Disinfecting Private Wells

About 12 million American households, roughly 15 percent of the U.S. population, draw their drinking water from private wells. Unlike public water systems, which are regulated by the U.S. Environmental Protection Agency (EPA), private wells are the responsibilities of homeowners. EPA standards do not apply to private wells. Consequently, well-owning homeowners are obliged to protect and maintain their water supplies to optimize the quality of the drinking water supplied to their families. Many factors contribute to the quality of private well water. Some, such as routine testing and treatment, and properly positioning wells relative to point sources of contamination (including in-ground septic systems), are within the control of homeowners. Others, such as regional ground water quality and flooding episodes are much less so. For the private well owner, good quality drinking water depends on a multi-barrier approach to contamination that includes well monitoring and maintenance, locating the well away from points of contamination, and protecting the watershed from excessive pollutant and sediment runoff. This article focuses on one of the most important tasks a well-owner undertakes to assure high quality drinking water: disinfecting the well.

Ground Water Facts

Ground water represents the accumulation
of rainwater and melted snow and ice that filters down through soil,
sediments and bedrock to concentrate below the Earth’s
A watershed is a drainage
area from which the waters of a stream are drawn; it includes the ground
water in the drainage area.
About 98% of the available fresh water
on Earth is ground water.
Every day, the U.S. uses about 83.3
billion gallons of ground water.

Why Disinfect?

Groundwater is not 100 percent pure water. Because it collects in the tiny pore spaces within sediments and in the fractures within bedrock, ground water always contains some dissolved minerals. And because there is some life form occupying virtually every geological niche, there are many naturally occurring microorganisms in ground water. According to the National Ground Water Association (NGWA), most waterborne microbes are harmless and many are actually beneficial. Some, however, are pathogenic. Bacteria, for example, from the intestinal tracts of people and warm-blooded animals, such as E. coli, can cause disease and death. These pathogens may enter ground water through septic tank overflow or through contaminated runoff from woodlands, pastures and feedlots. Routine testing of private well water and, when needed, chemical disinfection are critical to maintaining a safe private water supply and avoiding the ravages of pathogenic microbes. Infection with E. coli, for example, can result in stomach discomfort, diarrhea, serious illness and even death.

Monitoring for Microbials

The first step in maintaining a safe water supply is regular monitoring for the presence of harmful microorganisms. NGWA recommends well owners have their well water tested at least once every year and also after significant flooding. Water should be analyzed by a qualified laboratory and include a bacterial analysis. Commercial laboratory test costs range from about $50 to about $150, depending upon the number of analyses requested. However, some county health departments may offer private well water testing at a lower cost. Alternatively, there are several “do-it-yourself” test kits available in the cost range of approximately $10 – $30. NGWA recommends test kits that are simple to use with no mail-in requirements. “Total coliform” is a “catch-all” category in water testing used to determine the microbiological quality of drinking water. This category includes a large group of bacteria that inhabits the soil and the intestines of warm-blooded mammals, including humans. While it is true that most coliform bacteria do not cause disease, it is also a fact that they are not normally found in ground water. Therefore, if tests reveal the presence of coliform bacteria, there is an indication that pathogenic bacteria, viruses and protozoa also may be present in the water.

The Human Cost of Insufficient Well Water Disinfection

In a tragic example of the consequences of insufficient well water disinfection at the municipal level, the small town of Walkerton, Ontario suffered an outbreak of waterborne disease in the year 2000 that left seven people dead and 2,300 ill. A particularly rainy spring that year contributed to the leaching of fertilizer manure into one of the town’s wells. The well was situated close to a farm. E. coli and C. jejuni bacteria in the manure contaminated the well water, which was neither being monitored nor disinfected sufficiently. The grim episode serves as a case study in the importance of diligent monitoring and sufficient disinfection.

Flood Risks

Floodconditions render well water particularly vulnerable to microbial contamination, especially if the wellhead (the part of the well above the ground surface) becomes submerged, allowing dirty water to flow into the well. Under normal circumstances, rain water and melted snow trickle gradually into the ground through the tiny spaces between grains of sediment. This action results in the natural filtration of ground water, in which particles, even bacteria, are separated out of the water by a “sieve effect.” During periods of flooding however, natural filtration is bypassed and wells can become contaminated rapidly. Shallow wells are at greater risk for contamination than deep wells during floods. According to the EPA, wells that are more than 10 years old or less than 50 feet deep are likely to be contaminated following a flood, even if there is no apparent damage. Disinfecting the Well If microbial contamination is discovered in private well water, immediate disinfection is required. This task can be carried out either by ground water professionals or by the homeowner using an array of information resources available from state and local health departments and government agencies. The most commonly used well water disinfectants are sodium hypochlorite (chlorine bleach) and calcium hypochlorite (chlorinated swimming pool disinfectant). Before disinfecting a well it is important to ascertain, to the extent possible, that the well is located and constructed such that it is protected from contamination sources. The following section lists the necessary supplies and a procedure for disinfecting private wells.

