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Flooded Private Water Supply

Shock chlorinating private wells

​​​​​​​​​​​​​​​​​What if I think harmful bacteria are in my well water?

If you think your private well water is contaminated (unsafe to use) because of bacteria, you can contact your l​ocal public health inspector for advice. Or you can shock chlorinate your well to kill harmful bacteria. This information is about shock chlorination.

How do wells get contaminated with bacteria?

There are many reasons a well could become contaminated with bacteria.

Surface water and shallow groundwater have harmful bacteria in them. This means that the following wells have a higher risk of getting contaminated:

  • shallow wells
  • wells that are connected to surface water
  • wells surrounded by porous material (like sand or gravel)
  • wells that were poorly constructed

Changes in the aquifer (the underground layer of rock that holds water) or changes to the well as it gets older can also put it at risk of getting contaminated.

Deep wells have less risk of contamination because the water source is more protected from harmful bacteria in surface water. But if E. coli or total coliforms are found in a groundwater well through a proper water sample, then the water is contaminated no matter how deep the well is.

Surface water that pools next to or over top of the wellhead (the part of the well you can see above ground) may also cause contamination.

Before you shock chlorinate, you have to find out the reason your well is contaminated. That way can fix the problem and stop it from happening again.

How does shock chlorination work?

Shock chlorination (pdf) is a way to kill harmful bacteria and any bacteria you don’t want in your water (such as bacteria that reduce iron or sulphur). To shock chlorinate, you add a concentrated chlorine solution to the water in the well casing (material that supports the sides of the well). Some of this water will move outside the well and into the aquifer. Then you flush the chlorinated water through the system and let it sit for a certain amount of time.​

Does shock chlorination always work?

Shock chlorination doesn’t always work. The chlorine only disinfects (kills germs) in the water that it’s had contact with for the right amount of time. If you don’t know or don’t fix the source of the problem, the well can become contaminated again.

Shock chlorination may not work in wells that have never been shock chlorinated or have never been regularly cleaned and maintained. Over time, bacteria can attach to the well casing and grow, forming a slimy layer called biofilm. ​Biofilm can make shock chlorination not work as well. You may need to hire a licensed well driller to find and scrub away the biofilm before you shock chlorinate the well.

What if shock chlorination doesn't work?

If water samples show you still have bacteria in your well water after you shock chlorinate, you need to find out why. If you can’t find the reason or fix it, shock chlorinating again likely won’t help to make your water safe.

Think about:

  • installing a water treatment system that can disinfect properly
  • continuing to boil your water before you use it
  • using another source of drinking water

You can also drill a new well (which is expensive) and properly plug the old well. But if the aquifer is the source of the problem, which is sometimes hard to know, the new well may also be contaminated.

When should I sample the water after shock chlorination?

Test the water again at least 7 days after the chlorine has been flushed from the well. If the sample has no bacteria, test water again 14 to 30 days after shock chlorination. This is because you want to get a sample that shows the natural, untreated water from the aquifer.

It’s also a good idea to get another water sample within 3 months or after snow melts (in the spring), whichever comes first. This is to make sure the shock chlorination and anything else you did to fix the problem worked.

If you couldn’t find the cause of the problem, the well water may still be unsafe.

When can I drink the water after shock chlorination?

You can drink the well water after you do all these things:

  • shock chlorinate your well
  • flush the chlorinated water out of the system
  • test a new water sample at least 7 days after the chlorine has been flushed from the well and the results show no bacteria

For more information

If you have any questions about your water, contact Alberta Health Services Environmental Public Health​.

Current as of: February 18, 2021

Author: Environmental Public Health, Alberta Health Services