Disinfecting Wells After a Disaster

A picture of a well pump with water coming from the spigot into a barrel.

If you suspect that your well may be contaminated by germs from flood waters or another source, contact your local, state, or tribal health department or agricultural extension agent for specific advice on inspecting and disinfecting your well. If possible, use a contractor with experience in disinfecting and servicing drinking water wells. Use this guidance after checking with local authorities for flood precautions for private wells in your area. If you do not get your water from a well, see Keep Food and Water Safe for information on clean, safe drinking water during a natural disaster or emergency.

IMPORTANT: Fuel and other chemical releases and spills are common during floods. If your water smells like fuel or has a chemical odor, contact your local, state, or tribal health department to request a chemical analysis of your water before using it. Until you know the water is safe, use bottled water or some other safe supply of water. Boiling or disinfecting water contaminated with toxic chemicals or fuels will not make it safe.

Safety Precautions

Clear hazards away from wells before cleaning and disinfecting them. Follow these precautions:

  • Turn off all electricity to the well area before clearing debris. Do not attempt to repair the water system unless you are experienced with this type of work: electrical shock can occur. Inspect all electric connections for breaks in insulation and for moisture. Connections must be dry and unbroken to avoid electric shock.
  • Carefully inspect the area around the well for hazards such as power lines on the ground or in the water; sharp metal, glass, or wood debris; open holes; and slippery conditions.
  • Do not enter the well pit. Gases and vapors can build up in well pits, creating a hazardous environment. Clear debris from dug wells using buckets, grappling hooks, nets, and long-handled scoops.
  • Before the power is turned back on for the well, a qualified electrician, well contractor, or pump contractor should check the equipment wiring system.
  • Wear protective goggles or a face shield when working with chlorine solutions. Chlorine solutions may cause injury to the eye, irritate skin and damage clothing.
  • Work in well-ventilated areas and avoid breathing vapors when mixing and handling chlorine solutions.
  • Warn users not to drink or bathe in water until all the well has been disinfected.
Disinfection of Bored or Dug Wells

Bored and dug wells can be difficult to disinfect because the shallow depth and inadequate protection from flood water can allow contaminants to re-enter the well.

IMPORTANT: Bored or dug wells contaminated with fuel or toxic chemicals will not be made safe by disinfection. If your water smells like fuel or has a chemical odor, contact your local, state, or tribal health department for specific advice.

Follow these steps to disinfect bored or dug wells:

  1. If the well has an electrical pump, turn off all electricity and clear debris from around the top of the well.
  2. Repair the electrical system and pump if needed. Contact a qualified electrician, well contractor, or pump contractor if you are not experienced with this type of work.
  3. Start the pump and run water until it is clear. Use the outside faucet closest to the well to drain potentially contaminated water from the well and keep unsafe well water out of the interior household plumbing. If there isn’t a pump, bail water from the well with a bucket until water is clear.
  4. If the well is connected to interior home plumbing, close valves to any water softener unit.
  5. Use Table 1 to determine the amount of liquid household bleach needed to disinfect the well. Use only unscented bleach. For a table in metric units, please see Table 1.1: Approximate Amount of Bleach for Disinfection of a Bored or Dug Well (Metric).

Table 1. Approximate Amount of Bleach for Disinfection of a Bored or Dug Well
  Depth of Water Diameter of Well
 0.5 foot  1 foot  2 feet  3 feet  4 feet  5 feet
 10 feet  1/2 cup  1-3/4 cups  7 cups  1 gal  1-3/4 gal  2-3/4 gal
 20 feet  1 cup  3-1/2 cups  14 cups  2 gal  3-1/2 gal  5-1/2 gal
 30 feet  1-1/2 cups  5-1/4 cups  1-1/4 gal  3 gal  5-1/4 gal  8-1/4 gal
 40 feet  2 cups  7 cups  1-3/4 gal  4 gal   7 gal  11 gal
 50 feet  2-1/2 cups  8-3/4 cups  2-1/4 gal   5 gal  8-3/4 gal  13-3/4 gal

Notes:

  • Use only unscented household liquid chlorine bleach.
  • Bleach concentrations are generally between 5-6% and 8.25%.
  • Quantities given in this table are approximate and are rounded to the nearest practical measurement. Amounts given are calculated in accordance with reaching a chlorine concentration of > 100 mg/L

Key:

  • gal: gallon
  • 1 cup = 8 fluid ounces
  • 1 gallon = 16 cups
  1. Using a 5-gallon bucket, mix the bleach from Table 1 with 3-5 gallons of water (12-19 liters).
  2. Add the bleach water mixture to the well. Avoid all electrical connections. Attach a clean hose to an outside faucet and use it to circulate water back into the well for thorough mixing. If there isn’t a pump, mix water by pouring it back into the well using a bucket.
  3. Rinse the inside of the well casing with a garden hose or bucket for 5-10 minutes.
  4. Open all faucets inside the home and run the water until you notice a strong odor of chlorine (bleach) at each faucet. Turn off all faucets and allow the solution to remain in the well and plumbing for at least 12 hours.
  5. After at least 12 hours, attach a hose to an outside faucet and drain the chlorinated water onto an area without plants or vegetation, such as a driveway. Continue draining until the chlorine odor disappears. Avoid draining into open sources of water (streams, ponds, etc.).
  6. Turn on all indoor faucets and run water until the chlorine odor disappears.
  7. Until well water has been tested, boil it (roiling boil for 1 minute) before using or use another alternative water source. Wait at least 7-10 days after disinfection, then have the water in your well tested. Water testing cannot be done until all traces of chlorine have been flushed from the system.
Disinfection of Drilled or Driven Wells

