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Disinfection of Drilled or Driven Wells After an Emergency

Working on a well after a natural disaster can be hazardous. Disasters such as earthquakes, fires, and floods can damage well piping and electrical systems. Unless you are highly skilled, electrical repairs are best conducted by a qualified electrician or well contractor.

After a disaster, it is safest to drink bottled water until you are certain that your water is free of contaminants and safe to drink.

If extensive flooding has occurred or you suspect that the well may be contaminated, DO NOT drink the water. Use a safe water supply like bottled or treated water. Contact your local, state, or tribal health department for specific advice on wells and testing.

IMPORTANT: Fuel and other chemical releases and spills are common during flood events. If your water smells like fuel or has a chemical odor or if you live in an area where the potential for a release of fuels, pesticides, or other hazardous chemicals is high, contact your local health department for specific advice. Water contaminated with fuel or toxic chemicals will not be made safe by boiling or disinfection. Until you know the water is safe, use bottled water or some other safe supply of water.

Drinking water wells are best disinfected by a well or pump contractor. If you suspect that your well might be contaminated, contact your local or state health department or agriculture extension agent for specific advice on disinfecting your well. Use this guidance after checking with local authorities for flood precautions for private wells in your area.

Safety Precautions to Take Before Starting

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.
  • 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 disinfection steps have been completed.

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 the potentially contaminated water from the well and keep the 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 below 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) below.
  6. Using a 5-gallon bucket, mix the bleach from Table 2 with 3-5 gallons of water (12-19 liters).
  7. Remove the vent cap.
  8. 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.
  9. Rinse the inside of the well casing with a garden hose or bucket for 5-10 minutes.
  10. 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.
  11. 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.).
  12. Turn on all indoor faucets and run water until the chlorine odor disappears.
  13. 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.

 

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 ¾ tbsp 3¼ tbsp ½ cup ¾ cup 1¼ cups 7 cups 1 gal
20 feet 1½ tbsp 6½ tbsp 1 cup 1½ cups 2½ cups 14 cups 2 gal
30 feet 2¼ tbsp 9¾ tbsp 1½ cups 2¼ cups 3¾ cups 1¼ gal 3 gal
40 feet 3 tbsp 13 tbsp 2 cups 3 cups 5 cups 1¾ gal 4 gal
50 feet 3¾ tbsp 1 cup 2-½ cups 3¾ cups 6¼ cups 2¼ gal 5 gal
100 feet 7½ tbsp 2 cups 5 cups 7½ cups 12½ cups 4½ 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

 

Table 2.1. Approximate Amount of Bleach for Disinfection of a Drilled or Driven Well (Metric)
Depth of Water Diameter of Well Casing
5 cm 10 cm 15 cm 20 cm 25 cm 0.6 m 0.9 m
3 m 12 mL 48 mL 118 mL 177 mL 296 mL 1.66 L 3.78 L
6 m 24 mL 96 mL 236 mL 354 mL 592 mL 3.32 L 7.56 L
9 m 36 mL 144 mL 354 mL 531 mL 888 mL 4.98 L 11.34 L
12 m 48 mL 192 mL 472 mL 708 mL 1.18 L 6.64 L 15.12 L
15 m 60 mL 240 mL 590 mL 885 mL 1.48 L 8.30 L 18.90 L
30 m 120 mL 480 mL 1.18 L 1.77 L 2.96 L 16.60 L 37.80 L
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:
  • cm: centimeter
  • m: meter
  • 1 m = 100 centimeters
  • mL: milliliter
  • L: liter
  • 1 liter = 1000 milliliters

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 Hotline (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 may need to bypass the unit until completing the disinfection process.

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