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How To Shock Chlorinate and Sanitize Wells With Chlorine To Kill Bacteria
To shock chlorinate a well, bypass your softener and filters, add enough chlorine to bring the water in the well to about 100 ppm, and run it back into the well until every tap smells of chlorine. Let it stand at least 12 hours, preferably 24 (never longer than 36 hours), then flush until a low-range test shows no chlorine. For example, a 6-inch well with 100 feet of water holds about 150 gallons and needs about 1 quart of 6% household bleach, or 3 oz of dry chlorine (about 105 pellets); use the calculator below for your well’s diameter and water depth.
Work Out How Much Chlorine to Add
How much chlorine do I need?
Enter your tank size, or your well casing and water depth, and get the exact amount of bleach or dry chlorine to add.
Use NSF/potable-grade calcium hypochlorite. Do not use pool Tri-Chlor tablets or stabilized pool chlorine.
Check the bottle before you measure. Household bleach is no longer a single strength — it now runs from about 6% to 8.25% sodium hypochlorite, and the percentage is printed on the label. Use fresh, plain, unscented liquid bleach. Avoid scented, splashless, color-safe and gel bleach, which contain thickeners and additives that do not belong in drinking water.
Add this much liquid chlorine
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What to do with it
Before you start. Bypass the softener, filters and any other treatment equipment. Wear goggles and gloves, and keep the area ventilated — chlorine fumes in an enclosed well house or tank can be dangerous. Never pour chlorinated water into a septic system, onto plants, or into a creek or pond.
What you need on hand
Shock chlorination only tells you something if you can measure the result. A high-range kit confirms you actually hit your target and reads correctly while levels are still high; a low-range kit tells you when the chlorine is truly gone. You want both — a low-range kit alone can read zero on water that is still heavily chlorinated. If bacteria come back after two or three weeks, the source needs continuous disinfection rather than another shock.
The high-range kit reads to 80 ppm. If you dosed to 100 ppm it will be over range at first, which is itself useful information — it tells you the chlorine is there and working. It comes into range as the chlorine is consumed and drops, which is when you actually want the number.
Why we ask what strength your bleach is
Chlorine bleach is not all the same strength, and the amount you need depends entirely on which one is in your hand. Household bleach sold today generally runs between 6% and 8.25% sodium hypochlorite. Liquid pool chlorine is roughly twice as strong, at 10% to 12.5%. You will also see 5.25% quoted in older references and on some product labels, which was the common household strength for many years.
The percentage is printed on the bottle. Check it before you measure, and set it in the calculator — that one number is the difference between hitting your target and missing it. Dry chlorine is different: potable-grade calcium hypochlorite pellets and granules are a fixed 70% strength, so there is no percentage to set for them.
Two other things affect what you are actually adding. Bleach loses strength on the shelf, so a jug that has been in the garage since last summer is weaker than the label says; buy fresh for a job like this. And use plain, unscented liquid bleach only. Scented, splashless, color-safe and gel bleaches contain thickeners and additives that have no place in drinking water.
Chlorine demand, and why your residual drops
The numbers above are what it takes to reach your target in clean water. Iron, manganese, sulfide, tannins and organic matter all consume chlorine on contact — that consumption is called chlorine demand, and it is the reason a well that tested at 100 ppm in the morning reads 20 ppm by evening.
This is why the test kit matters more than the calculation. Measure the residual after the contact time. If it has fallen well below your target, the demand was higher than the dose and the job needs repeating.
Read a zero with suspicion while chlorine could still be high. Above about 10 ppm, chlorine bleaches the reagent rather than coloring it, and the result looks like no chlorine at all. Dilute the sample with equal parts chlorine-free water, retest and double the reading, or use a high-range kit.
Watch: using the calculator step by step
Before You Start
A common concern for homeowners with well water is how to properly disinfect their system, especially after receiving a positive test result for coliform bacteria.
Coliform bacteria in your well or plumbing system may indicate contamination from surface water, septic tank leakage, or inadequate disinfection after drilling or recent maintenance. This can also mean harmful bacteria and viruses may be present. In addition to health risks, other nuisance bacteria—such as iron and sulfur bacteria—can produce foul tastes, odors, and slime, and even clog pumps and plumbing.
Shock chlorination is a common method used to disinfect wells by adding chlorine bleach to kill bacteria and sanitize the system. In this guide, we’ll walk you through how to shock chlorinate a well safely and effectively.
It is important to periodically monitor your well for contamination. The EPA recommends testing private water supplies annually for coliform bacteria to detect problems early.
A positive test indicates the likely presence of disease-causing bacteria and viruses, possibly from surface water influence, septic leaks, or improper disinfection after drilling or servicing.
While iron and sulfur bacteria are not health threats, they can cause odors, tastes, and clogging in pumps and water systems.
