Last checked: October 21, 2026

Shock chlorinating a well

Shock chlorination means putting a strong dose of chlorine into the well and plumbing to kill bacteria. It's the usual first step after a positive coliform test, but it doesn't always fix the problem. Here's what state and county agencies say, with a source for every claim.

When to shock chlorinate

Penn State Extension lists three situations:¹

• After a new well is built (many drillers do this as standard practice)
• After work on an existing well or installing a new submersible pump
• After a positive test for coliform bacteria

Bacteria are a common problem. Penn State says a survey of 450 Pennsylvania private wells found about 35% had coliform bacteria.¹

How often it works

Shock chlorination is meant for one-time contamination events, such as a new pump or surface runoff after unusually heavy rain.¹ In a Penn State study of wells with coliform bacteria, shock chlorination plus a sanitary well cap removed the bacteria for a year in only 15% of wells. It worked best when there were few coliform bacteria (fewer than 10 colonies per 100 mL) and no E. coli.¹

If bacteria keep coming back, the well may need repairs or continuous disinfection (see below).

Key points from official procedures

Use the full procedure from your state or county. Some points they stress:

• Bypass treatment equipment. High chlorine levels can damage or exhaust some equipment. Penn State says to check with your treatment company or manuals. Minnesota's procedure starts by bypassing softeners, reverse osmosis and other devices.¹²
• Use the right chlorine. Minnesota calls for unopened, unscented household bleach with no additives, made within six months, and says not to use pool or hot tub disinfectants. It also warns against solutions stronger than 200 ppm, which reduce effectiveness.²
• Mind the casing. Livingston County, MI says not to pour liquid bleach into a steel well casing (use granular chlorine). It says to have a licensed well contractor do the job if the cap has one large center bolt or the wellhead is buried.³
• Contact time. Minnesota says a minimum of two hours, preferably six hours or overnight.² Livingston County says at least eight hours or overnight, and not to drink, bathe, cook or wash clothes with the water during that time.³
• Where the flush water goes. Minnesota says not to discharge chlorinated water into or near your septic system, landscaping or water bodies.²
• Safety. Minnesota says never to mix bleach with other chemicals, and to call a licensed contractor if you aren't confident working safely around electricity.²
• Springs. Penn State says shock chlorinating springs is difficult and rarely successful. Don't attempt it if the overflow drains to a stream, pond or wetland.¹

Full procedures: PA DEP, Disinfection of Home Wells and Springs, Minnesota Department of Health, Well Disinfection, Livingston County, MI, and the National Ground Water Association, which Penn State also points to.¹

Retesting

• Penn State: retest for total coliform about 10–14 days later. If it's clear, test again 2–3 months after that.¹
• Minnesota: test after the bleach is flushed out, before using the water for drinking or cooking, and retest 2–4 weeks after disinfection, because coliform can regrow.²

Use an accredited or state-certified lab. See best way to test well water.

If bacteria come back

Penn State says that if bacteria come back in either follow-up test, a continuous disinfection system will be necessary, such as ultraviolet light or chlorination.¹

Where to get tested and treatment options

Related guides

• How often to test a private well
• How to read your well water test results