How much bleach do I need to shock my well?

For a 6-inch diameter well: 2 pints for 50 feet deep, 1 quart for 100 feet, 1.5 quarts for 150 feet, 2 quarts for 200 feet. Use unscented household bleach at 6-8.25% sodium hypochlorite. For an 8-inch casing, multiply by 1.75.

How do I disinfect a well with bacteria?

Follow the EPA shock chlorination method: calculate bleach by well depth and diameter, mix with 5 gallons of water, pour into the casing, run water into the well to circulate, open each faucet until you smell chlorine then close it, let it sit 8-24 hours, then flush until the chlorine smell clears. Retest 2 weeks later.

Can I drink water after shocking my well?

Not until you flush the system and the chlorine smell clears from all taps. After flushing, wait 5-14 days before collecting a water sample for a bacteria test. Residual chlorine in the water produces false negative results on bacteria tests if you test too soon.

How long after shock chlorination can I test water?

Wait at least 5-7 days after completing the flush before collecting a water sample. Most labs recommend 10-14 days for the most reliable results. Testing too soon risks a false negative from residual chlorine in the system.

Does shock chlorination permanently fix bacteria in a well?

Shock chlorination addresses a one-time contamination event. It's not a permanent solution for ongoing contamination. If you test positive again 4 weeks after treatment, you have a persistent source problem, such as a cracked casing or surface water intrusion, that requires professional well inspection.

How to Shock Chlorinate a Well: EPA Method Step by Step

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.