Shock Chlorination of Wells

AEX-318
Agriculture and Natural Resources
Date: 
06/10/2019
Karen Mancl, Professor, Food, Agricultural and Biological Engineering

If a water test reveals a well is contaminated with bacteria, a series of actions are needed to protect the health of the family and visitors to the home.

First, don’t panic. Bacterial contamination of private wells is common. Studies show that more than 40 percent of individual water wells are contaminated with E coli bacteria.

Second, better protect the well from contamination. It is important to remember that the groundwater is seldom contaminated. Wells can be contaminated if water pools around the well and is funneled down the outside of the casing into the well.  Well protection diagram with approved seal 8 inches above ground level and 25 foot minimum casing

Take these well-protection steps:

  • Make sure the well casing extends at least 8 inches above the ground surface. Very old wells may be buried down in a well pit. If this is the case, the well needs an upgrade to install a pit-less adapter so the casing can be extended above ground.
  • The ground must slope away from the well, so no puddles form around the well casing.
  • Properly constructed wells have the space between the borehole and the casing filled with a grout seal. Since grouting is not required, some people skip this important step in well protection.  

Third, keep the plumbing clean when servicing the water supply. When installing new plumbing fixtures, repairing or extending pipes or pulling out the pump for service, the system is opened up to contamination. Carefully rinse all the plumbing components with a chlorine bleach solution, to sanitize the plumbing.

Finally, shock chlorinate the system to eliminate bacterial contamination. This process takes about a day to complete, so the home will be without water for 24 hours. The goal of shock chlorination is to rinse the entire plumbing system with a strong chlorine solution that is 100 to 400 times stronger than the amount of chlorine found in “city water.” The chlorinated water needs to flow throughout all of the plumbing and stand in the pipes for 12 to 24 hours before being flushed out.

Before shock chlorinating a well, take the following precautions:

  • Check on water treatment equipment. Most, like water softeners, iron filters, water heaters and pressure tanks, should be chlorinated. However, carbon or charcoal filters and RO (reverse osmosis) units can be damaged by chlorine, so check the manufacture’s requirements.
  • Wear protective equipment, like safety glasses, gloves and a protective apron. Strong chlorine solutions, if splashed, can be corrosive to eyes and skin, and damage clothing.
  • Never mix chlorine and ammonia, because it forms a toxic gas.
  • Do not use “fresh scent” bleach or bleach with detergent to shock chlorinate a well.
  • Protect your septic system and your lawn from too much chlorinated water when flushing the systems. Connect a hose wherever possible to the faucets to discharge the highly chlorinated water onto a gravel or hidden area of the yard away from the septic system. The chlorine might start to “bleach” the grass.

To shock chlorinate a well:

Pour chlorine down into the well
Directing water back into the well, to wash down sides of well casing
  1. Determine the depth of water in the well. From the well log, look up the well depth and the depth to water table. Subtract the depth to the water table from the well depth to estimate the depth of water in the well. If you do not have a well log, it may be on file with the Ohio Department of Natural Resources. You can search online at water.ohiodnr.gov/search-file-well-logs.
  2. Measure the diameter of the well casing.
  3. Measure the amount of chlorine bleach or HTH from the tables below, pour into a bucket, and dissolve in about 3 gallons of water.
  4. Fill a 5-gallon bucket with clean water and set aside if needed for cleanup.
  5. Connect a clean hose to a faucet in the system.
  6. Take the top off of the well.
  7. Pour in the chlorine solution.
  8. Turn on the hose and direct the water back into the well, washing down the sides of the well casing.
  9. Go into the house and turn on every tap until you smell chlorine then turn them off.
  10. Let the water stand in the pipes for at least 12 and up to 24 hours.
  11. Flush the system until the strong chlorine smell dissipates. Start with the outside hose and let it run on the ground away from the septic system. Turn on each indoor faucet and let them run until the system is flushed. If possible, connect a hose to inside faucets to direct flushed water outside away from the septic system.
  12. Wait about two weeks and retest the well for bacteria. If shock chlorination did not eliminate the problem, a continuous disinfection system might be needed.

To find out more about well testing and water treatment, check the website for the Ohio State University Soil Environment Technology Learning Lab at setll.osu.edu/node/12/water-supply.

Table 1. Volume of Household Chlorine Bleach Needed to Shock Chlorinate a Well of Different Depths and Diameters

Water Depth in Well (in feet)

Casing Diameter

 

4 inches

6 inches

8 inches

10 inches

12 inches

10

½ cup

1 cup

1½ cups

1 pint

2 pints

25

1 cup

1 pint

2 pints

3 pints

4½ pints

50

1 pint

1 quart

2 quarts

3 quarts

1 gallon

100

1 quart

2 quarts

1 gallon

1½ gallons

2 gallons

150

3 pints

3 quarts

1½ gallons

2 gallons

3 gallons

 

Table 2. Weight of HTH (High-test Hyochlorite) Needed to Shock Chlorinate a Well of Different Depths and Diameters

Water Depth in Well (in feet)

Casing Diameter

 

4 inches

6 inches

8 inches

10 inches

12 inches

10

-

-

-

-

-

25

-

-

-

¼ pound

½ pound

50

-

-

1/3 pound

½ pound

¾ pound

100

-

1/3 pound

¾ pound

1 pound

1½ pounds

150

¼ pound

½ pound

1 pound

1½ pounds

4 pounds