What Is Shock Chlorination of a Well?

Shock chlorination is the standard first fix when a well tests positive for bacteria, or after work on the well. It’s a one-time cleanup, not ongoing treatment.

How does it work?

You mix plain household bleach into the well, run it through every faucet until you can smell chlorine, let it sit, then flush it all out. The Minnesota Department of Health aims for 50 to 200 ppm of chlorine and a contact time of at least two hours, preferably six or overnight. The Wisconsin DNR uses 100 to 300 ppm and at least overnight, preferably 24 hours. That’s far stronger than tap water: the EPA’s limit for chlorine in public drinking water is 4 mg/L, and continuous chlorination of a well aims for about 0.3 to 0.5 mg/L (Penn State Extension).

Both guides say to keep the chlorinated water away from the septic system, and Wisconsin warns never to mix chlorine with ammonia products and not to drink or bathe in the water while it’s heavily chlorinated. Follow your own state’s instructions, or hire a licensed well contractor.

Does it work?

For one-time contamination, such as bacteria introduced with a new pump, yes: Penn State says that done properly, it kills all the bacteria in the well. But if a defect keeps letting bacteria in, they come back. Penn State recommends retesting 10 to 14 days later and again two to three months later, and says continuous disinfection is necessary if bacteria return. Our guide to bacteria in well water covers what to check first and when UV makes sense.