Well Water Program

If you need to test your water for bacteria go to the Water Tests page.


Received a positive test for bacteria?

Review these helpful publications:


More about bacteria in well water


Useful chlorination formulas

These recommendations are from the Private Water System Handbook, Midwest Plan Service, Iowa State University, MWPS-14, 1992

All of these formulas are for common household bleach, like Clorox, Purex, or a generic brand. The label should indicate that the bleach contains 5-6% Sodium Hypochlorite. Do not use bleach with additives or special scents - just plain laundry bleach.

Solution for cleaning surfaces such as the inside of a holding tank, cistern, or spring box. After construction, cleaning, or maintenance, flush the cistern thoroughly with water to remove sediment. Use a stiff brush or broom to thoroughly wash all inside surfaces. A recommended solution is given below. Note: Do not go into a holding tank (or septic tank) without positive air flow - toxic gasses can settle in these tanks. If possible, do not go in the tank at all.

1/4 cup of 5.25% chlorine laundry bleach per 10 gallons of water

Solution to shock chlorinate a large volume of water.

Note: If you need to disinfect water in a holding tank, you may consider draining the tank, cleaning as described above, and then refilling it with sanitary water (like from a well that is free of bacteria).

3 pints of 5% chlorine bleach per 100 gallons of water = 200 mg/l (ppm) chlorine concentration

If your system is plumbed such that you must shock chlorinate the water in the holding tank to disinfect your drinking water system, then clean out the tank as much as possible before shocking the system.

  • Remove debris and sediment (try a wet-dry vacuum)
  • Scrub the walls with a very strong chlorine solution (1/2 gallon cholorine laundry bleach per 5 gallons of water)
  • Pump the system to remove the water with suspended matter from the holding tank
  • Then add bleach to water to shock chlorinate.

Note: Chlorination isn't effective if the water isn't already fairly clean, so don't bother trying this if the water is cloudy or has a lot of suspended matter in it.

8 cups (1/2 gallon) of chlorine bleach per 1000 gallons of water

Mix proper amount of chlorine and water together thoroughly and let stand for 20 minutes. If the water does not have a slight chlorine smell, repeat the dosage and wait another 15 minutes. If after the second dosing, the water still does not have a chlorine odor, it isn't suitable as a drinking water source. If the chlorine taste is too strong, let it stand exposed to the air or pour it from one container to another several times.

You may be able to use the water source if you filter it first. Pour a gallon of water through a paper filter or clean cloth.  To treat one gallon of water, use 10 drops of chlorine bleach, mix, wait 30 minutes, smell for chlorine.  If no odor, repeat the treatment.  If after the second dose of bleach, there is still no odor, don't consume the water.


See EPA's fact sheet for information on wells and septic systems

If you need to disinfect drinking water temporarily, such as after a flood or any time bacteria is found in your water, read about using chlorine bleach or boiling to make your water safe:

Emergency Disinfection of Drinking Water (US Environmental Protection Agency)