Your water test came back positive for total coliform. Before you buy anything, know this: the standard first response is shock chlorination — flooding the well and plumbing with a strong chlorine solution, letting it kill everything, then flushing it out. It costs about $20 in bleach, takes a weekend, and fixes the majority of one-time contamination events. Here's the full procedure and the mistakes that make people think it "didn't work."
(Here because of a sulfur smell rather than a bacteria test? Run the 5-minute rotten-egg diagnosis first — the most common cause is the water heater, and shocking the well won't touch it.)
Step 1: Find the entry point first
Walk the wellhead before touching bleach. A loose or cracked cap, a casing that doesn't rise proudly above grade, pooling surface water, or recent well work are the usual suspects. Fix what you find — chlorinating without closing the door just schedules the next contamination.
Step 2: Calculate your dose
Target about 200 ppm chlorine, and no stronger: Minnesota's health department notes that going above 200 ppm makes disinfection less effective. Practical math: a standard 6-inch-diameter well holds about 1.5 gallons per foot of standing water. Water depth × 1.5 = gallons; add your pressure tank and water heater gallons, plus about 50 for the pipes. Then use roughly 3 pints of plain unscented 5.25% household bleach per 100 gallons — about 2⅔ pints if the label says 6%, 2 pints at 8.25%. (100 ft of standing water ≈ 150 gal ≈ 4½ pints for the well itself.) The well volume & bleach calculator does this math for any casing size. Use a fresh jug — bleach loses strength within months, and Minnesota advises bleach made within the last six months — and never the scented, splashless, color-safe, or "with cleaners" varieties.
Step 3: Bypass what chlorine destroys
Bypass carbon filters, RO systems, UV units, and the water softener — Wisconsin's DNR advises sanitizing a softener separately, following its manual. Plan to replace cartridge filters after the final flush. Full details in our treatment equipment guide.
Step 4: Dose the well and circulate
Gloves and eye protection on, and the pump's breaker off before you remove the well cap — keep bleach off the wiring. Pour the bleach (diluted in a few gallons of water to protect components) down the casing, and rinse the casing walls. Then switch the breaker back on and run a garden hose from an outside spigot back into the well for 15–30 minutes — you're circulating chlorinated water to mix the entire column and wash the casing.
Step 5: Pull chlorine through every pipe
In the house, open every fixture — hot AND cold, showers, outside spigots, the washing machine — until you smell chlorine at each one, then shut it off. The hot side matters: the water heater is a warm, dark bacteria apartment complex, and skipping it is a common reason shocks "fail."
Step 6: Wait 12–24 hours
No water use — that's the contact time doing the killing. Overnight is the practical schedule. Warn the household; one 3 a.m. toilet flush won't ruin it, but a morning of showers will. Exception: if your well is in an arsenic area, check your state's rules before shocking. Wisconsin's DNR, for example, says not to exceed 100 ppm there or leave the solution in the well more than 30 minutes.
Step 7: Flush — outside first, gently on the septic
Flush the heavily chlorinated first water through outdoor hoses onto gravel or an unplanted area — away from your septic system (a chlorine slug kills the tank's bacteria) and away from plants and surface water. When the outdoor smell fades, finish flushing indoor fixtures, spread out if you can, because a large flush can also hydraulically overload a septic system. Test strips beat noses: flush until strips read zero.
Step 8: Retest — and read the result correctly
Retest for coliform 1–2 weeks after the flush, drinking bottled water until it's clean. Testing sooner gives a false pass. Then:
- Clean, and still clean at a follow-up test 2–3 months later: it was a one-time event. Back to the normal testing schedule.
- Bacteria return: there's an open door — cap, casing, grouting, or a failing septic system nearby. Fix the source; for stubborn cases, continuous UV disinfection ($700–$1,500) is the durable answer.
Frequently Asked Questions
How much bleach do I use to shock a well?
Aim for about 200 ppm of chlorine throughout the well and plumbing. That's the usual extension target, and stronger won't disinfect better. With regular unscented 5.25% bleach, that's about 3 pints per 100 gallons (about 2⅔ pints if the label says 6%, 2 pints if it says 8.25%). A typical 6-inch well holds ~1.5 gallons per foot of standing water, so 100 feet of water is ~150 gallons, or about 4½ pints, plus more for the pressure tank, water heater, and pipes. When in doubt, your county extension office publishes exact tables.
How long after shocking a well can I drink the water?
Wait until the chlorine is fully flushed (no smell, ideally verified with test strips), then retest for bacteria 1–2 weeks after chlorination. Drink bottled water until that retest comes back clean — testing too soon gives a false pass because residual chlorine masks regrowth.
Did shock chlorination fix my well permanently?
It disinfected the system once — it did not fix whatever let bacteria in. If a retest is clean and is still clean at a follow-up test 2–3 months later, the contamination was likely a one-time event (flood, repair work). If bacteria return, you have an ongoing entry point — commonly a cracked cap, casing, or nearby septic issue — and the durable fix is repairing it and/or installing continuous UV disinfection.
Can I shock a well with a water softener or filters installed?
Bypass carbon filters and RO units (chlorine ruins RO membranes, and carbon just soaks it up), and bypass the water softener too, then sanitize it separately following its manual. Remove or bypass UV lamps' prefilters per the manual, and replace all cartridge filters after the flush.