Rotten-egg or musty smell that comes back after a shock? Iron and sulfur bacteria often return. The J-PRO-38 Hydrogen Peroxide Well Water System injects hydrogen peroxide as you use water to oxidize hydrogen sulfide and iron and keep the odor down. It treats odor; it is not a replacement for shock chlorination when coliform is present.
Shock chlorination can eliminate these harmful bacteria and nuisance organisms, causing bad smells and slime.
You can test for bacteria using a home coliform test kit or by sending a sample to a licensed lab.
Shock Chlorinate & Sanitize a Well & Home Piping System Whenever:
- A new well has been constructed
- Anytime a well is opened for repairs
- Floodwater has entered a well
- A new holding tank, pump, or pressure tank has been installed
- A new pipeline or other piping or plumbing work has been done
- Tests indicate the presence of coliform bacteria
- Odors or slime caused by iron or sulfur bacteria are present
Bypass Your Equipment and Work Safely
CAUTION: Shock chlorination may temporarily discolor water and loosen iron and sediment, which can damage pipes and fixtures. Avoid running such water through water softeners, heaters, or filters.
If unsure or if the well is hard to access, contact a licensed well driller or pump contractor for professional chlorination.
Note of Caution:
Shock chlorinating a well or piping system may loosen up the scale, iron deposits, and other materials, which can clog fixtures, appliances, and valves in the piping system.
Care should be taken when flushing the piping, and all aerators removed to prevent clogging. In some extreme cases of corroded piping, the piping may fail and start to leak after this procedure. Chlorination will not remove nitrate or other contaminants.
Disinfecting a storage tank or cistern rather than a well? See how much chlorine to add to a storage tank to kill bacteria.
How to Shock Chlorinate a Well: Which Type of Bleach To Use
The best way to shock chlorinate and sanitize wells is to use a combination of dry pellets and dry chlorine granules mixed with water. The dry pellets drop down past the pump and ensure the entire well is sanitized.
In addition, liquid bleach made from potable water grade powdered bleach is poured into the well and then recirculated until the chlorinated water has made its way through the well pipe, pressure tank, and pipes in the house if desired.
You can use regular household bleach (non-perfumed type), which kills bacteria and certain viruses. Most household bleach sold today is 6% to 8.25% sodium hypochlorite, not the old 5.25%, so check the label and enter the strength in the calculator above. Laundry bleach can contain contaminants such as benzene and heavy metals, so only use it as a last resort if you can not find the powdered bleach and pellets.
You can also use pool chlorine, which is 10% to 12.5% sodium hypochlorite, roughly 1½ to 2 times as strong as household bleach. Do not use pool pellets, which are not designed for potable water.
How to Shock Chlorinate Sanitize Wells Using Dry Pellets or Powdered Bleach?
The dry pellet method (70% calcium hypochlorite) can be used for well, storage tanks, or cistern sanitation. The number of tablets and granules used will depend on the amount of water in the system to be sanitized. Half of the total treatment will be completed with chlorine pellets, and the other half will be completed with granular chlorine slurry.
A 100-ppm chlorine concentration is ideal for sanitizing a well. To produce a 100 PPM chlorine concentration, use 2 oz. sanitizer pellets or granules for every 100 gallons of water in the system. (2oz. granules = 70 Pellets) If the water has a high iron or sulfur content more chlorine may be needed to achieve 100-ppm chlorine residual.
Instructions
- Bypass the water softener, other filters, or purification equipment. Clean the well house, spring house, or storage tank by removing debris and scrubbing interior surfaces with a strong chlorine solution (½ gallon household bleach or 0.2 oz dry bleach granules per 5 gallons of water). Pump out any suspended solids if possible.
- Remove the well inspection plug or cap carefully. Measure water depth if possible and use the table below to determine the sanitizer amount. If unsure, seek professional help.
- Mix granules in 5 gallons of water (add product to water, not water to product) and pour into the well. Drop tablets into the well if applicable.
- Circulate the water by connecting a garden hose to an outdoor sillcock after the pressure tank and running water back into the well for about 15 minutes until a strong chlorine odor is detected. If not, add more bleach and repeat.
- Let it stand at least 12 hours, preferably 24. Don’t leave it in the system longer than 36 hours. Then flush outdoor faucets until a low-range test shows no chlorine, followed by indoor faucets. (A low-range kit alone can read zero on heavily chlorinated water, so confirm with a high-range kit that the shock level has come down first.)
Do not discharge high chlorine water into septic systems or onto lawns.
Note: Chlorine may cause colored water and loosen iron or organic material that can clog pumps. Do not continue pumping if the water flow stops. Don’t leave it in the system longer than 36 hours.
- Return equipment to service.
- After it has stood at least 12 hours, preferably 24 (never longer than 36 hours), safely discharge chlorinated water away from plants and water bodies. Remaining household plumbing water can be discharged into the septic system.
