When Should You Shock Chlorinate a Water Well?

July 24, 2019

Shock chlorination is the easiest, cheapest way to sanitize well water that has been contaminated.

Sourcing your home’s water from a well means that you likely get to enjoy water that tastes far better than city-supplied water. But it also means that you are responsible for maintaining your system to ensure that you continue to enjoy safe, healthy water. Failing to monitor water quality can expose you and your family to harmful bacteria that can make you sick, or even cause damage to your well.

This is why all well water must occasionally be treated with chlorine bleach, through a process called “shock chlorination.”

What is shock chlorination?

Shock chlorination is the easiest, cheapest way to sanitize well water that has been contaminated with:

  • Bacteria that could cause harmful health effects, such as E. coli.
  • Iron bacteria, which cause buildups of rust and slime.
  • Anerobic, sulfur-reducing bacteria, which produce hydrogen sulfide gas. These bacteria are notorious for giving water an unpleasant “rotten egg” taste and smell.

Chlorinating your well is quite simple, as all it requires is adding chlorine bleach to the water in your well. By adding chlorine to your well, the pH of the water is raised to a point that is toxic to bacteria and pests, killing them in a matter of hours.

When should you shock chlorinate a well?

If you have performed a water test that has come back positive for coliform bacteria, then it’s time to chlorinate you well. If you haven’t tested your well in a few years, it’s a good idea to test your water. But homeowners often wait until signs of a problem surface before testing their water. Common symptoms of bacterial growth in well water include:

  • Foul odor or taste: As mentioned above, a rotten-egg smell is a tell-tale symptom of the presence of sulfate-reducing bacteria.
  • Stained laundry: If your whites are turning a light orange color, this can be a sign of bacterial contamination.
  • Blockage or Reduced Flow: Iron bacteria gradually deposit rust throughout a well system. Over time, this buildup can reduce the flow of water through the casing, screens, and pipes.
  • Slime: As iron bacteria build up, they tend to collect together in groups, forming deposits of slime. These slime deposits can often be easily seen inside the well cap, or anywhere water sits for a while, such as inside a toilet tank.

A water test will indicate whether the issues you are seeing are symptoms of bacteria in your well.

How much bleach do you need to chlorinate a well?

Before you do anything, be sure to catch up on any bathing or laundry you need to do, and make sure you have adequate drinking water on hand. Once you have shocked a well, avoid bathing or doing laundry for at least 24 hours, if not longer.

The amount of bleach needed to shock a well depends on the size of your well. you need to calculate how much bleach you need. The rule of thumb is to add 3 pints of bleach (3/8 gallon), plus 3 pints for every 100 gallons of water in your well.

If you’re not sure how much water your well holds, you can calculate that yourself. Refer to the documentation provided to you by the company that drilled your well to determine its full depth. If you’ve lost the paperwork, contact them.

Once you know how deep your well is, determine the distance between the ground’s surface and the top of the water level. Subtract this from the full well depth. The resulting number is the water depth of your well. For instance, if your well is 300 feet deep, and the water starts 30 feet down, then the water depth is 300 – 30 = 270 feet. Now, refer to the following table.

Drilled/Bored Well Storage Capacity – Gallons Per Foot:

  • 4” Diameter: 0.7 gallons
  • 5” Diameter: 1.0 gallons
  • 6” Diameter: 1.5 gallons
  • 7” Diameter: 2.0 gallons
  • 8” Diameter: 2.6 gallons
  • 9” Diameter: 3.3 gallons
  • 10” Diameter: 4.1 gallons
  • 12” Diameter: 5.9 gallons
  • 16” Diameter: 10.5 gallons
  • 20” Diameter: 16.3 gallons
  • 24” Diameter: 23.5 gallons
  • 28” Diameter: 32.0 gallons
  • 32” Diameter: 41.8 gallons
  • 36” Diameter: 52.9 gallons

Multiply the water depth of your well by the gallons per foot indicated above. This is the total water volume in your well. For example, if the water is 270 feet deep, and your well is 6” in diameter: 270 x 1.5 = 405 gallons.

As we said before, you need 3 pints of bleach per 100 gallons of water, plus an additional 3 pints. In this case, 405 gallons is pretty close to 400, so we’ll just round down to 400. Divide the number of gallons of water by 100, then multiply by 3, and then add 3, to get to the number of pints of bleach you need. In this case:

  • 400 gallons ÷ 100 = 4
  • 4 x 3 pints per 100 gallons = 12 pints
  • 12 pints + 3 pints = 15 pints

In the case of our example, we need 15 pints of bleach, or 1 7/8 gallons.

Important note: You do not need special bleach to shock a well. In fact, you should avoid any special scented or gel bleaches. Don’t buy any the fancy stuff. Chances are, the cheapest generic bleach you can find will be perfect.

How do you chlorinate a water well?

Once you have your bleach and know how much you need, clean out any debris in the well house or storage tank. Switch off the pump circuit breaker and remove the well cover.

Before you do anything else, you’ll want to sanitize the area just inside the well cap, as bacteria often builds up there, and failing to remove it will result in recontamination. Make a mixture of 10 parts water to 1 part bleach. Use this mixture to scrub as much of the inside surface of the well as possible. Scrub the cap as well. If the cap is in bad shape and will not form a seal when replaced, purchase a new one.

Now, you’re ready. Take the amount of bleach that you calculated above and pour it into the well. If you can, try and coat all the interior walls of the well with the chlorine, to sanitize the exposed surface.

Turn the breaker on, then run a garden hose—make sure the end is clean—into the well and turn on the hose faucet. Allow the hose to run until you can smell chlorine in the water. This may take anywhere from ½ hour to 2 hours. Then allow the hose to run for a few more minutes. Take the opportunity to use the hose to rinse the inside surfaces of the well casing, washing bleach and any bacterial buildup into the chlorinated water where they can be sanitized.

The water in your well is now nicely chlorinated, but the water in the pipes throughout your home is still contaminated. One at a time, will need to run every faucet in and outside your home, both hot and cold, until you can smell chlorine in the water. Run all shower faucets and showerheads. Flush all your toilets. Dishwashers and laundry machines should be run once as well. If running a faucet does not produce a chlorine smell, pour an additional 3 to 4 pints of chlorine into the well and try again.

Once you have run all your home’s taps, turn the pump circuit breaker off, and leave everything as it is for 24 hours. The chlorine in the water will gradually sanitize your system, killing all the bacteria in the water.

How do you flush a water system after chlorinating it?

After you’ve allowed 24 hours to pass, turn the circuit breaker back on, and run each of your outside faucets, one by one, until you can no longer smell chlorine in the water. For the first outdoor faucet, this may take an hour or two. Be sure to run the hose out to a safe area where the chlorinated water won’t harm plants or animals.

If you have a low production well, run the hose for an hour, turn it off for two hours, and repeat as necessary. Then run each indoor faucet until you can’t smell chlorine. Run your laundry machine and dishwasher once more and flush all toilets again.

This entire process can take quite a long time, sometimes as much as 2 or 3 days, especially with low-production wells where you can’t flush the system as quickly. But once you have completed the flush process, your water should be safe for drinking, bathing, and laundry. If you want to err on the cautious side, purchase a chlorine test kit from a swimming pool shop to directly measure the chlorine level.

Wait a week, and then run another water quality test. Your water should now be bacteria-free. But if the test comes back positive for bacteria, or you start to see signs of water contamination any time within a few months after chlorinating your well, you have a more serious problem that shock chlorination can’t fix. In this case, you need to contact Watson Well and have our professionals service your well.

California Groundwater Association
National Groundwater Association