How to Shock a Well With Bleach (Step by Step + Bleach Calculator)
A positive coliform test, a flood, a pump replacement or a sudden rotten egg smell all lead to the same job: shock chlorinating the well. Done right, it kills the bacteria living in the casing, pump and pipes in about a day, using nothing more exotic than plain household bleach.
Done wrong, it either does nothing or damages your system. This guide gives you the exact bleach amount for your well with a calculator, the step by step method I use in the field, and what to do when the bacteria come back.
Boreholeflow is reader supported. When you buy through links on this page, I may earn a commission at no extra cost to you. Read the full disclosure.
| Situation | Target chlorine | Contact time | Retest |
|---|---|---|---|
| Routine yearly disinfection | 50 ppm | At least 12 hours | Optional |
| After pump, pipe or tank work | 100 ppm | 12 hours | 7 to 14 days later |
| Positive coliform or E. coli test, or flooding | 200 ppm | 12 to 24 hours | 7 to 14 days after chlorine is gone |
| Iron bacteria or sulfur bacteria | 250 ppm or more | 24 to 48 hours | Repeat if slime or smell returns |
When You Should Shock a Well
- Your water test shows total coliform or E. coli. This is the most common reason, and the one that matters most for health.
- After any work on the well. Pulling the pump, replacing the pressure tank or repairing a pipe can introduce bacteria.
- After flooding. If floodwater covered the wellhead, assume the well is contaminated until a test says otherwise.
- New wells. Drillers disinfect new wells before handover, and you should test before you drink.
- Smell, slime or staining. A rotten egg smell that comes and goes, or orange slime in the toilet tank, often means iron or sulfur bacteria.
If you are not sure whether you have bacteria, test first. My guide to the best well water test kits covers lab mail in kits that test for coliform, and the rotten egg smell guide helps you separate bacteria from dissolved gas.
How Much Bleach to Shock a Well (Calculator)
Enter your well depth, the depth to water and your casing size. The calculator works out how much water is in the well and the plumbing, then gives the bleach volume for the chlorine level you need.
Uses about 0.041 gallons of water per foot of casing per square inch of diameter (1.47 gal/ft for 6 in). Use plain unscented bleach only, never splashless or scented. Confirm the result with a high range chlorine test strip.
Bleach Amounts Table by Casing Size
For a quick check without the calculator, here is how much bleach treats every 100 ft (30 m) of standing water at 200 ppm. Double it for 200 ft of water, and add extra for your pressure tank and water heater.
| Casing diameter | Water per foot | Water per 100 ft | 8.25% bleach for 200 ppm | 6% bleach for 200 ppm |
|---|---|---|---|---|
| 4 in (100 mm) | 0.65 gal (8.1 L/m) | 65 gal | 2.5 cups | 3.5 cups |
| 5 in (125 mm) | 1.02 gal (12.7 L/m) | 102 gal | 4 cups | 5.5 cups |
| 6 in (150 mm) | 1.47 gal (18.2 L/m) | 147 gal | 5.5 cups | 8 cups |
| 8 in (200 mm) | 2.61 gal (32.4 L/m) | 261 gal | 2.5 quarts | 3.5 quarts |
| 10 in (250 mm) | 4.08 gal (50.7 L/m) | 408 gal | 4 quarts | 5.5 quarts |
| 12 in (300 mm) | 5.88 gal (73.0 L/m) | 588 gal | 5.5 quarts | 8 quarts |
These numbers are based on the volume of a cylinder, about 1.47 gallons per foot for a 6 inch casing. Health department tables often round up, which is fine, because a little extra chlorine is safer than too little.
What You Need
- Plain unscented bleach. Regular unscented bleach only, never splashless, scented or color safe, which contain additives you do not want in a well.
- High range chlorine test strips. Strips that read up to 500 ppm confirm that the chlorine actually reached every tap.
- Gloves and goggles. Chemical resistant gloves and safety goggles, because concentrated bleach burns eyes and skin.
- A clean bucket, funnel and garden hose long enough to reach from an outside tap back to the wellhead.
- Stored drinking water for 24 to 48 hours, because you will not be able to drink from the tap.
High range chlorine test strips (0 to 500 ppm)
Normal pool strips stop at about 10 ppm, which is useless for shocking. High range strips let you confirm 50 to 250 ppm at the hose, and then confirm the chlorine is gone before you retest for bacteria.
- Reads from 0 up to about 500 ppm
- Results in seconds
- Also useful for checking chemical injection systems
The only way to know the shock actually worked through the whole system.
You need a separate low range strip to confirm the water is clear afterwards.
Bacteria test kit for well water
Shock chlorination is only finished when a lab test comes back clean. Order a coliform test before you start so it is ready for the retest a week or two later.
- Tests for total coliform and E. coli
- Certified labs give results you can trust
- Some kits also cover nitrate and metals
Proves the well is safe to drink again.
Samples must reach the lab within a set time.
How to Shock a Well With Bleach: Step by Step
- Bypass your treatment equipment. Put carbon filters, softeners and reverse osmosis units on bypass, because chlorine this strong ruins carbon and resin. Remove cartridge filters from their housings.
- Turn off power to the pump at the breaker, then remove the well cap or the plug in the well seal. Wear gloves and goggles from this point on.
- Mix and pour the bleach. Dilute the amount from the calculator in about 5 gallons (20 litres) of clean water, then pour it into the well through a funnel.
- Restore power and recirculate. Connect a garden hose to the nearest outside tap and run it back into the well, washing the inside of the casing. Keep it running until you smell strong chlorine at the hose, usually 15 to 60 minutes.
