Shock chlorination

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 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.

1. Bypass or disconnect any treatment equipment — softener, carbon filter, UV, RO. Chlorine at this strength damages resin, saturates carbon and destroys RO membranes.
2. 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.
3. Mix the bleach with water in a bucket and pour it into the well, washing down the casing walls as you go.
4. Recirculate: run a hose from an outside tap back into the well for 15–20 minutes to mix the solution thoroughly.
5. Open every tap in the house, hot and cold, until you smell chlorine at each one, then close them.
6. Let it stand at least 8 hours; 12 to 24 is better. Do not drink or use the water during this time.
7. 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.
8. Wait 7 to 10 days, then retest for bacteria. Testing sooner can give a false negative from residual chlorine.

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.

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.

UV disinfection→