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How to Shock-Chlorinate or Disinfect Your Private Water Supply or Water Well (Chlorine Disinfection)

petri dish sample showing bacteria
An Enumeration (Count the Colonies) Test Method for Total Coliform Bacteria.

Unlike public water supplies that are regularly tested to ensure the water is safe to drink, individuals or families using private water supplies are responsible for testing for contamination.

If test results indicate that bacterial contamination is occurring, shock-chlorination or disinfection is the most widely suggested method for initial treatment. Shock-chlorination (disinfection) is the one-time introduction of a strong chlorine solution into the entire water distribution system (well, pump, distribution pipeline, etc.).

When to Shock-Disinfect the Well

Shock-chlorination (disinfection) is recommended:

  • When lab results indicate the presence of Bacteria.
  • Upon completion of a new well or after pump replacement or repair.
  • When the distribution system is opened for repairs or maintenance or when the system has not been used on a regular, i.e., weekly basis.
  • Following contamination by flood water, a contamination incident, or damage to the system.
  • To control Iron, Slime, and Sulfate-reducting Bacteria and other nuisance issues including discolored water, high iron, manganese, turbidity, black water, or even odors.

Shock-chlorination (disinfection) is recommended in these circumstances to ensure that any bacterial contamination is controlled.


How to Shock-Disinfect a Well Safely

Safety Practices

Before you begin the shock-chlorination process, run some fresh water into a five-gallon container. If concentrated chlorine accidentally comes in contact with your eyes or skin, use this fresh water to flush the affected area for 10-15 minutes. If you get some of the chlorine solutions in your eyes, see your doctor after thoroughly flushing the affected eye.

A second safety practice is to wear appropriate safety clothing and equipment. Wear goggles to avoid contact with the strong chlorine material and your eyes. Wear a pair of rubber gloves to protect your hands and rubber boots on your feet. To prevent discoloration of your clothing, wear a waterproof suit, coveralls ,or a full-length apron.

During the Shock-Disinfection process, do not use the water for bathing, drinking, washing clothes or dishes, or for any other potable or sanitary use. Also, do not use the water to irrigate your lawn or crops.

Preparations

Begin the shock-chlorination (disinfection) procedure by:

  • Being sure that the well construction is adequate to prevent direct entry of contaminants.
  • Finding and eliminating any obvious source of potential contamination, such as a poorly installed well cap, a well that is below grade, or a well with damaged casing or leaky pitless adapter.
  • If you are changing a pump, piping, or electric lines, making sure to disinfect the components prior to installing in the well.
  • Isolating portions of the system that may be degraded by the strong chlorine solution, such as a carbon filter/ water softener/ and many reverse osmosis treatment units. When doing a shock-disinfection we strongly recommend using Well-Safe Sanitizer.

The best way to prevent a water supply from being contaminated by bacteria or pathogens is to eliminate the bacteria's access to the water source. Controlling access to the water supply by contaminants is difficult if the water supply is a pond, spring, or other surface water. In some cases, sealing up cracks in well pits, spring houses (or spring boxes), and other potential points of entry will suffice. Be sure to remove all debris (leaves, twigs, etc.) from the spring house, well pit, or storage reservoir.

Well Chlorination (Disinfection):

Shock-chlorination or shock-disinfection of the well consists of mixing sufficient quantities of a chlorine-based chemical with the well water to create a solution containing 200 milligrams per liter (mg/l), or parts per million (ppm), of chlorine throughout the entire system (well, distribution pipeline, water heater, pressure tank, and other equipment).

Remember that chlorine is very volatile so it is dangerous to work with in confined areas.

Make sure the work area is well-ventilated. Prepare a mixture of one-half gallon of household bleach without additives per 5 gallons of fresh water. Disinfect the well pit, spring house, or other portions of the distribution equipment that may contribute bacteria to the water supply (pump, motor, pressure tank, and exposed wiring conduits) as follows:

Drain as much water from the system as possible. For systems with pressure tanks containing a bladder, the rubber air-water separator inside the tank could be damaged by the chlorine solution. Check the manufacturers' recommendations to determine if the pressure tank should be bypassed. For pressure tanks without bladders, release the air so that the tank can be filled with chlorinated water. Drain water from the water heater so that chlorinated water can be circulated through the hot water pipelines.