- Check the strength. Dip a high range test strip in water from the hose. If it reads well below your target, add more bleach and recirculate again.
- Chlorinate the plumbing. Open every hot and cold tap one at a time, including showers and outside taps, until you smell chlorine, then close it. Flush each toilet once.
- Replace the well cap and wait. Let the chlorine sit for 12 to 24 hours, or 24 to 48 hours for iron bacteria. Do not use the water during this time.
- Flush the system. Run an outside hose to a gravel area away from your septic tank, lawn, garden and any stream until the chlorine smell is gone. Then run the indoor taps briefly.
- Retest. Wait until a low range strip shows no chlorine, then take a bacteria sample. Most health departments suggest waiting 7 to 14 days.
Do Not Send Strong Chlorine Into Your Septic Tank
A few gallons of chlorinated water from flushing toilets will not hurt a septic system, but hundreds of gallons will kill the bacteria that make it work. That is why the main flush goes out through an outside hose.
Point the hose at gravel or a driveway, not your vegetable garden, lawn or a creek. Highly chlorinated water kills plants and fish.
Shocking a Well for Iron Bacteria
Iron bacteria are harder to kill than coliform because they hide inside a slimy biofilm on the casing and pump. You will often see orange or brown slime in toilet tanks, and the water may smell swampy or like rotten eggs.
Use a higher dose of about 250 ppm or more, scrub the casing with recirculation for longer, and leave the chlorine for 24 to 48 hours. Many wells need two or three treatments, or a professional cleaning with a brush and surge block, before the problem stays away.
If iron bacteria keep returning, a permanent fix is often continuous chlorination with a chemical feed pump. My guide to the best chemical injection pumps explains how these systems work, and the iron filter guide covers dissolved iron.
Can You Shock a Well With Hydrogen Peroxide?
Some contractors use hydrogen peroxide for iron and sulfur bacteria because it leaves no chlorine taste and breaks down into water and oxygen. The catch is that the useful strength is around 35%, which causes serious burns to skin and eyes.
The 3% peroxide from a pharmacy is far too weak for a well. For a homeowner, bleach is cheaper, safer to handle and better documented, so I recommend leaving peroxide treatments to licensed well contractors.
What About Chlorine Pellets?
Calcium hypochlorite pellets and tablets are sold for well disinfection and are much stronger than bleach. They work, but they can sit on top of the pump and concentrate chlorine in one spot, which can corrode stainless parts.
If you use them, dissolve them in a bucket first and follow the label dose exactly. For most home wells, liquid bleach and the recirculation method give a more even result.
Why the Bacteria Came Back
| Problem | Likely cause | Fix |
|---|---|---|
| Positive test again within weeks | Surface water getting into the well | Fix the well cap, grout or slope; see the sanitary well caps guide |
| Positive after heavy rain | Shallow well or cracked casing | Have the well inspected, consider a liner or UV |
| Smell or slime returns | Iron or sulfur bacteria biofilm | Repeat at a higher dose, then continuous chlorination |
| Positive only at one tap | Contaminated faucet or filter housing | Clean or replace the fixture and retest |
| Chlorine never reached the taps | Too little bleach or no recirculation | Recalculate with the tool above and repeat |
Shock chlorination is a one time cleanup, not protection. If bacteria keep coming back, a UV water purifier gives continuous protection without chemicals, as long as the water is filtered first.
Whole house UV water purifier
A UV system disinfects every drop as it enters the house, so a future contamination does not reach your taps. It pairs well with a sediment filter in front of it.
- Sized by flow rate, typically 8 to 15 GPM for a home
- No chemicals or taste
- Lamp replaced once a year
Permanent protection against coliform and E. coli.
Needs clear water and power, and it does not fix the source.
How Often Should You Shock Your Well?
You do not need to shock a healthy well on a schedule. Test for bacteria once a year, and shock only when a test is positive, after work on the system, or after flooding.
Repeated shocking without fixing the cause just hides the problem. A clean test, a sealed wellhead and good drainage around the well are what keep the water safe.
Frequently Asked Questions
How much bleach do I need to shock a 200 foot well?
For a 6 inch well with 150 feet of water, you need about 9 cups of 8.25% bleach for 200 ppm, plus a little more for the pressure tank and water heater. Use the calculator above for your exact depth and casing size.
How long should bleach sit in a well?
Leave it for at least 12 hours, and 12 to 24 hours after a positive coliform test. For iron bacteria, 24 to 48 hours works better.
Can I use splashless bleach to shock my well?
No. Splashless, scented and color safe bleach contain thickeners and additives that you do not want in a drinking water well. Use plain unscented bleach only.
How long after shocking a well can I drink the water?
Wait until the chlorine is fully flushed and a bacteria test comes back clean, which usually takes one to two weeks. Use bottled or boiled water until then.
Will shocking my well hurt my septic system?
A small amount from flushing toilets is fine, but the main flush should go through an outside hose to gravel, away from the septic tank, lawns and streams.
Why does my well keep testing positive for coliform?
Usually surface water is getting in through a damaged cap, a cracked casing or poor drainage around the well. Fix the source, then shock again, or add a UV purifier for continuous protection.
Sources: University of Nevada Cooperative Extension, Shock Chlorination: Estimating the Amount of Bleach Needed; Tacoma Pierce County Health Department, Shock Chlorinate Your Well; Oregon Health Authority, Well Disinfection; Iowa State University Extension, Shock Chlorinating Small Water Systems.
