Coliform in Your Well? Shock Chlorination, Explained
You sent off your annual well sample, and the lab came back with two words that make people nervous: total coliform present. Before you start boiling every pot of water in the house forever, take a breath. A positive coliform result is common, it's usually fixable in an afternoon, and the fix has a name: shock chlorination.
Ask any well driller who also does service work and they'll tell you they run this procedure constantly — after a repair, after a flood, or just because a routine test flagged bacteria. Let's go through what coliform actually tells you, how shock chlorination disinfects the whole system, and the one result that means you've got a bigger problem than a jug of bleach can solve. And Joe, naturally, has a cameo.
What a coliform hit really means
Total coliform bacteria are mostly harmless organisms that live in soil and vegetation. They aren't usually the thing that makes you sick — they're an indicator. Their presence means there's a pathway for surface stuff to reach your well water, which means disease-causing organisms could take the same path. That's why labs test for them: they're the canary, not the coal-gas.
The more serious version is E. coli, a specific type of coliform that lives in the gut of warm-blooded animals. A positive E. coli result points to actual fecal contamination and means don't drink the water until it's resolved and re-tested. Plain total coliform without E. coli is a warning to disinfect and find the entry point.
How shock chlorination works
The idea is simple: flood the entire well and plumbing with a strong chlorine solution, let it sit long enough to kill everything living in there, then flush it all out. The steps, in plain order:
- Calculate the dose based on your well's depth and diameter — you need enough unscented household bleach to bring the whole water column to a strong chlorine concentration.
- Pour it in and circulate it by running a hose from a spigot back down into the well casing until you smell chlorine coming back out.
- Pull the chlorine through every fixture — run each tap, hot and cold, until you smell bleach at it, then shut it off. That disinfects the pipes, the water heater, everything, not just the well.
- Let it sit, ideally overnight (8 to 24 hours), with no water use.
- Flush it out — run the water outside first (away from your septic drain field and plants) until the chlorine smell is gone, then finish inside.
- Wait and re-test after a week or two of normal use to confirm the bacteria are actually gone.
The CDC publishes a clear homeowner walkthrough with the dosing tables and safety notes — worth reading before you start, because the exact bleach amount depends on your well: CDC — disinfecting wells.
A few things people get wrong
| Mistake | Why it bites you |
|---|---|
| Using scented or splashless bleach | The additives don't belong in drinking water; use plain unscented sodium hypochlorite |
| Skipping the fixtures | You disinfect the well but leave bacteria hiding in the water heater and lines |
| Dumping the flush into the septic all at once | A big slug of chlorine can knock back the bacteria your septic tank needs — flush outside |
| Not re-testing | You assume it worked; the only proof is a clean follow-up sample |
When bleach isn't the answer
Here's the part that matters most: if you shock the well, re-test, and coliform comes back — especially if it keeps coming back — the problem isn't a one-time contamination event. Something is structurally letting surface water in. That usually means a cracked or short well casing, a missing or damaged well cap, a casing that doesn't stick far enough above grade, or surface water pooling around the wellhead and running down alongside the pipe. Repeat disinfection just resets the clock; you have to fix the actual opening. That's when it's time to bring in a licensed well contractor to inspect the casing and seal.
Joe — the septic-ignoring, well-neglecting legend himself — got a coliform hit, shock-chlorinated like a champ (he does love a project with a jug of bleach), and it came back clean. Six months later: coliform again. Shocked it again. Clean, then dirty. Joe treated it like a chore that just needed doing twice a year. Turns out the actual problem was his well cap, which a lawnmower had cracked years ago, letting bugs and runoff stroll right in. Joe was disinfecting a bucket with a hole in it. A twelve-dollar cap ended a two-year cycle.
So: one coliform hit, no E. coli? Shock it, flush it, re-test — that's often the whole story. But if it keeps coming back, stop reaching for the bleach and start looking at the wellhead. The water's only as clean as the seal keeping the outside out.