Fort Collins

Boulder/GNC Water Well

Your water treatment experts

Water, especially well water, should be tested for potability on a yearly basis. Various laboratories use different methods to accomplish this, but the most common is the Colilert test. Test results are commonly given in a Pass/Fail format, with more extensive reports listing a value with the units C/100 mls or colonies per one hundred milliliters. Too numerous to count (TNTC) is a result that can be returned if there are too many colonies present to count by hand. If the result of the Potability test is failure, (or, 1 or more C/100mls or TNTC), the laboratory will recommend chlorinating you water supply. Chlorinating is an excellent first step in solving most bacteria contamination problems. The following paragraphs describe step by step instructions about how to use chlorine to disinfect your water system.



1. To chlorinate the well:

  1. Remove the well cap, concrete lid, or pipe plug to allow direct access to the well. Attach one end of a hose to an outside faucet and place the other end into the well, so it discharges into the well.
  2. Run water through the hose and into the well at a rate of about 3-5 gpm. While you are running the water into the well, pour the amount of bleach determined with the chart in section 3, into the well. Be sure to use only unscented Sodium Hypochlorite bleach. The chart is based on using 5.25% concentration, but it is okay to use up to 6% (as is common with modern bleach) using the same values. After Chlorinated water starts to flow from the hose, continue circulating water for about 10 minutes, spraying the walls of the well, well cap (or seal), or concrete casing. Do not spray directly on any electrical connections. The garden hose can be shut off and put away at this point.
  3. At the house, open one cold water faucet at a time, letting it run until chlorinated water flows. Repeat for all cold water fixtures including toilets, washing machine, dishwasher, exterior hose bibs, yard hydrants, showers and baths.
  4. After all cold water is chlorinated, do the same for all the hot water fixtures. Chlorine should take longer to flow from the hot water as it will be diluted by the hot water heater. Again include washing machine, dishwasher, showers and baths and all faucets.
  5. The Chlorinated water must stand in the pipes for a minimum of eight hours to effectively treat bacteria, and a maximum of 36 hours to protect the plumbing. Use no water during this period. You will need an alternative source of drinking and cooking water. Do not use chlorinated water directly on plants and do not wash clothing. At the end of this period you may be able to flush the pipes per section 4.


2. If you have a cistern or other type of storage tank, it's possible that this is a source of contamination. Airborne bacteria can enter the cistern, if not properly sealed, and propagate.

  1. You may follow steps A through E above, but be aware that the volume of the cistern (from 100 to 3000 gallons) is a factor in the dilution of the chlorine. Add to the amount determined in step B; one cup of bleach per 31 gallons of stored water. A large amount of time is necessary for circulating the chlorine through larger cisterns, so it may be more beneficial to chlorinate the cistern directly.
  2. You may want to chlorinate the cistern directly if you know the contamination source is there, if it is not possible to chlorinate the well, or if the size of the cistern necessitates additional chlorine. Use a garden hose as described above but direct it into the cistern. Add one cup of 5.25% bleach (unscented) per 31 gallons of stored water while continuing to circulate the water. When chlorinated water is flowing from the garden hose, wash down the walls and lid of the tank before sealing it back up. Continue as per 1c above.


3. The volume of water treated predominantly determines the amount of chlorine you will use in this process. Well drillers and pump installers are required to use 100 parts per million (ppm)as a minimum, to disinfect the well after their work. This is significantly higher than a household treatment will require. Generally, 1+ppm of free chlorine is sufficient for household treatments. A chlorine test kit may be used to measure the concentration. The chart below shows the concentration needed for 100 ppm in 100 feet of water as recommended by the USDA.

USDA Recommendations:

Liquid chlorine required to dose 100 feet of water at 100 ppm.

Casing Diameter


Volume/100 ft.


5.25% Sodium hypochlorite











4. Your water will be quite strongly chlorinated when you finish the treatment. To remove the chlorine and have your water chlorine free, the well and/or cistern must be emptied of the chlorinated water. Typically discharging water to an area without plants can flush the majority of chlorine out. Avoid excessive discharge of high levels of chlorine to a leach field or sewer as this can dramatically effect the chemistry of your waste water system. Chlorine will dissipate when water is aerated, so some people will water plants with misting irrigation devices and the equivelent. In the winter, or when water supplies are limited, you may want to have an alternative source of water for drinking, cooking, watering inside plants, etc. for use until the chlorine is at a more tolerable level. Another Potability sample may be taken twenty-four hours after chlorine is no longer detected in the water.

5. Coliform may be treated on an on-going basis by one of the methods outlined below.

  1. According to the Safe Drinking Water Act, continuous chlorinating is mandatory for public drinking water supplies.
  2. Private wells are exempt from this law and may also use the following water treatment methods to disinfect water:
  • Ultraviolet Water Sterilization
  • Iodine Injection
  • Ozone Injection
  1. Micro-filtration

Various Authors. Rewritten 03/15/02 Joann Foss



Member of:


Boulder GNC Water Well apparel and other cool stuff, like this tote bag with pictures of an up and coming water well tech!.