Private Well Disinfection Supply List

  • EITHER sodium hypochlorite (unscented chlorine laundry bleach containing 5 to 6% sodium hypochlorite) OR calcium hypochlorite (swimming pool granules containing 65 to 70% calcium hypochlorite-available at hardware stores and pool supply stores)
  • A two-gallon or larger bucket
  • A garden hose long enough to reach as far as possible into the well
  • A funnel that fits into the end of the garden hose

Private Well Disinfection Directions

  1. Determine the appropriate amount of chlorinating chemical needed to obtain 100 parts per million (ppm) available chlorine1 for routine disinfection or 500 ppm for emergency post-flood disinfection using Chart A for sodium hypochlorite or Chart B for calcium hypochlorite2.

Chart A: Quantities of Liquid Household Bleach3 (5-6%) Required for Well Disinfection [T =
tablespoon; C = cup; Q = quart; G = gallon]

Well diameter in feet (inches)
Depth of water in well
0.333 feet (4 inches) 0.5 feet(6 inches) 0.666 feet(8 inches)
Routine Disinfection Post-Flood Routine Disinfection Post-Flood Routine Disinfection Post-Flood
10 21/2 T 3/4 C + 1/2 T 1/2 C 21/2 C 3/4 C 3 3/4 C
20 1/2 C 21/2 C 3/4 C 33/4 C 13/4 C 2 Q + 3/4 C
50 1 C 1 Q + 1 C 21/3 C 2 Q + 32/3 C 41/3 C 11/4 G + 12/3 C
100 2 C 2 Q + 2 C 41/2 C 1 G + 61/2 C 1/2 G 21/2 G
150 3 C 3 Q + 3 C 1 Q + 3 C 2 G + 3 C 3/4 G 33/4 G
200 1 Q 1 G + 1 Q 2 Q + 3/4 C 21/2 G + 33/4 C 1 G 5 G

Chart B: Quantities of Dry Calcium Hypochlorite Pool Chemical4 (67%) Required for Well Disinfection [T = tablespoon; C = cup]

Well diameter in feet (inches)
Depth of water in well
0.333 feet (4 inches) 0.5 feet(6 inches) 0.666 feet(8 inches)
Routine Disinfection Post-Flood Routine Disinfection Post-Flood Routine Disinfection Post-Flood
10 1 T 5 T 1 T 5 T 1 T 5 T
20 1 T 5 T 2 T 1/2 C + 2 T 3 T 3/4 C + 3 T
50 2 T 1/2 C + 2 T 3 T 3/4 C + 3 T 5 T 11/2 C + 1 T
100 3 T 3/4 C + 3 T 6 T 13/4 C + 2 T 3/4 C 33/4 C
150 4 T 11/4 C 1/2 C 21/2 C 1 C 5 C
200 6 T 13/4 C + 2 T 2/3 C 31/3 C 11/4 C 61/4 C

2. If water is muddy or cloudy, as after a flood, run water from an outside spigot with a hose attached until the water becomes clear and sediment-free. 3. Divide the appropriate amount of chlorinating chemical among three or four bucketsful of water, mixing thoroughly. 4. Remove the well casing cap being careful not to contaminate the cap or let any debris fall into the well. Place one end of the garden hose as far as possible into the well. Place the funnel into the other end of the hose and pour the contents of each bucket through the hose while alternately raising and lowering the hose to disperse the disinfectant throughout the water supply. 5. When the correct amount of disinfectant has been added, close the well cover if the well has no pump. If the well has a pump, draw the chlorinated water through all the fixtures and outlets until the smell of chlorine is noticed. This will ensure that all piping and fixtures are disinfected. 6. Leave the chlorinating solution in the entire water supply system for at least twelve hours, but preferably, overnight. 7. Flush the chlorinated water completely out of the water supply system by opening a tap. The system will be flushed when the chlorine odor is no longer detectable. 8. After 7 to 10 days, sample water for coliform bacteria. If bacteria are detected, repeat disinfection procedure as many times as necessary until bacteria are no longer detected. If water becomes contaminated again after a short time, it is important to identify and remove the source of contamination.


Whether maintained by professionals or the homeowner who has access to approved procedures, there are significant responsibilities associated with private well ownership. Well owners should adopt a multi-barrier approach to safeguard their drinking water from contaminants that includes regular monitoring for waterborne pathogens and prompt disinfection when needed. It’s a responsibility that can mean the difference between illness and health for families relying on private
well water.

End Notes
1To disinfect iron bacteria, 1000 ppm chlorine is required. To obtain
1000 ppm chlorine, multiply all “Routine Disinfection” quantities by 10.
2Charts A and B are based on the Missoula City-County
Health Department’s “Well Disinfection Procedure”
3Use fresh bleach that does not contain detergent or other
additives. 4Handle with caution since dust will
irritate the eyes, nose, mouth and skin. Calcium hypochlorite is highly corrosive when