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How to Treat Well Water for the Presence of Coliform Bacteria

Private water wells can be contaminated quite easily. Coliform bacteria are organisms which may be present in well water. Animal feces, raw surface water, soil and decaying plants normally contain varying amounts of coliform.

These bacteria can migrate into well water and cause the water to become biologically polluted. Water that is found to contain coliform is considered unsafe to drink and must be treated.

Test your well water for the presence of coliform bacteria. Many state health departments provide well water sampling kits for the homeowner to test the source of drinking water. University extension offices, as well as commercial businesses also provide testing kits for a small fee. Once you have determined that your well contains the coliform microbes, you will need to chemically treat the water. The most common way to do this is through the use of ordinary household bleach. This is also known as shocking the well.

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Determine the amount of water in your well. If you do not have records on your water well, you can contact the company that drilled the well and ask for that information. Many counties also keep records of the drilling permit and the driller's log, which contain information on the depth of the well and the level of the water table. If you are unable to obtain these records, just make an estimate based on averages for your location. You will use this information to figure the amount of chlorine necessary for disinfection.

Remove any carbon filters from your water supply line. Carbon filters will interfere with the chemical action of the chlorine bleach. Remove all items from the vicinity of faucets and outside hose bibs that may be damaged or discolored by the bleaching action of the chlorine.

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Establish the amount of chlorine bleach needed. Take the number of feet of water in the well and divide it by 66. This will give you the number of quarts of bleach you need. For example, if your well is 200 feet deep and the water table is at 50 feet, you will use a little over 2 quarts of bleach, since you can estimate that you have about 150 feet of water in the well.

Pour the bleach down the well shaft, allowing it to run along the inside edges of the pipe. Let the bleach stay in the well for thirty minutes. Do not run any water during this time.

Open all the faucets and hose bibs until the smell of chlorine is present at each outlet. Then shut off all faucets and refrain from running any water for at least twelve hours. This will allow the chlorine to thoroughly penetrate the water in the well and throughout the entire plumbing system.

Retest your water within two days to make certain the bacteria have been eradicated. You will need to take extra measures if your water sample still contains coliform bacteria. Try to determine the source of the contamination. This can be caused by the soil around the well opening becoming recessed enough to hold water and debris which can seep into the well. You can easily correct this by raising the soil around the base of the well cap to create adequate draining away from the casing. More extreme measures may be called for if the bacteria are found to be migrating from a nearby septic system or livestock area.


Perform the water safety test at least once per year if your initial test comes back free of bacteria. However, wells that have been found to test positive for coliform bacteria must be treated and retested until the problem is resolved.

The Environmental Protection Agency (EPA) provides additional information regarding the safety of drinking water. Call the EPA's Safe Drinking Water Hotline at (800) 426-4791


Take safety precautions when working with chlorine bleach. Use goggles, rubber gloves, and avoid inhaling the fumes. Wear old clothes that will not be damaged by the bleach.