed a pH of 9.5.
Corrosion protection: Using cathodic protection or corrosion-inhibiting chemicals to protect
casing and equipment while acidizing.
Acid enhancers: Special polymers, usually mineral dispersants, which increase the effectiveness
of an acid by improving the solubility of the mineral deposit as initiated by an
acid reaction.
Steps in Residential Well Cleaning
Local geologic conditions will impact on the well’s condition, so the contractor will most likely
need to employ a range of options to accomplish the best work for the customer.
Let us discuss open borehole “open hole” and cased completions in wells. The open borehole
wells are holes drilled into the aquifer with the casing placed in the upper portion of the hole to
stabilize unconsolidated formation materials and facilitate the installation of grout for the sealing
of the well to prevent surface contamination. The second type is the screened well which consists
of a casing and screen or perforated pipe in the water bearing zones.
Specifically, wells are constructed to fit the geology of the area, with the open borehole well
being placed in consolidated formations and the screened well placed where geology is unconsolidated, such as alluvial deposits, plains, or river basins.
The construction of the well will necessitate some changes in the technology of well cleaning.
The decision to clean the well should be based on one or more factors, such as poor production,
observational reports dealing with water quality, or field or laboratory analysis. Because biological
fouling can lead to heavy blockage with mineral accumulation, taste and odor problems, red water,
and possible coliform or pathogen contamination, a simple assessment of biological activity can be
sufficient to determine the need for cleaning. The simplest assessment for biological fouling is to
direct the testing to the anaerobic bacterial population. When the anaerobic bacterial community is
abundant in the well, the well sanitation is in poor condition and in urgent need of cleaning. If the
contractor suspects that the “well problem” is due to the sanitary condition of the well, he should
take along a test kit or the means to sample the well water to provide evidence to the well owner of
biological buildup in the well. If he performs his own testing, he should be trained in appropriate
methods.
Several versions of user-friendly tests can provide a useful assessment of biological activity and
fouling within the well structure. This fouling leads to blockage through mineral precipitation and
the accumulation of contaminants of both a chemical and biological nature. This precipitation and
accumulation of contaminants also enhances corrosion of the well structure and taste and odor in
the water. The well contractor should consult with a microbiologist familiar with biofouling and
biocorrosion on how to choose and use the right tests. The sample for biofouling analysis ideally
should be drawn from a tap between the well and the system’s tank, so as to establish the condition
8
of the water in the well. The sample can also be taken from open pump discharge or from a sterile
bailed sample. However, if such options are not available, the test can be taken from any tap within
the system under evaluation, recognizing that the sample quality is compromised.
The sample should be collected in a sterile glass or plastic container. The container should be
filled completely and capped immediately. The contractor must not cough or touch the water sample, the container’s neck, or inside of the cap. The sample should be collected in midstream and
the faucet or orifice should not come in contact with the sample container. The sample should be
collected in the morning or at a time when the well has sat idle for at least eight hours. Water from
the down piping should be discharged and the water representing casing water should be collected.
This may require running the pump for 10 to 30 seconds or a calculated period of time before
collecting the sample. The sample should be delivered to a laboratory or used in a field kit within
12 hours. The sample should be kept at ambient ground water to room temperature and in a dark
space.
The water well system contractor should conduct a flow test to check flow and (if possible) the
drawdown. If the contractor is unfamiliar with these, NGWA recommends consulting the Manual
of Water Well Construction Practices. It is essential that well tests be performed properly to obtain
valid information.
All wells will require the temporary removal of the pump and isolation of water treatment or
conditioning equipment to accomplish complete cleaning. Any equipment removed from the well
should be secured from exposure to bacterial or chemical contamination. When this equipment is
ready to be reinstalled, it should be cleaned and disinfected appropriately, as well.
Check the apparent structural integrity of the casing and screen with a downhole camera.
Determine the real bottom of the well compared to the assumed bottom of the well so as to be
able to determine that all accumulated well debris has been removed.
A mechanical process or combination of mechanical processes should be used to begin the well
cleaning. The water well system contracting professional will use his experience to determine the
best selection from among many options.
Brushing and surge block surging is used for cased and screened wells. Jetting is the method of
choice in open borehole wells in some friable formations as surge blocks and brushes can harm
some of the more fragile borehole formations, such as Cretaceous or Triassic rocks in the central
states. Over much of the North American Great Lakes region, surge blocks and brushing are preferred methods that are used very successfully in open rock boreholes (carbonates and sandstoneshale sequences) and screened wells alike. Water or chemical jetting is preferred. Air jetting
should be avoided, although single-pipe air development is successfully used in open borehole
completions.
