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Procedure for Sanitizing or “Shock Chlorinating” a Well

Friday, March 23rd, 2012 | Maintaining a Home | No Comments

If you use a well for drinking water, ensuring that the water is safe to drink should be a high priority. Well water can be the best water for your health provided that you take certain precautions. Well water quality can be negatively affected by factors such as: “bio-film” build-up in pipes and equipment, iron bacteria plugging up pump and equipment, natural events such as flooding and spring runoff can cause contamination, hydrogen sulfide present in water can cause smelly water, and anytime a well cap is removed the well should be sanitized before it is put back in service. Sanitation, or “shock chlorination”, is needed for these reasons; and, annual maintenance sanitation of a well is also recommended.

Because conditions can change with time, shock chlorination of the well two weeks prior to a well test is a good idea. Without shock chlorination, you may get a “false positive” bacteria test result. “False” because it may not be a true measure of water quality and may reflect the condition of your pipes and equipment instead. This can be a major issue for a home seller being served by a well. Real estate agents should encourage home owners who have homes served by a well to perform this procedure when putting the property up for sale. The use of liquid household bleach for shock chlorination is generally not the best choice because it is less stable and often contains additives, but straight sodium hypochlorite (the key component in household bleach) could be used in a time constrained period. Sodium hypochlorite solution (10%-15%) is available from local hardware stores or places that sell “well equipment”.

The best choice for shock chlorination is granular or pellets such as ChlorPel available from Grainger and other sources. Caution: these treatment pellets are not the same pellets used to treat swimming pools and spas. These pellets are designed specifically for treatment of drinking water and will cost more than swimming pool treatment pellets.

The procedure is as follows:

1. Check the pH of the well water. The ideal pH for shock chlorination is 6-7. If necessary, pre-treat the well water with an acid before shock chlorinating (do not mix acid and chlorine at the same time). This can be done with acid, such as muratic or phosphoric, or dry acids like sulfamic or sodium bisulfate.
2. Determine the amount of water to be sanitized refer to the table below.
3. Take cap off well casing and drop the proper amount of pellets into the casing. It is very important that when pellets are dropped into the well, cistern, etc., that the pellets get to the water and are not lodged on wiring, pitless adaptor, etc. Make sure that the pellets can be heard hitting the water.
4. Drop 1/2 of the pellets into the well. Dissolve the remaining pellets in a few gallons of water and pour into the well.
5. To mix the concentration in the well, run water from a hydrant or faucet (plumbed to the well) into the well for about 15 minutes. This re-circulates the well water, and after a short period of time there will be a strong chlorine odor. Turn off the water.
6. Run water through all taps, spigots, etc., connected to the water system until the smell of chlorine is present. Allow treated water to stand 12-24 hours. After this time, flush all taps through the outside hose bibb. You do not want chlorine going in to the septic system as it can kill the beneficial bacteria in the system.
7. After the water pipe system has become completely flushed with clean water, a bacteria test can be done.  Ideally, this test should not be performed sooner than five days after disinfection to be certain the water has no continuing chlorine contamination and that the water test is representative of the water normally used for drinking.


2 – 10″ diameter wells
Well # pellets/ cups pellets/
Diameter(in) 100 ft water 100 ft water

Diameter # Pellets Cups
2 9
3 20
4 36
5 57 1/4
6 82 1/3
8 146 2/3
10 228 1

If you need more information, or, assistance with a well bacteria test please call me at 865-453-9965.
Tom Maides
Accu-Spec Inspection Services PC