Wells · In brief
Disinfecting a Well: When, Who, and When to Retest
Well disinfection, or shock chlorination, follows new construction, repairs, pump work or a bacteria hit; leave pump and wiring to a contractor, then retest 7 to 10 days later.
By the Ranch House Water desk · Updated · 3 min read
Shock chlorination puts a strong dose of chlorine through the well and household pipes to kill bacteria, then flushes it out. California's well standards treat it as routine: a well that supplies drinking water must be disinfected after construction, after repairs and after any pump job, before it goes back into use.
When does a well need it?
Besides new wells and pump jobs, the common trigger is a lab result. The State Water Board's GAMA booklet says that when coliform bacteria are present, the answer is to find and remove the cause, disinfect, and test again. Floods and other disasters count too; the EPA advises testing right after flooding or any repair. A rotten-egg smell from hydrogen sulfide is another case where shock chlorination is one listed treatment, covered in the rotten egg smell in water brief.
Who should do the work?
Any job that involves the pump, wiring or opening the well belongs to a licensed well or pump contractor. After a disaster, the CDC says well work should be done by a well or pump contractor and that wiring should be checked before the power goes back on. The CDC's own procedure for drilled wells uses unscented bleach measured by water depth and casing width, runs every tap until chlorine can be smelled, holds it for at least 12 hours, and drains the water outdoors away from plants, streams and ponds. San Luis Obispo County's own disinfection handout was not available to us, so ask County Environmental Health for its current version.
When can the water be tested again?
Wait at least 7 to 10 days after disinfecting, the CDC says, and sample only after all traces of chlorine have been flushed. Until a lab confirms the result, drink bottled or boiled water. Accredited labs are covered in testing your well water, and living on a private well explains which results to keep on file.
Sources
- DWR — Combined Well Standards, Section 11 (archived copy) — disinfection after construction, repair or pump work
- CDC — How to Disinfect Wells After an Emergency (May 29, 2025) — contractor work, wiring check, well pit gases, procedure outline, 12 hours, 7 to 10 day wait, fuel and chemicals
- State Water Board GAMA — A Guide for Private Domestic Well Owners (June 2025) — coliform response, shock chlorination for hydrogen sulfide
- U.S. EPA — Protect Your Home's Water — test after flooding or repairs