Shock Chlorination: How to Disinfect a Private Well After a Bad Bacteria Test
You sent off a water sample, and the lab came back saying total coliform — or worse, E. coli — is present. First thing: don't panic, but do stop drinking it straight until it's sorted. Total coliform bacteria are usually a sign that surface water or something is getting into your well; E. coli means fecal contamination and you switch to bottled or boiled water for drinking right now.
The standard first response for a private well is shock chlorination. It's exactly what it sounds like — you flood the well with a strong dose of chlorine to kill off what's living in there, let it sit, then flush it all out. It's a job a careful homeowner can do in an afternoon, and it's worth understanding even if you hire it out.
Before you dump any bleach: find out how it got in
A well is supposed to be a sealed system. Coliform showing up usually points to a way in — and if you don't close it, the bacteria come right back. Look at the obvious stuff first:
- The well cap. Is it cracked, loose, or missing the vent screen? A bad cap lets in insects, surface runoff, and rodents.
- The casing. Does it stick up at least a foot above grade, or did someone bury it or grade the yard down to it? Water needs to run away from the casing, not toward it.
- Recent work or flooding. New pump, new pressure tank, or a well that got submerged all reintroduce bacteria. We covered that in the flooded-well post.
How shock chlorination works, step by step
The idea is to get a strong chlorine solution into every part of the system — the well, the pump, the pipes, the pressure tank, and every faucet — and hold it there long enough to do the job. Here's the general procedure. Your state or county extension office publishes a table matched to your well's depth and diameter; use theirs for the exact bleach amount.
- Use plain, unscented household bleach (sodium hypochlorite, around 5–6%). No scents, no splashless, no color-safe additives — those aren't meant to go in drinking water.
- Bypass or protect sensitive gear. Route around any carbon filters, and know that chlorine is hard on some water softeners and treatment media — check before you send chlorinated water through them.
- Pour the bleach in through the well cap or a sanitary access port, following your extension's dosing table for your well volume.
- Circulate it. Run a hose from an outside spigot back into the well casing and let the water recirculate for 15–30 minutes to mix the chlorine and rinse the casing walls.
- Pull it through the house. Open every faucet, inside and out, hot and cold, one at a time until you smell chlorine at each, then shut it. Don't forget the outdoor spigots, the washing machine line, and any outbuildings.
- Let it sit. Leave the chlorinated water in the whole system, undisturbed, for at least 12 to 24 hours. Don't drink it, don't shower in it, don't run the dishwasher.
- Flush it out. Run outside spigots (onto a spot that won't kill your garden or reach the septic drain field all at once) until the chlorine smell is gone, then finish flushing indoor lines. This can take a while — go easy so you don't run the well dry or overload your septic system.
The part people skip: retesting right
Here's where a lot of folks trip up. They shock the well, get one clean test a week later, and call it done. One clean sample right after a chlorine treatment can be misleading — you may just be measuring the leftover disinfection.
| When | What to do |
|---|---|
| ~1–2 weeks after shocking | First bacteria retest, after chlorine has fully cleared |
| A few months later | Second retest to confirm it's actually staying clean |
| Ongoing | Routine annual coliform testing |
If coliform comes back after a proper shock — especially if it keeps coming back — that's your well telling you there's a real breach or a source you haven't closed. At that point it's worth getting a licensed well contractor to look at the casing, seal, and grouting rather than chlorinating on repeat.
Joe — the same fellow whose septic tank we're always bailing out — got a coliform hit and figured he'd handle it. Dumped a jug of the good-smelling lavender bleach down the well because that's what was under the sink, ran the hose for five minutes, and drank a glass that afternoon to prove a point.
His retest a week later came back clean, so he declared victory. Trouble was, his well cap had a finger-sized gap in the vent screen the whole time. By spring the coliform was back, right on schedule. The bleach was never the problem. The open door was.
Bottom line
Shock chlorination is a proven, do-it-yourself-able fix for a bacteria hit — plain unscented bleach, a good long contact time, and a thorough flush. But treat it as half the job. Find and fix how the bacteria got in, and retest twice before you trust it again. A well that keeps failing after a proper shock needs a contractor's eyes on the casing, not another jug of bleach.
For the official homeowner guidance on private wells and water testing, start here: Private Drinking Water Wells (EPA). Then grab your own state or county extension's shock-chlorination table for the exact bleach dose — it's matched to your well depth and diameter, which is the number that actually matters.