Backwash and clean-water softeners, sand filters, and iron removal filters with a strong chlorine solution. Do not chlorinate activated-carbon filters since these filters will remove the chlorine until they become overloaded. Activated-carbon filters, water softeners, and many whole-house, point-of-entry, and point-of-use filters should be removed or by-passed until after chlorine has been flushed from the system.

Table 1 | The volume of water contained per foot of well depth.

Well casing diameter
(inches)
Water volume per foot of
water depth (gallons)*
4 0.65
6 1.47
8 2.61
10 4.08
12 5.88
18 13.22
24 23.50
30 36.72
36 52.87


* Volume of water calculated as the volume of a cylinder multiplied by 7.48 gallons/cubic foot. You can try our Wellbore Volume Calculator.

Calculating the Amount of Chlorinating Chemical to Put Into the Well

Step 1 | Determine the depth of water in the well.

The company that constructed the well should be able to provide you with the well depth and water level. For example, let's say that you have a well that is 100 feet deep, and the water level is at 40 feet from the top of the casing. The well contains approximately 60 feet of water (100 – 40 = 60 feet).

Step 2 | Determine the volume of water in the well.

You measured the inside diameter of the well and it was 6 inches. Find the gallons per foot of depth for a 6-inch well in Table I. For our example we would multiply the depth of the water in the well (60 feet) by 1.47 gallons of water per foot of water depth (from Table I) to get 88.2 gallons of well water (60 x 1.47 = 88.2gallons of water in the well).

Step 3 | Estimate the volume of water in the distribution system.

Total up the water storage in the system, including the water heater, pressure tank, etc., and add 50 gallons for the pipeline. If you have a 30-gallon hot water heater and a 10-gallon pressure tank, you need to add 90 gallons for the distribution system.

Step 4 | Determine the water contained in the entire system.

Add the water volume in the well to the water contained in the distribution system to get 180 (178.2) gallons.

Step 5 | Determine the amount of chlorine product required for a 200 ppm solution.  

Our preferred method and chemical in Shock Well Disinfection is an NSF approved product for drinking water. This product has 65 - 75% available chlorine and only 4 ounces of this dried chemical is needed to treat 100 gallons of water. Therefore in our example, we would need to add 8 (7.2) ounces of this approved chemical. 

Only in emergency situations, some users and contractors will use laundry bleach. If using laundry bleach, use a bleach that doesn’t contain scents of coloring agents. Standard bleach has 5.25 percent available chlorine. You will need 3 pints of bleach per 100 gallons of water in the well and distribution system. For our example, you would need to have at least 6 (5.4) pints. You can also use our Wellbore Calculator and disinfection calculator.

Table 2 | Amount of chemical required to create a chlorine concentration of about 200 ppm

Chemical Name Amount per 100 gallons of water*
Liquid Laundry Bleach (5.25% NaOCl) 3 pints
Commercial Strength Bleach (12-17% NaOCl) 1 pint
Chlorinated Lime (25% CaOCl2) 11 ounces
Dairy Sanitizer (30% CaOCl2) 9 ounces
High-test calcium hypochlorite** (65-75% Ca(OCl)2) 4 ounces


* Well water containing iron, hydrogen sulfide, or organic substances may require more chlorinating chemical to create a 200 ppm solution. Chlorine combines readily with these materials, making some of the chlorine unavailable as a disinfectant.

** Concentrated hypochlorite is available as a powder and as a pellet (Well Safe Sanitizer Kit).

The process works best if the pH is between 6.5 and 7.5 and when the ORP (Oxidation Reduction Potential) is raised to 650 to 700 mv. (Report on Using ORP)

Step 6 | Introduce the chlorine material into the well and distribution system.

The best way to introduce chlorine material into the well is to dissolve a portion of the dried chlorine chemical in a 5-gallon bucket of fresh water. Be sure the bucket is plastic and has been thoroughly washed. Take the remaining dried pelletized chemical and pour into the well. The pellets will sink to the bottom of the well.  Then pour the chlorine solution into the well. 