Where these methods are not readily available, economical, or feasible (as with very small
diameter wells) to deploy, airlift surging is a highly effective cleaning method for residential wells
that can be accomplished with rental-class compressors available in many communities. Airlifting
is also an effective adjunct to surge block cleaning. Surging should be accomplished with a light
pulsing that moves water up and down inside the well. Other methods such as Airburst or Sonar Jet
may be used as well.
Once the surging (airlift or surge block or both) has been completed, the water in the borehole
should be evacuated immediately to prevent settlement of suspended debris in the well environment. Evacuate or pump all the loosened debris from the well bottom. It is imperative that this
action be completed to the bottom of the well.
Following or in conjunction with a mechanical process, the water well system contracting professional should use an appropriate chemical cleaning process.
9
“
All wells will
require the temporary
removal of the pump
and isolation of
water treatment
or conditioning
equipment to
accomplish
complete cleaning.
“
When using a chemical cleaning process,
the prepared cleaning
solution should be
equal to 1.5 to 2
times the standing
well volume (SWV).
The contractor will
need to be familiar
with formation characteristics as many
rock formations will
not accept more than
1.5 times SWV, while
in more cavernous
formations, 2 times
may not be nearly
enough. Most
screened wells will
accept 2 times SWV.
When using a chemical cleaning process, the prepared cleaning solution should be equal to 1.5
to 2 times the standing well volume (SWV). The contractor will need to be familiar with formation
characteristics as many rock formations will not accept more than 1.5 times SWV, while in more
cavernous formations, 2 times may not be nearly enough. Most screened wells will accept 2 times
SWV. This solution should be tremied into the well evenly throughout the well. It is excellent
practice to use approximately 20% of the solution to clean the area above the water line.
All chemicals mixed for well cleaning should always be neutralized above ground when pumped
back out after cleaning.
When using an acid solution, be sure to maintain a pH of 3 or lower throughout the duration of
cleaning. Additional acid should be added if the pH rises above 3 before the solution is pumped
from the well. If the pH rises above 4, more than half of the dissolved debris will drop out of solution and be prevented from being pumped from the well.
If the solution is to be left in the well overnight to improve dissolution of contaminants, then
some surging should be applied the following day prior to pumping out any residual cleaning
solution.
A water well system contractor may elect to conduct a downhole inspection of the cleaned well
prior to disinfection.
Well Disinfection
Well cleaning should then be followed by a thorough disinfection of the well and its immediate
environment.
It is difficult to test following the cleaning process and prior to well disinfection. It is recommended to move directly from cleaning right to disinfection as testing for bacteria is difficult at
this point due to disturbance in the well. Warning: Be sure all the acid is pumped from the well as
hypochlorite used for disinfection may release chlorine gas if the pH is 5 or below. The pH of the
discharge water should be checked several times to ensure that pockets of acid do not remain.
The water well system contracting professional should consult the National Ground Water Association’s Manual of Water Well Construction Practices for guidelines for well disinfection.
This document was prepared as the result of regular meetings of a task group composed of
representatives from among the NGWA membership and dually affected individuals. These
volunteers included:
Ricky Layman
James C. Hefty, CWD/PI
Lawrence H. LaChance, MGWC
Kevin B. McCray, CAE
John H. Schnieders, Ph.D.
Stuart A. Smith, CGWP
Gilman G. Violette
Neil Mansuy
Todd E. Hunter
James R. Sparrow
Michael Vaught
Tom Platzer
Stephen Ordway
Clifford L. Grieves
Michael Wiedorn
Michael Barcelona, Ph.D.
Tom Smith
Abnel Estaba
Kieth Larrimore
John W. Pitz, CPI
Robert W. Masters (NGWA staff)
Compiled by: Jonathan Jenkins (NGWA staff)
10
Disclaimer:
This publication is a collaborative effort to try to set forth best suggested practices on this topic
but individual situations and local conditions may vary, so members and others utilizing this
publication are free to adopt differing standards and approaches as they see fit. The Association
assumes no liability or responsibility for the contents of the publication.
References
NGWA’s Illustrated Glossary of Driller’s Terms, NGWA Press, 2004
1
Ibid.
2
Copyright © 2008 by National Ground Water Association Press
ISBN 1-56034-079-7
NGWA Press
Published by: NGWA Press
National Ground Water Association
601 Dempsey Rd.
Westerville, OH 43081-8978
Phone/ 614 898.7791
Fax/ 614 898.7786
Email/ [email protected]
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Revised 11/2009