- Retest for coliform bacteria 1–2 weeks after the chlorine is completely flushed out. If problems persist, consult a water professional.
(If you want to read more about Chlorinate Sanitize Wells, check out this post)
Shock Chlorinate With Liquid Chlorine Bleach
- Clean the well house, spring house, or storage tank by removing debris and scrubbing interior surfaces with a strong chlorine solution (½ gallon household bleach per 5 gallons of water).
- Determine the amount of chlorine needed using the chart below. If unsure of well depth, contact your well driller.
- Mix the chlorine with 10 times as much water before pouring down the well.
- Remove the well cap or air vent carefully and pour the diluted chlorine into the well. If unsure, seek professional help.
- Protect yourself by wearing safety goggles, rubber gloves, and a chemical-resistant apron.
- Circulate the chlorinated water by running water from an outdoor faucet back into the well for about 15 minutes until a strong chlorine odor is detected.
- Open all indoor faucets and outdoor fixtures until the chlorine smell is present.
- Let it stand at least 12 hours, preferably 24. Don’t leave it in the system longer than 36 hours.
- Flush the system by running an outdoor faucet until a low-range test shows no chlorine, then do the same for indoor faucets. (A low-range kit alone can read zero on heavily chlorinated water, so confirm with a high-range kit that the shock level has come down first.) Avoid discharging chlorinated water onto lawns or septic systems.
- Backwash water softeners, flush water heaters, and replace filters as needed.
- Retest for coliform bacteria 1–2 weeks after the chlorine is completely flushed out. If bacteria persist, consult a professional.
Bacteria keep coming back? A shock is a one-time treatment. If retests keep showing coliform, the well needs continuous disinfection: the J-PRO-38 Chlorinator Well Water Package injects chlorine automatically every time the pump runs.
Liquid Bleach Quick Reference for Wells (6% Household Bleach, 100 PPM)
| Well Casing Diameter | Distance From Water Level to Bottom of Well (Water Depth). Amount of 6% household bleach for 100 PPM, worked out for the deepest water in each column. Numbers are rounded for easier measuring. | |||||
| 0′ – 50′ | 50′ – 100′ | 100′ – 200′ | 200′ – 300′ | 300′ – 400′ | 400′ – 500′ | |
| 4″ | 7.5 fl oz | 1.9 cups | 3.7 cups | 1.4 quarts | 1.9 quarts | 2.3 quarts |
| 6″ | 2 cups | 1 quart | 2 quarts | 3 quarts | 1 gallon | 1.3 gallons |
| 8″ | 3.5 cups | 1.7 quarts | 3.5 quarts | 1.3 gallons | 1.7 gallons | 2.2 gallons |
| 10″ | 1.4 quarts | 2.7 quarts | 1.4 gallons | 2.1 gallons | 2.7 gallons | 3.4 gallons |
| 12″ | 2 quarts | 3.9 quarts | 2 gallons | 3 gallons | 3.9 gallons | 4.9 gallons |
| 24″ | 2 gallons | 3.9 gallons | 7.8 gallons | 12 gallons | 16 gallons | 20 gallons |
| 36″ | 4.4 gallons | 8.8 gallons | 18 gallons | 26 gallons | 35 gallons | 44 gallons |
Every column assumes 6% household bleach. For pool chlorine (10-12.5% sodium hypochlorite) use about half, or use the calculator above for the exact amount for any strength.
Well System Diagram
Wells with submersible pumps have pipes that either enter through the top or through the side. Many wells are underground or in vaults in areas with freezing temperatures.
Resources:
- See the EPA resource for home water wells: https://www.epa.gov/safewater/pwells1.html
- Consider a home chlorination system to keep your well water disinfected.
- See all our well water treatment systems
Frequently Asked Questions (FAQs)
1. What precautions should I take before chlorination starts?
If you are not familiar with your well or well construction, or how to shock chlorinate a well, consider hiring a licensed well driller to shock chlorinate.
2. When Can I Use the Water Again?
Make sure to flush 4 well volumes before using the water:
- 4” well diameter has 70 gallons per 100 ft of depth
- 6” well diameter has 150 gallons per 100 ft of depth
- 8” well diameter has 260 gallons per 100 ft of depth
Retest for coliform bacteria 1–2 weeks after the chlorine is completely flushed out. Test for total coliform and E. coli bacteria.
If the test results show the absence of coliform bacteria, the water is safe to drink.
However, if test results show the presence of coliform bacteria, the source(s) of contamination should be identified and eliminated through a licensed well driller/contractor, or a continuous disinfection treatment system should be installed.
3. What Type of Chlorine Bleach Should Be Used?
For best results, use powdered bleach or pellets that are certified for drinking water. However, it is also OK to use unscented household chlorine bleach found in supermarkets; most is 6% to 8.25% sodium hypochlorite now, so check the label and enter the strength in the calculator above.
