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Flooded Private Water Supply

Shock Chlorination Information for Private Water Wells

​​​​​​​​​​​​​​This information is for a private water well owner that has a well with a known or suspected microbial contamination. If you think your well has been contaminated, you can contact your local public health inspector for advice. If you want to shock chlorinate your well, see the handout Shock Chlorinating Your Well.

What causes a well to become contaminated with bacteria?

Wells can become contaminated a number of ways. Surface water and shallow groundwater are considered microbiologically contaminated. This means that shallow wells, wells that are hydraulically connected to surface water, wells surrounded by porous material (e.g., sand or gravel), and wells that were poorly constructed are at more risk of contamination. Some wells may be more likely to become contaminated over time due to changes in the aquifer or changes to the well itself as it gets older.

Deep wells are felt to be at less risk of contamination, as the source of water tends to be more protected from microbiological contaminants on the surface. However, for any groundwater wells, if E. coli and/or total coliforms are found in a properly taken water sample then the drinking water is said to be contaminated no matter how deep the well is.

Surface water that pools next to or over top of the well head may also cause microbiological contamination. Before you shock chlorinate, you have to find out what’s causing or leading to the contamination (see Common Reasons for Failed Bacteriological Results in a Groundwater Supply).

What is shock chlorination?

Shock chlorination is done to kill harmful bacteria and/or nuisance bacteria (e.g., bacteria that reduce iron or sulphur). A very concentrated chlorine disinfection solution is added to the water in the well casing. Some of this water will penetrate slightly beyond the well intake into the aquifer. The chlorinated water is then flushed through the distribution system and left to sit for the time specified in the handout Shock Chlorinating Your Well.

Does shock chlorination always work?

No, it doesn’t always work. Only the water that’s come into contact with the chlorine for a specific amount of time will be disinfected. If the source of contamination isn’t known and fixed, once the treated water has been turned over (or flushed out of the system) and replaced with the natural untreated water, the well may become contaminated again. That’s why it’s important to find and fix the source(s) of contamination (see Common Reasons for Failed Bacteriological Results in a Groundwater Supply).

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

When will the water be drinkable again after shock chlorination?

After the shock chlorination is done, the chlorinated water has been flushed out of the system, and the water has been re-sampled (and there is no bacteria), the well water can be used for drinking.

When should I sample the water after shock chlorination?

Test the water again at least 7 days after the chlorine has been purged from the well. Based on average well diameter, depth, and water usage, after 7 days the water that was first treated will be removed from the system. Therefore, the 7-day test should represent untreated, natural water, unless usage in those 7 days has been very low. If the sample has no bacteria, the water has to be tested again 14 to 30 days after the shock chlorination. This is because you want to get a sample that reflects the natural, untreated water from the aquifer. If both tests show there’s no bacteria in the well water, it can be used for drinking.

Along with the above tests, to make sure the contamination hasn’t come back, it’s a good idea to collect another microbiological sample within 3 months or after snow melt (in the spring), whichever comes first, to confirm that the shock chlorination worked and any other actions you took (e.g., fixed the problem that caused the contamination) worked.

If you weren’t able to find the cause of the contamination, the well water may still be at risk.

What if the shock chlorination doesn't work?

If the water samples show that you still have bacteria in your well water after the shock chlorination, you need to look into the reason for the contamination again. If the reason for the contamination can’t be found or fixed another shock chlorination likely won’t work over the long term, leaving your water supply at risk.

Some things to think about doing include:

  • installing a water treatment system that can disinfect properly
  • keep boiling your water
  • using another source of drinking water
  • drilling a new well (expensive), and properly plugging the contaminated well (If the aquifer is susceptible to contamination, which is sometimes hard to know, the new well may also show signs of contamination.)

For More Information

The Alberta Agriculture and Forestry website has a lot of information about water well, workbooks, and e-learning tools (fact sheets, videos, etc).You can find the manual for the course at the Water Wells that Last page.

If you have any questions about your water, call an Environmental Public Health Office in your area:

  • Calgary: (403) 943-2295
  • Edmonton: (780) 735-1800
  • Grande Prairie: (780) 513-7517
  • Lethbridge: (403) 388-6689
  • Red Deer: (403) 356-6366

For 24/7 nurse advice and general health information, call Health Link at 8​11.

Current as of: March 7, 2018

Author: Environmental Public Health, Alberta Health Services