Investigation of Bacterial Contamination of Private Wells

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Bacterial contamination is the most common water quality problem faced by private well owners. This information sheet will lead you through the step-by-step procedure to deal with cases of bacterial contamination of private wells. 

Step 1: Results of the first sample

If the results of the first sample do not indicate the presence of any total coliform bacteria or E. coli, the water is considered suitable for consumption and no further action is required. 

If the results of the first sample indicate the presence of any level of bacteria, the water needs to be tested again to confirm the initial results. If the first sample results indicate that there are more than 10 total coliforms present or any E. coli, the water is considered unfit for consumption and should be boiled, or an alternate source used until the problem has been rectified. 

Before retesting the water, inspect the well (see suggestions below under the heading “Basic Considerations When Inspecting a Well), and flush the water thoroughly by running the tap for a couple of hours if possible.  Regardless of the level of bacteria present in the initial sample, unless an ultraviolet disinfection system is installed, two consecutive samples, showing no presence of bacteria, should be obtained before resuming normal use of the water. 

Step 2: Results of the second sample

If the results of the second sample show no presence of bacteria, a third sample should be collected to confirm these results prior to resuming normal use of water. If the third sample shows no presence of bacteria, the water may be considered safe to consume. However, a follow-up sample in three or four months is recommended. If the second sample still shows the presence of bacteria, the well should be considered contaminated. No further sampling is recommended at this point, and the well and plumbing system should be disinfected prior to any further action. 

Step 3: Disinfection of the well and plumbing system

Disinfection of the well will help determine if contamination is a result of a gradual buildup of a thin layer of bacteria over time, or if there is an ongoing problem with the well and contaminants are still entering the water supply. It is important to follow the correct procedure for disinfection. An information sheet on the procedure is available at Access PEI sites.

Step 4: Sampling after disinfection

Once a well has been disinfected, it should not be sampled until at least 48 hours after any trace of chlorine has dissipated from the water. When the water is retested after disinfection, if the results do not indicate the presence of bacteria, the well should be
sampled one more time to confirm the water is free of bacteria. If the results of either of these two tests after disinfection still indicate the presence of bacteria, the problem is most likely with the well. No further testing or disinfection is recommended, and water treatment, well reconstruction or a new well may be necessary. At this point, you should contact a groundwater technician for further advice. (The department can be reached toll free at 1-866-368-5044.) Until the problem has been fully remedied, any water to be used for drinking or food preparation should be boiled.

What are the basic considerations when inspecting a well?

One of the main goals of proper well construction is to ensure that bacteria and other contaminants at or near the ground surface are kept out of the well. The following are steps to address the most common ways a well can be contaminated:

If the top of the well is buried or is in a well pit which may flood, contaminants can enter the well even if a proper well seal has been installed. To remedy the problem, have a well driller add additional well casing to the top of the well so it extends above ground level, and install a pitless adaptor to provide a water tight connection between the
well and the water line to the house. Also ensure that there is a proper vermin-proof well cap at the top of the casing.

Surface water collecting near a well can result in bacterial contamination. Ensure that down spouts for rain gutters are directed away from the well, and that the ground is graded so water does not pond near the well.

If your well is in the basement, ensure there is a proper well seal installed on the top of the casing, and that the top of the casing is high enough above the basement floor that the well will not become flooded.

Wells with an inadequate length of casing are particularly vulnerable to contamination.  Current well construction practices require a minimum of 40 feet of casing, with the casing sealed in the well bore with a material called bentonite. For information about the casing on your well, check the water well report supplied by the well driller when the well was constructed.

If the top of your well is above ground, remove the cap and check for the presence of insect nests, spiders, etc. which can contaminate the water. The use of a vermin proof well cap will keep these pests out of the well.

If the top of the well is accessible, listen carefully to see if you can hear water running into the well. This should be done with the well “at rest” and also after the well has been pumped for 10 minutes. If you hear water running into the well, it may indicate that surface water is running down along the outside of the casing and into the well. In this case, you may need to have the casing re-installed with a better seal, or have additional casing added to the well.


Date de publication : 
le 5 Février 2024
Environnement, Énergie et Action climatique

Renseignements généraux

Ministère de l'Environnement, Énergie et Action climatique 
Immeuble Jones, 4e étage
11, rue Kent
C.P. 2000
Charlottetown (Î.-P.-É.) C1A 7N8

Téléphone : 902-368-5044
Sans frais : 1-866-368-5044
Télécopieur : 902-368-5830
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