How often should you shock chlorinate a well?
Updated July 2026
Shock chlorinate a well only when a test or event calls for it, never on a fixed calendar. The real triggers are a water test that shows coliform bacteria, any pump or plumbing repair that opened the system, flooding that reached the wellhead, or water that suddenly smells of sulfur. The treatment itself is a cheap DIY job: you pour unscented household bleach into the well, circulate it, let it sit in the pipes for 12 to 24 hours, then flush it out and retest a week or two later.
What changes the interval
| Your situation | Interval | Why |
|---|---|---|
| Positive coliform bacteria test | Shock now, retest in 1–2 weeks | Bacteria in the drinking supply must be disinfected and then confirmed gone |
| New well, pump, or plumbing work | Shock before regular use | Drilling and repairs introduce surface bacteria into the system |
| Flooding submerged the wellhead | Shock and test | Floodwater carries coliform and E. coli straight into the casing |
| Sudden sulfur or rotten-egg smell | Shock the sulfur bacteria | Chlorination knocks back the biofilm that produces the odor |
| Slime or iron staining in fixtures | Shock | Iron and sulfur bacteria colonize the well and coat the pipes |
| Water tests clean, no symptoms | Do not shock | Routine chlorination is unnecessary and just wastes the treatment |
| Tests keep failing after shocking | Install UV or chlorination | A $500–1,500 continuous system beats repeating one-time shocks |
The cost of skipping it
What skipping it costs: ignoring a positive coliform result leaves live bacteria in every glass, ice cube, and cooking pot, and coliform is the warning flag for a path that can carry E. coli and other pathogens into your drinking water. Left alone, iron and sulfur bacteria also corrode and clog plumbing over time. A DIY shock costs only a few dollars of bleach; a professional will do it for roughly $150–300; and a permanent UV or chlorination system for a recurring problem runs $500–1,500. A bout of waterborne illness costs more than all three.
How to do it (A few hours plus an overnight wait · DIY)
- Test the water first so you know what you are treating and have a before reading.
- Bypass any carbon filter, softener, or UV unit, and shut off the water heater if the instructions call for it.
- Pour the right amount of unscented 5.25% to 6% household bleach into the well for the volume of water in the casing.
- Recirculate by running a garden hose from an outdoor tap back into the well until you smell chlorine coming out.
- Open every tap indoors and out until each one smells of chlorine, then close them.
- Let the chlorinated water sit in the well and pipes for 12 to 24 hours without drinking it.
- Flush the system outdoors until the chlorine smell is gone, then retest for bacteria in 1 to 2 weeks.
Knowing the interval is the easy half. OnOtto reminds you at the right interval, and it won't let you snooze it into next year. Tasks persist until they're done, and completing well shock treatments schedules the next one automatically.
Put this on autopilot, free for 30 daysFrequently asked questions
How much bleach do I use to shock a well?
The dose depends on your casing diameter and how deep the water column is, so there is no single number. A widely used rule of thumb is roughly 3 quarts of plain 5.25% to 6% unscented bleach per 100 gallons of water in the well, and your health department or lab can give you a chart for your exact well dimensions. Use only regular unscented bleach, never scented, splashless, or gel formulas, which contain additives you do not want circulating through your plumbing.
How long after shocking a well can I drink the water?
Not until it is flushed clean and confirmed safe. First let the chlorine sit in the system for the 12-to-24-hour contact time, without drinking any of it. Then run the taps outdoors until the chlorine smell is fully gone, which can take hours on a deep well. After that, wait one to two weeks and retest for bacteria. Only drink the water once a fresh test comes back clean; the chlorine leaving is not by itself proof the bacteria are gone.
Why does my well keep failing bacteria tests after shocking?
Shock chlorination is a one-time disinfection, not a barrier. If the source that let bacteria in is still open, a cracked casing, a loose or missing well cap, or a septic field too close, the contamination simply returns after the chlorine clears. Fix the physical entry point first. If the well keeps testing positive even after you have sealed the obvious paths, the durable answer is continuous treatment: a UV system or an in-line chlorinator, typically $500–1,500 installed.
Does shock chlorination fix a rotten-egg smell?
Often, but sometimes only for a while. The rotten-egg odor usually comes from sulfur bacteria, and a shock knocks their population back, so the smell fades. Because the bacteria can regrow from what survives deeper in the aquifer, the odor may return over weeks or months. Persistent sulfur smell tends to need an ongoing fix such as a UV system, aeration, or, if the odor is worst on hot water only, replacing the anode rod in the water heater.