Updated October 16, 2026: An earlier version of this guide gave two bleach amounts that did not agree (3 cups in one place, 1–3 gallons in another). The dosage table below now follows Ohio State Extension, and every amount on this page matches it. We also added test-strip checks and expanded the retest and "bacteria came back" sections.
Your well tested positive for coliform bacteriaColiform BacteriaA group of bacteria used as indicators of potential contamination. Total coliform presence suggests a pathway for pathogens to enter your well.EPA MCL: 0 (no coliform should be present)Learn more → (germs that signal contamination). Shock chlorination is the first fix. It kills bacteria in the well, the pipes, and the pressure tank.
The job takes about 2 hours of work plus a 12–24 hour wait. You need household bleach, a bucket, a hose, and chlorine test strips[1].
Quick Answer: How to Shock a Well
Look up your bleach amount in the table below. Dilute it, pour it in the well, and recirculate until every faucet smells of chlorine. Wait 12–24 hours. Flush until the chlorine is gone. Retest for coliform 2–4 weeks later. If bacteria come back, find out how they are getting in.
When to Shock Chlorinate a Well
- After a positive coliform or E. coli test.
- After flooding, or after surface water got into the well.
- After a pump replacement, well repair, or any work that opened the well.
- After new plumbing goes in.
- When a well has been sitting unused, such as in a home you just bought[2].
How Much Bleach to Shock a Well
The amount depends on two numbers: how wide the casing is, and how many feet of water stand in it. That second number is not the total well depth. It is the total depth minus the static water level (how far down the water sits). Both are on your well log[1].
Example: a 200-foot well with water starting at 60 feet holds 140 feet of water.
Bleach dosage table
Amounts of regular 5.25–6% household bleach, from Ohio State Extension[1]:
| Feet of water | 4-inch casing | 6-inch casing | 8-inch casing | 10-inch casing | 12-inch casing |
|---|---|---|---|---|---|
| 10 ft | ½ cup | 1 cup | 1½ cups | 1 pint | 2 pints |
| 25 ft | 1 cup | 1 pint | 2 pints | 3 pints | 4½ pints |
| 50 ft | 1 pint | 1 quart | 2 quarts | 3 quarts | 1 gallon |
| 100 ft | 1 quart | 2 quarts | 1 gallon | 1½ gallons | 2 gallons |
| 150 ft | 3 pints | 3 quarts | 1½ gallons | 2 gallons | 3 gallons |
Your casing diameter is on the well log too, or you can measure across the top of the casing. Dug wells are far wider than this table covers. Ask a well contractor how to dose one.
If your bleach is 8.25%
Many bleach brands now sell a concentrated 8.25% formula. It is about 1.6 times stronger than 5.25% bleach. Use about two-thirds of the amount in the table.
Use bleach from an unopened bottle with no scent, no "splash-less" formula, and no additives. Bleach loses strength over time, so use a bottle made within the last 6 months. Never use pool or hot tub chemicals[2].
Why more is not better
These amounts aim for a strong but safe solution. The Minnesota Department of Health advises 50–200 ppmPPM (Parts Per Million)A unit of concentration equivalent to milligrams per liter (mg/L). Used for most common contaminants like iron, manganese, and nitrates.Learn more → (parts per million) of chlorine, and no more than 200 ppm[2]. Extra bleach does not kill more bacteria. It corrodes the pump, wiring, and casing, and it takes hours longer to flush out.
Do not know your water depth and cannot find the well log? Your state well program or the driller may have a copy. Minnesota's guidance is to use the total well depth if you truly cannot find the water level, which will overestimate a little[2]. Test strips tell you whether you landed in range.
Supplies You Need
| Item | Details |
|---|---|
| Household bleach | Unopened, unscented, 5.25–8.25% sodium hypochlorite. |
| Bucket | For diluting the bleach before it goes in the well. |
| Garden hose | To recirculate water back into the well. |
| Chlorine test strips | High-range strips that read up to 200 ppm, plus the ability to read 0 ppm when flushing. |
| Rubber gloves and eye protection | Bleach burns skin and eyes. |
Step-by-Step: How to Shock Chlorinate a Well
Step 1: Turn off and bypass treatment equipment
Put softeners, filters, and RO units in bypass. Chlorine can damage softener resin and carbon filters. Turn off the UV lamp if you have one, and drain or bypass the water heater if your local guidance says to.
Step 2: Dilute the bleach
Measure your bleach from the table. Mix it into about 3 gallons of water in a bucket[1]. Straight bleach can corrode metal parts inside the well[3].
Step 3: Pour it into the well
Open the well cap or the vent opening. Pour the solution into the casing. Keep it off the wires and metal cap parts.
Careful: If pipes come up through the cap, do not unbolt it. On some wells the pump hangs from the cap, and the seal plate can fall into the well. Use the vent hole or call a well contractor[3]. Work outdoors and ventilate well pits, since chlorine gas can build up[2].
Step 4: Recirculate until you smell chlorine
Connect a hose to the outdoor faucet nearest the well. Run it back into the top of the well. When you smell chlorine at the hose, keep it running about 30 more minutes[2].
Check the water with a test strip. It should read at least 50 ppm. If it reads lower, add more diluted bleach and recirculate again[2].
Step 5: Pull chlorine to every faucet
Open every faucet in the house, hot and cold, one at a time. Run each until you smell chlorine, then close it. Include showers, tubs, the laundry, outdoor spigots, and the ice maker line. Flush each toilet.
