Safety note: Chlorine bleach is a chemical irritant. Work in a ventilated area. Wear eye protection and gloves. Keep children away from the well area during this procedure. Never mix bleach with other chemicals. Do not run heavily chlorinated water to a septic system at high volume, it can harm the beneficial bacteria the system needs to function.
Shock chlorination is the EPA-recommended method for disinfecting a private well after a bacteria contamination event. It’s a temporary high-dose chlorine treatment that kills bacteria throughout the well casing, pump, and distribution pipes.
It addresses one-time contamination events. It’s not a permanent solution. If you test positive, shock chlorinate, and test positive again 4 weeks later, you have an ongoing source problem that requires professional investigation.
When Shock Chlorination Is Needed
Shock chlorinate your well after any of these situations:
- A positive E. coli or total coliform test result from a certified lab
- Any flooding event that reached or submerged the well casing
- New well construction or newly drilled well (builder may have done this, but verify)
- Well pump replacement or any repair that opened the casing to the outside
- A well that has not been disinfected in more than 5 years and is newly purchased
Do not shock chlorinate based on taste or smell alone. Get a lab confirmation of bacterial contamination first. Shock chlorination puts significant chlorine into your plumbing and your aquifer and should be done with a confirmed reason.
What You Need
- Unscented household bleach, 6-8.25% sodium hypochlorite. Check the label. Do not use splash-less, scented, or “ultra” formulations with added thickeners.
- A clean plastic bucket (5 gallon)
- A funnel
- A garden hose connected to an outdoor spigot
- Measuring cup
- Rubber gloves
- Eye protection
How Much Bleach to Use
The amount depends on your well’s depth and casing diameter. These are EPA-based calculations using 6% bleach.
For a 6-inch diameter casing:
- 50 feet deep: 2 pints
- 100 feet deep: 1 quart
- 150 feet deep: 1.5 quarts
- 200 feet deep: 2 quarts
For an 8-inch casing, multiply the above by 1.75. For a 4-inch casing, use about two-thirds of the 6-inch amount.
If you don’t know your well depth, check your well completion report (usually filed with the county or provided by the driller). If you genuinely can’t find it, use the higher end of the range for your estimate.
Step-by-Step Procedure
Step 1. Put on gloves and eye protection before handling bleach.
Step 2. Remove the well cap carefully. Set it aside somewhere clean.
Step 3. Measure the calculated amount of bleach and mix it into 5 gallons of water in the clean bucket. Adding bleach to water (not water to bleach) reduces splashing.
Step 4. Pour the bleach solution into the well casing using a funnel.
Step 5. Connect the garden hose to an outdoor spigot. Run water into the open well casing for 15-20 minutes. This circulates the bleach solution through the water column so it reaches the pump and lower sections of the casing.
Step 6. Replace the well cap.
Step 7. Go inside and open each faucet one at a time, including outdoor spigots, until you detect a strong chlorine smell at that tap. Once you smell it, close that faucet. Work through every faucet in the house. This confirms that chlorinated water has reached all the distribution pipes.
Step 8. Let the chlorinated water sit in the well and all pipes for a minimum of 8 hours. Overnight (12-24 hours) is better. Do not use water during this period.
Step 9. After the waiting period, run an outdoor spigot (not connected to your septic system’s drain field) until the chlorine smell clears. This may take 1-3 hours of continuous running. Running to an outdoor area, a ditch, or a storm drain is preferable to flushing large chlorinated volumes through your septic system.
Step 10. Once the outdoor water smells normal, briefly flush each interior faucet to clear the pipes. Use the water briefly for non-drinking purposes (toilet flushing) for a few hours until residual chlorine drops further.
After the Procedure
Wait at least 5-7 days before collecting a water sample for bacteria testing. Residual chlorine in the water will produce a false negative on bacteria tests. Most labs and the EPA recommend waiting 10-14 days for reliable results.
Shock chlorination is not considered complete without a negative retest. Document the date you performed it, the bleach amount, and the wait time. File the retest result alongside your contamination test result.
If the Retest Is Positive
A positive bacteria test 2-4 weeks after proper shock chlorination means one of two things: re-contamination from an ongoing surface water pathway, or a structural problem with the well (cracked casing, failed well cap, insufficient depth) that allowed re-entry of bacteria.
This requires a licensed well contractor. A camera inspection of the casing and an evaluation of the well cap and casing integrity are the starting points. This is beyond what DIY chlorination can address.
For wells where bacteria is a recurrent problem, a UV water purifier installed after filtration provides continuous protection between annual tests, though it doesn’t fix the underlying source issue.
See the well water testing guide for guidance on finding a certified lab and collecting samples correctly. The bacteria in well water page covers health risks and long-term management options.