Well pumps · water quality
How to shock-chlorinate a well
Shock chlorination puts a strong dose of chlorine through a private well and household plumbing to disinfect it — commonly after a positive coliform test, after a pump or well repair, after flooding, or as part of tackling sulfur and iron bacteria. It is a process to follow carefully, with your state or county health department’s instructions as the rulebook.
When shock chlorination is used
- A well water test comes back positive for total coliform bacteria.
- After a new well, pump replacement, or any work that opened the well.
- After floodwater reached the wellhead.
- As part of addressing iron or sulfur bacteria — see rotten egg smell and iron bacteria.
A positive E. coli result or a flooded well is more serious: use bottled water and contact your health department and a licensed well contractor before trying a DIY shock.
Before you start
- Get local instructions. Health departments publish step-by-step guides with dose tables by well depth and diameter.
- Store water. Plan for 12 to 24 hours or more with no drinking water from the tap.
- Bypass treatment equipment. Put the softener in bypass (see softener bypass valves) and bypass or remove carbon filters and RO units — strong chlorine can damage resin, membranes, and media. Ask the maker about treatment equipment that should be sanitized separately.
- Protect the septic system. Plan to flush chlorinated water outdoors, away from the septic drain field, gardens, and surface water.
- Electrical safety. Turn off power to the pump before opening the well cap, and keep the wiring dry.
The general steps
- Mix the chlorine per your local instructions, using plain unscented bleach (no splash-less or scented types) or a product labeled for well disinfection.
- Add it to the well through the cap or vent opening, with the pump power off.
- Recirculate. Restore power, run a hose from an outside spigot back into the well, and run it until you smell chlorine at the hose. Rinse down the inside of the casing as you go.
- Draw it through the house. Open each tap, hot and cold, one at a time until you smell chlorine, then close it. Include toilets, outside spigots, and the water heater.
- Let it sit for the contact time your health department gives — commonly overnight or longer.
- Flush from an outside spigot onto gravel or a non-sensitive area until the chlorine smell is gone, then flush indoor taps. Use chlorine test strips to confirm.
- Return treatment equipment to service once the chlorine is gone.
- Retest with a certified lab after the chlorine is fully flushed, on the schedule your health department recommends, before drinking the water again.
If it keeps coming back
Repeated positive tests usually mean contamination is getting in — a damaged or unsealed well cap, a cracked casing, poor grading around the wellhead, or a nearby source. Shocking treats the symptom; a licensed well contractor can inspect the well construction and recommend a sanitary cap, repairs, or continuous disinfection.
When to call a pro
- The well was flooded, tests positive for E. coli, or keeps testing positive — call a licensed well contractor and your county health department.
- You cannot safely open the well cap, or the wiring at the wellhead is damaged.
- You need a pump, cap, or casing repaired before disinfecting.
Start with a licensed well contractor and your county health department for disinfection and testing. For Grundfos residential well pumps and pump questions, R.E. Merrill & Associates has handled industrial and municipal pumps and fluid systems since 1987 — contact R.E. Merrill & Associates.
Related across the site
- Iron in well water and softeners — treatment after the water is clean
- Leak sensors for the basement, softener, and well tank
Sources
- State and county health department private-well shock chlorination guides — doses by well size, contact time, flushing, and retesting.
- U.S. EPA private drinking water well guidance — testing and disinfection after flooding.