Follow these steps:

  1. If the well is equipped with an electrical pump, turn off all electricity and clear debris from around the top of the well.
  2. Repair the electrical system and pump if needed. Contact a qualified electrician, well contractor, or pump contractor if you are not experienced with this type of work.
  3. Start the pump and run water until it is clear. Use the outside faucet closest to the well to drain potentially contaminated water from the well and keep unsafe well water out of the interior household plumbing. If there isn’t a pump, bail water from the well with a bucket or other device until the water is clear.
  4. If the well is connected to interior home plumbing, close valves to any water softener units.
  5. Use Table 2 to determine the amount of liquid household bleach needed to disinfect the well. Use only unscented bleach. For a table in metric units, please see Table 2.1: Approximate Amount of Bleach for Disinfection of a Drilled or Driven Well (Metric).

Table 2. Approximate Amount of Bleach for Disinfection of a Drilled or Driven Well
  Depth of Water Diameter of Well Casing
2 inches 4 inches 6 inches 8 inches 10 inches 24 inches 36 inches
 10 feet 3/4 tbsp 3-1/4 tbsp 1/2 cup 3/4 cup 1-1/4 cups 7 cups 1 gal
 20 feet 1-1/2 tbsp 6-1/2 tbsp 1 cup 1-1/2 cups 2-1/2 cups 14 cups 2 gal
 30 feet 2-1/4 tbsp 9-3/4 tbsp 1-1/2 cups 2-1/4 cups 3-3/4 cups 1-1/4 gal 3 gal
 40 feet 3 tbsp 13 tbsp 2 cups 3 cups 5 cups 1-3/4 gal 4 gal
 50 feet 3-3/4 tbsp 1 cup 2-1/2 cups 3-3/4 cups 6-1/4 cups 2-1/4 gal 5 gal
100 feet 7-1/2 tbsp 2 cups 5 cups 7-1/2 cups 12-1/2 cups 4-1/2 gal 10 gal

Notes:

  • Use only unscented household liquid chlorine bleach.
  • Bleach concentrations are generally between 5-6% and 8.25%.
  • Quantities given in this table are approximate and are rounded to the nearest practical measurement. Amounts given are calculated in accordance with reaching a chlorine concentration of > 100 mg/L

Key:

  • tbsp: tablespoon
  • gal: gallon
  • 1 cup = 8 fluid ounces = 16 tablespoons
  • 1 gallon = 16 cups
  1. Using a 5-gallon bucket, mix the bleach from Table 1 with 3-5 gallons of water (12-19 liters).
  2. Remove the vent cap.
  3. Pour the bleach water mixture into the well using a funnel. Avoid all electrical connections. Attach a clean hose to the nearest outside faucet and use it to circulate water back into the well for thorough mixing.
  4. Rinse the inside of the well casing with a garden hose or bucket for 5-10 minutes.
  5. Open all faucets inside the home and run the water until you notice a strong odor of chlorine (bleach) at each faucet. Turn off all faucets and allow the solution to remain in the well and plumbing for a minimum of 12 hours.
  6. After at least 12 hours, attach a hose to an outside faucet and drain the chlorinated water onto an area without plants or other vegetation, such as a driveway. Continue draining until the chlorine odor disappears. Avoid draining into open sources of water (streams, ponds, etc.).
  7. Turn on all indoor faucets and run water until the chlorine odor disappears.
  8. Until well water has been tested, boil it (rolling boil for 1 minute) before using or use another alternative water source. Wait at least 7-10 days after disinfection, then have the water in your well sampled. Water sampling cannot be done until all traces of chlorine have been flushed from the system.
Sampling After Disinfection
  • Wait at least 7 to 10 days to test the water after disinfection to ensure that the chlorine has been thoroughly flushed from the system. Until well water has been tested, boil it (rolling boil for 1 minute) before using or use another alternative water source.
  • Contact your local health department for water sampling and testing information or contact your state laboratory certification officer to find a certified lab near you. You can also get water sampling information from the U.S. Environmental Protection Agency’s Safe Drinking Water HotlineExternal (800-426-4791).
  • Sample the water for total coliform and either E. coli or fecal coliform bacteria to confirm that the water is safe to drink.
  • If results show no presence of total coliforms or fecal coliforms, the water can be considered safe to drink.
  • Follow up with two more water tests, one in the next 2 to 4 weeks and another in 3 to 4 months.
  • Check the safety of your water over the long term: continue to monitor bacterial quality at least twice per year or more often if you suspect any changes in your water quality.

If results show the presence of any coliform bacteria, repeat the well disinfection process and test again. If tests continue to show the presence of bacteria, contact your local health department for assistance.

Disinfection Issues and Concerns

The disinfection process may damage water softeners due to the large amounts of chlorine used. Follow your manufacturers’ instructions for appropriate methods to disinfect your softener unit. You will need to bypass the unit until completing the disinfection process.

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