Shock chlorination
The short answer
Shock chlorination — shocking a well, or simply chlorinating a well with bleach — disinfects it by filling the well with a strong chlorine solution, circulating it through the plumbing, and letting it stand before flushing it out. Well chlorination is the first response to a positive bacteria test and to sulfur or iron bacteria. How quickly the problem returns tells you whether it fixed anything.
Health & safety
Never mix bleach with other cleaning products — the gases produced are dangerous, especially in a well house. Wells and well pits are confined spaces: do not climb into one. Keep chlorinated flush water away from plants, septic drainfields and watercourses.
When it's the right move
Shock chlorination is the correct first response to a positive coliform test, to iron or sulfur bacteria, and routinely after any work that opened the well — a new pump, a repaired casing, a replaced well cap.
It is also a genuine diagnostic, which is the part usually left out. Chlorination kills bacteria and clears dissolved gas temporarily, so what happens afterwards separates two very different problems. If a sulfur smell stays away for months, it was bacterial and you have your answer. If it returns within days, the hydrogen sulfide is dissolved in the groundwater and no amount of disinfection will change that — you need permanent treatment.
Where it is not the right move is as a repeated ritual. A well that needs shocking every few months has something wrong with it physically — a cracked casing, a failed seal, surface water finding a route in — and repeatedly disinfecting it treats the symptom while the fault continues.
The procedure
The principle is straightforward: get chlorine everywhere the water goes, at sufficient strength, for long enough. The commonly cited dose is around 3 pints of unscented household bleach per 100 gallons of water in the well, doubled where contamination is heavy.
Working that out for your own well means converting casing diameter and standing water depth into gallons before you touch the bleach, which is the step people most often guess at. Guessing low leaves bacteria alive; guessing high wastes a day of flushing and risks your treatment equipment.
- Bypass or disconnect any treatment equipment — softener, carbon filter, UV, RO. Chlorine at this strength damages resin, saturates carbon and destroys RO membranes.
- Calculate the water volume in the well: casing diameter and the depth of standing water. Your well log has the figures, or a contractor can measure.
- Mix the bleach with water in a bucket and pour it into the well, washing down the casing walls as you go.
- Recirculate: run a hose from an outside tap back into the well for 15–20 minutes to mix the solution thoroughly.
- Open every tap in the house, hot and cold, until you smell chlorine at each one, then close them.
- Let it stand at least 8 hours; 12 to 24 is better. Do not drink or use the water during this time.
- Flush the system by running an outside hose to waste until the chlorine smell is gone, keeping the discharge away from plants, septic drainfields and watercourses.
- Wait 7 to 10 days, then retest for bacteria. Testing sooner can give a false negative from residual chlorine.
The safety points that actually matter
This is a job people do routinely and safely, but the failure modes are serious enough to be specific about.
Never mix bleach with any other cleaning product, particularly anything containing ammonia or acid. The gases produced are genuinely dangerous in an enclosed space, and a well house is an enclosed space.
Wells, well pits and cisterns can accumulate gases and are confined spaces in the technical sense. Do not climb into one. If the work cannot be done from outside, that is a job for someone with the right equipment and training.
Chlorinated water at shock concentration will kill vegetation and disrupt a septic system's biology. Discharge it well away from planting, drainfields, ponds and streams.
- Use plain unscented household bleach. Scented, thickened and 'splashless' formulations contain additives you do not want in a well.
- Wear eye protection and gloves; this is a much stronger solution than normal cleaning use.
- Ventilate well houses and basements while working.
- Do not drink, cook with or bathe in the water while it stands. Arrange an alternative supply for the day.
- Disconnect treatment equipment first — replacing an RO membrane or a carbon bed costs far more than the exercise saves.
If it comes back
A well that tests positive again after a proper shock has an ongoing route for contamination, and finding it matters more than repeating the treatment.
The usual culprits are physical: a well cap that is cracked, loose or not vermin-proof; a casing that does not extend far enough above grade; ground sloping toward the well rather than away; a failed sanitary seal; or a nearby septic system too close or failing. A well contractor can inspect for all of these, and a camera survey will find casing damage that nothing else will.
Where the well is sound but the water is persistently vulnerable — a shallow or dug well, or a property that floods — continuous UV disinfection is the appropriate answer. That is protection rather than repair, and it only works on water that has been properly filtered first.
Frequently asked questions
- How do you chlorinate a well?
- Bypass the treatment equipment, work out the volume of standing water from the casing diameter and water depth, mix plain unscented bleach with water and pour it in while washing down the casing, then recirculate with a hose for 15 to 20 minutes. Open every tap until you smell chlorine, let it stand 8 to 24 hours, flush to waste away from plants and septic drainfields, and retest for bacteria after 7 to 10 days.
- Can you shock a well with regular household bleach?
- Yes, and plain unscented household bleach is the right product. Avoid scented, thickened and splashless formulations — they contain additives you do not want in a drinking water well. Pool shock is also used, but it is a different concentration, so the dose is not interchangeable with a bleach dose.
- How long does well chlorination take from start to finish?
- Budget two days for the work and around a fortnight before you know it worked. Mixing and circulating takes an hour or two, the chlorine needs to stand 8 to 24 hours, and flushing can take several hours. The bacteria retest that confirms it has to wait 7 to 10 days so residual chlorine does not mask the result.
- Is shocking a well the same as a well shock treatment?
- Yes — shocking a well, well shock treatment and shock chlorination all describe the same procedure. It is distinct from continuous chlorination, which injects a low chlorine dose permanently rather than a single strong one, and from UV disinfection, which uses no chemicals at all.
- How long after shock chlorination can I drink the water?
- Once the chlorine has been fully flushed and you can no longer smell it. But you should not rely on the water being safe until a bacteria retest comes back clear, taken 7 to 10 days later so residual chlorine does not mask the result.
- How much bleach do I need to shock a well?
- About 3 pints of plain unscented household bleach per 100 gallons of standing water in the well, doubled where contamination is heavy. You need the casing diameter and standing water depth to work out the volume.
- Do I need to disconnect my water softener before shock chlorinating?
- Yes. Chlorine at shock strength damages ion exchange resin, saturates carbon filters and destroys RO membranes. Bypass or disconnect all treatment equipment first.
- How often can you shock chlorinate a well?
- As often as needed, but needing it repeatedly is the signal to stop and investigate. A well requiring shocking every few months has a physical defect letting contamination in, and finding that is more useful than disinfecting again.
- Will shock chlorination fix a rotten egg smell permanently?
- Only if the cause was sulfur bacteria. If the hydrogen sulfide is dissolved in the groundwater itself, the smell returns within days and needs permanent treatment. How fast it comes back is the diagnosis.