Attach a hose to the water hydrant or faucet nearest the well and run water through the hydrant and back into the well. This will thoroughly mix the chlorine solution throughout the well.  During this process, make sure to wash down the insides of the well casing and make sure there are no pellets or solid chlorine powder in contact with a pitless adapter that may be in your well.

Regardless of how you introduce the chlorine material into your well, start and stop the pump several times to ensure that the chlorine is thoroughly mixed with the well water. Recirculate the water until a strong chlorine smell has been noted for at least five minutes.

If your system has a UV disinfection system, whole-house water treatment unit, water treatment tanks or point-of-use devices, these need to be by-passed.  If your home has any inline-filters on appliances, the appliances should not be used or the lines to the inline-filters should be closed. After the chlorine has been placed in the well and the casing, etc., has been washed down, move around the water distribution system and open each faucet (hot and cold), hydrant, or other water outlet. Allow water to flow until a strong chlorine odor reaches that position in the system. Then close the valve at that location. Do this with all faucets, hydrants, and other outlets in the system.

If a strong chlorine odor is not detected at each site, add more chlorine to the well. This may be an indication that your well contains Iron, Hydrogen Sulfide, or organic materials that are using up the chlorine. The reason for the lack of odor is caused by an oxidizing reaction between the chlorine and something in the water.

Step 7 | Let the chlorine disinfect the system.

The most difficult step is to refrain from using water from the well so that the chlorine can disinfect the system. The system should remain idle for at least 2-3 hours, preferably overnight.

Step 8 | Flush the system to remove the chlorine.

After the water system chlorination has been completed, the entire system must be emptied of chlorine and thoroughly flushed with fresh water.  If your home uses a septic system or on-site wastewater management system. We recommend that you first flush the chlorine from the well by connecting to an outside faucet and running the water to a non-grassy area or wooded landscape. Do not run this water to a stream, lake, pond, or wetland. After most of the chlorine has been flushed from the well, open up the other faucets or hydrants to remove the chlorinated water that is in the distribution system.  

NOTE: Distribute the waste water on gravel roads or other areas without plants or aquatic life, which it might harm. Do not allow the bulk of the chlorinated water to enter the septic system.

Step 9| Retest the water supply for bacterial contamination.

The final step is to retest the water to ensure that the water source is Bacteria-free. Take a water sample 1-2 weeks after shock-chlorinating the well, using the same procedures as before. Though most shock-chlorination treatments are successful, do not drink the water until the laboratory results confirm that no bacteria are present. Retest the well every month for 2-3 months to be sure contamination is not recurring. If test results are negative, an annual water analysis program can be reinstated.

If the water supply continues to develop bacterial contamination problems after being shock-chlorinated, continuous chlorination or disinfection may be an option. Other options include repairing the well or constructing a new well. It may be necessary to abandon the water source. Procedures for properly abandoning a well may be obtained by your local Department of Environmental Protection. You may want to contact a licensed water well contractor to perform these duties. For more specific information we suggest you Contact Us.

Remember

We recommend using the Well Sanitizer Pack for Shock-Disinfecting water wells.  The product is very effective, is easier to use than liquid chlorine, and doesn’t have the negative qualities of standard household bleach. 

Liquid chlorine compounds, such as standard household bleach, are volatile so they will degrade with time. Household bleaches may contain scents or other additives that can compromise the quality of potable or drinkable water. When using chlorine bleaches, do not purchase bleaches that have scents or other additives. Do not add other cleaning materials to the chlorine solution. Some combinations of chlorine and acids or ammonia could produce dangerous gasses.

Make sure all work areas are well-ventilated.

Well pits do not meet most local well construction criteria because it is difficult to preclude contamination. The best option is to construct a new well using current construction criteria and/or consider cement grouting in the pit and extending the casing.

Ozone - It may be possible to develop an Ozonation system to disinfect the well. This can be done by either recirculating ozonated water back to the well or inserting an air line containing ozone directly into the well. The use of ozone creates special conditions and should be done by a professional. Ozone can adversely impact some types of piping, rubber, and other components of a private water system.

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