Well Disinfection: What To Do After Contamination Occurs

It is a common misconception that ground water is safe from contamination. This simply is not true. Wells can easily become contaminated if the proper maintenance isn’t taken and even with proper maintenance they can become contaminated during an emergency like a flood. See my article on Ground water and Wells for the proper maintenance of wells to prevent contamination.

Wells can become contaminated two separate ways. The first one is harder to control.  It happens when the entire aquifer becomes contaminated. This can happen when there is an environmental spill of something (anything really) that percolates into the aquifer in or around the recharge area. This could be on your property or a hundred miles away. It all depends on the aquifer.

How Contamination Enters A Well: From: http://www.health.gov.on.ca

The other one is the well itself leads to the contamination (like in the picture above). The water still comes from the surface, but instead of becoming filtered through the soil, it bypasses the soil and flows down the casing either on the inside or the outside. This bypassing means all the surface contamination and pathogenic organisms will now be in your well water.  The ability of water to bypass the natural  filtering of soil is increased with weather,  heavy rain, flooding and storm surges will all saturate the ground, making the soil separate a little and water flow easier.  Even a pool backwash or leaving a sprinkler on constantly can do this, anyone ever forget a garden hose was on next to the house?  Did it flood your basement? The same “flooding” can happen in your well.

So, there was a flood, your well water now smells weird and people are starting to get sick. What should you do? The first thing you should do is check for signs of infiltration. This will look like a depression in the ground around the well and wet casing walls inside your well.  The second thing you should do is get your water tested.  It is a good idea to get into the habit of testing your water on a regular basis.  The exception being if  you are on a municipal water system, then you are just wasting money by having your own tests done.

Knowing how contaminated your well is will be essential.  To know how contaminated your well is, you need to know what’s normal in your well and then, how much is it different.  The only way to do this is to have at least two samples; one before a problem and one after.  Take one sample immediately after a significant event like a flood or as soon as you suspect contamination for any reason. Once you know the level of contamination you will be able to measure your success or failure in disinfection.  As an aside, the best practice it to  also take a sample anytime something from the surface goes down into the well, even if you only took the pump out of the well for an inspection and are going to put it right back in, this is still a prime opportunity for contamination to occur.

Detailed instructions are following; please read them, this paragraph is a description, not instructions.  In general you open the well cap and pour in sodium hypochlorite, or a mixed solution of calcium hypochlorite (do not add the sediment at the bottom) and that should take care of any bacteria.  But it isn’t that simple, too much bleach can damage your well pump and it then become more of a problem to get rid of the chlorinated water after disinfection.

Below is a list of  volumes of disinfectant to be added to your well.  Each line represents three meters or ten feet of water in the well.  This is not the depth from the surface down to the water table, but the depth from the bottom of your well to the surface of the water (the bottom up to the water table).  Like the picture above, this table is also from http://www.health.gov.on.ca.  Your local health authority should also have information regarding wells in your area.

Volume of Bleach to Add for Every 3 Metres (10 Feet) of Water in the Well
Casing Diameter Volume of Unscented Bleach
(5.25% solution)
Millimetres Inches Millilitres
50 2 6
100 4 30
150 6 60
200 8 100
250 10 200
300 12 250
400 16 400
500 20 650
600 24 900
900 36 2000 (2 litres)
1200 48 3600 (3.6 litres)

Here is a list of instructions on how to disinfect your well.

  1. Calculate the depth of water in your well. (Take the necessary measurements)
  2. Find the volume of disinfectant required from the table above.  Pay attention to the % strength of the disinfectant.  I use 10% sodium hypochlorite at work. For me these volumes would be cut in half.  Also 5.25% is what you can expect from fresh bleach, if you have been storing it for a long time multiply by two for each month stored.
  3. Mix the water in your well, if possible use a pump and a hose to pump WELL WATER back into your well.
  4. Bypass any filtration devices, especially carbon filters as they remove chlorine (bleach)
  5. Run EVERY faucet in the building (and outbuildings) until you can measure a residual do this with a chlorine kit or by smell.
  6. If there is no chlorine residual, repeat steps 2-5 until a strong chlorine residual remains.
  7. Drain water heater and fill it with chlorinated well water.
  8. Flush and wash any and all filters (wash only for carbon filters).
  9. Wait for at least 12 hours (you can do this in as little as 1 hour if you super chlorinate)
  10. Flush chlorinated water outside, over the ground only. Never directly into surface water as it is often illegal.
  11. Avoid putting chlorinated water into your septic tank so you don’t disrupt the biological treatment.
  12. Take water samples.  I like to have two consecutive clean samples before I go back to normal.  The two samples are taken at least 48 hrs apart.  Until then DO NOT DRINK THE WATER WITHOUT TREATING IT.

There you have it, this disinfection procedure will cover a lot of  of potential contamination in your well.  It should get most living organisms and some chemicals.  If you do this entire procedure and you still have a problem then the best thing to do is ask someone with more knowledge and experience, I’t talking of course about a professional.  Professional laboratories, professional samplers, professional well drillers and professional water treatment specialists.  I have seen some problems need involvement from all these people.  Don’t be ashamed to ask for help when the problem is over your head, after all it is your life we are talking about.


5 thoughts on “Well Disinfection: What To Do After Contamination Occurs

  1. i’ve heard that these steps are good annual maintenance for a well – what are your thoughts? Disinfect annually or only after flooding/contamination occurs?

    • If your well is uncontaminated then leave it alone. Everytime you add anything to your well you risk adding contamination. IF however you have a recurring problems with the well then this procedure could/should be added to a routine maintenance schedule.

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