If a faucet never smells of chlorine, add more solution to the well and recirculate again[3].
Step 6: Wait 12–24 hours
Do not drink, cook, or brush teeth with the water during this time. Ohio State Extension recommends at least 12 hours and up to 24[1]. Overnight is the easiest way to do it. Stock bottled water first.
Step 7: Flush the chlorine out
Run an outdoor hose until you can no longer smell chlorine. Point it away from the septic system, gardens, lawns you care about, and streams. A lot of chlorinated water can harm your septic tank's bacteria, so send as little as possible down indoor drains[2].
Then run each indoor faucet until a test strip reads 0 ppm. Flushing can take 30 minutes or most of a day, depending on your well[2].
If your water flow drops sharply while flushing or recirculating, shut off power to the pump and call a well contractor[2].
When to Retest After Shock Chlorination
Wait until the chlorine is fully gone. Then wait 2 to 4 more weeks and send a new coliform sample to a state-certified lab[2]. Ohio State's guidance is about 2 weeks[1].
The wait matters. A sample taken too early can come back clean only because leftover chlorine is still killing bacteria. That tells you nothing about whether the source is gone.
| Retest result | What it means | Next step |
|---|---|---|
| Negative for coliform | The shock worked. | Test again in a few months, then yearly. Replace filter cartridges that sat in the system before the shock. |
| Total coliform positive, E. coli negative | Bacteria are still getting in or survived. | Shock a second time. Inspect the cap, vent screen, and casing. |
| E. coli positive | Fecal contamination is reaching the well. | Use bottled or boiled water. Call a licensed well contractor and your health department. |
Want to watch for a comeback between lab tests? A home presence/absence coliform kit such as Safe Home's DIY Bacteria in Water Test gives a yes/no result in 6 to 72 hours, according to the maker[4]. Use it for the months after a shock. It doesn't replace the certified-lab retest above. See the Safe Home bacteria test on Amazon.
Not sure how to read the lab report? See how to read a water test report, or plan your next sample with our well water testing guide.
When Shocking the Well Is Not Enough
Shock chlorination is a one-time reset. It does not stop bacteria from getting back in. The Minnesota Department of Health notes that repeat attempts are normal when bacteria have been growing a long time[2]. Two failed shocks in a row usually point to one of these:
- A path for surface water. A cracked or loose cap, a missing vent screen, a damaged casing, or a failed grout seal. A licensed well contractor can inspect and repair these.
- Bacteria living in the well itself. Long-established growth may need the well cleaned, not just disinfected[2].
- A shallow or dug well near a source. Septic fields, livestock, and flooding can keep reaching it. Repair may not be possible.
If you cannot fix the source, you need continuous disinfection[1]. For most homes that means a UV purifierUV DisinfectionA chemical-free method that uses ultraviolet light to destroy bacteria, viruses, and other microorganisms by damaging their DNA. Does not change water taste or chemistry.Learn more → (a lamp that kills germs as water flows past). Compare models and sizing in our UV water purifier reviews. A UV unit needs clear water to work, so iron, manganese, or sediment have to be treated before it.
Frequently Asked Questions
How much bleach do I need to shock a well?
It depends on how much water is standing in the well and how wide the casing is. For 100 feet of water, Ohio State Extension lists 1 quart of household bleach for a 4-inch casing, 2 quarts for a 6-inch casing, and 1 gallon for an 8-inch casing. Those amounts assume regular 5.25-6% bleach. If your bottle says 8.25%, use about two-thirds as much.[1]
How long do I wait after shocking a well?
Let the chlorine sit 12 to 24 hours, then flush until no chlorine is left. Wait 2 to 4 weeks after that and retest for coliform. Do not drink the water until the retest comes back negative[2].
When should I retest my well after shock chlorination?
Retest 2 to 4 weeks after disinfection, and only after the chlorine is fully flushed out. A sample taken while chlorine is still in the water can show a false clean result.
Can I use too much bleach?
Yes. The Minnesota Department of Health advises staying between 50 and 200 ppm[2]. Much stronger solutions do not disinfect better. They corrode the pump, wiring, and casing, and take longer to flush.
What if bacteria come back after shocking the well?
Shock chlorinate a second time first. Repeat attempts are common when bacteria have been growing a long time. If coliform keeps returning, something is letting surface water in. Have a licensed well contractor inspect the cap, casing, and grout seal. If the well cannot be fixed, add continuous disinfection such as a UV purifier.
When should I shock chlorinate my well?
After a positive coliform test, after flooding, after a pump replacement or any work that opens the well, after plumbing work, and when a well has sat unused. It is a one-time disinfection, not an ongoing treatment.
References
- Mancl, K. "Shock Chlorination of Wells." AEX-318, Ohio State University Extension, 2019. https://ohioline.osu.edu/factsheet/aex-318
- Minnesota Department of Health. "Well Disinfection." Well Management Section, updated October 2024. https://www.health.state.mn.us/communities/environment/water/wells/waterquality/disinfection.html
- Oregon State University Well Water Program. "Shock Chlorination of Wells." OSU Extension Service. https://wellwater.oregonstate.edu/well-water/bacteria/shock-chlorination
- Safe Home Test Kits. "DIY Bacteria in Water Test Kit." Product page. https://safehometestkits.com/product/diy-bacteria-in-water-test/