Well depths and types (C) Carson Dunlop AssociatesWell Chlorination Procedure
when and how to shock a drinking water well

  • WELL CHLORINATION & DISINFECTION - CONTENTS: How to shock the well - well shocking procedure. When to shock or chlorinate a drinking water well & What is the procedure to shock or chlorinate a well? How much bleach or hypochlorite do you use to shock a well? When should well water be re-tested after shocking the well? Well shocking details for water filters & Well shocking details for water softeners. Chlorinating water - details for wat
  • POST a QUESTION or READ FAQs about well shocking or well sterilization procedures: how to deal with bacterial contamination in drinking water wells
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How to shock or disinfect a well:

This article explains how to shock a well, when, why, and exactly how to chlorinate a drinking water well. We provide a table explaining the quantity of bleach needed to disinfect a well, and a table comparing the 3 Common Well Water Disinfectants: Chloramine, Chlorine and Chlorine Dioxide.

We include a detailed description of the well shocking procedure using household bleach to sterilize well water and water equipment. The purpose of shock disinfection of a well system is to destroy bacterial contamination present in the well system at the time of disinfection and is not intended to kill bacteria that might be introduced at a later time. Page top sketch illustrating both deep and shallow water well construction and depths is provided courtesy of Carson Dunlop Associates.

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When and How to Shock or Chlorinate a Well

Unsanitary well opened by owner, needs to be shocked and sealed (C) Daniel FriedmanProcedure for Shocking a Well to (temporarily or maybe longer) "Correct" Bacterial Contamination in Well Water

This article includes adapted information from the New York Dutchess County Health Department's environmental laboratory, along with other expert sources and with annotations, expansion & annotation. You will find this well shock procedure advice consistent with the well shocking or well chlorination procedures recommended by most health authorities.

[Click to enlarge any image]

Article Series Contents

The purpose of shock disinfection of a well system is to destroy bacterial contamination present in the well system at the time of disinfection and is not intended to kill bacteria that might be introduced at a later time.

Our photo shows an owner who has lifted the loose, poorly-sealed well piping and cap right off of the steel well casing. This well needed repairs and it needed to be sterilized using the well chlorination procedure we discuss here.

Therefore it is vital that the well be constructed so that no new contamination may enter the well following completion of the shock disinfection. In order to achieve a satisfactory disinfection of the system, the bacteria must be brought in contact with a chlorine solution of sufficient strength and remain in contact with that solution for a sufficient time to achieve a complete kill of all bacteria and other microorganisms.

Details of the Well Chlorination Procedure - Exactly How to Shock a Well, Where to Put Chlorine, How Much Chlorine to Use to Shock the Well

Standard well casing (C) Daniel FriedmanIf drinking water has been tested and has not passed standards for safe drinking, or any time the building water supply system has been opened for repairs (such as replacing a submersible well pump or a jet pump foot valve), the well should be disinfected following these procedures, and should be re-tested as described below.

Our photo shows a standard modern 6" steel well casing - it's easy to spot at a property.

If you don't know where the well is located you'll have to find it before this well chlorination procedure can be best performed.

See WELLS CISTERNS & SPRINGS for articles that describe different types of water wells, what they look like, where they're found, and their operating characteristics.

It's possible to get chlorine into the well by sending it through the building piping and pump but that step won't sterilize the interior and sides of the well casing - so the procedure below is a better one.

Quantity of Chlorine or Household Bleach Needed to Shock a Well

Well Depth in Feet (6-inch diameter well casing) Volume of standard household bleach needed
Well depth 100' 3 cups Clorox or 2 oz. of granules.
Well depth 200' 6 cups Clorox or 4 oz. of granules
Well depth 300' 9 cups Clorox or 6 oz. of granules
Well depth 400' 12 cups Clorox or 9 oz. of granules
Well depth 500' 1 gallon Clorox or 12 oz. of granules.

Watch out: NOTE to be accurate in reaching the necessary concentration of chlorine in your well, treat the "depths" listed above as if they were the height of the actual column of water in your well (assuming a standard casing which is 1.5 gallons per foot of height). So if your well is 400 feet deep, but if 100 feet of it is air, your water depth is actually 300 ft. The actual quantity of water in a well bore when the well is at rest is defined as the static head.

Actual well water volume: See STATIC HEAD, WELL DEFINITION for a table of well water volumes per foot or per meter of depth.

Choices of well disinfectants are detailed at WELL DISINFECTANT CHOICES

More about how to measure the actual depth of a well is at DEPTH of a WELL, HOW TO MEASURE.

Watch out: for inadequate well disinfection: use enough bleach to reach the necessary concentration in the well and let the disinfectant remain in the well long enough (8-24 hours) - otherwise you may fail to adequately disinfect the well.

Watch out: for excessive well disinfection: don't significantly "overdose" the well with bleach or chlorine or you may find that you have to waste a lot of water and time flushing out the chlorine bleach at the end of the disinfection period.

Watch out: for difficult-to-sanitize wells or if the water well has been contaminated by area flooding extra steps and extra safety precautions needed. Details are at WELL DISINFECTION after a FLOOD

If you need to disinfect in-building water equipment such as a water tank or water heater or water softener, see WATER TREATMENT EQUIPMENT DISINFECTION

Reader Question/complaint: My well ran out of water during the well-shock and flush-out procedure

You need to WARN people = re water discharge/well shocking. i ran my well out of water and made the situation WORSE. Tell people not to run the water outside only for a half hour or so at a time!!!! NOT until Chlorox smell is gone - can take forever o get rid of Chlorox smell.... well needs to recharge. Now, listening to you folks, I have an even BIGGER problem. - M.P.

Reply: Indeed the well flush-out procedure can disclose another problem with poor well flow rate or poor well recovery rate

Thank you for the note complaining about discovering that your well could run dry during an attempt to flush out the chlorine odor from a bleach-shock of the well. We were of course sorry to read that your well shock process disclosed a second fault - a well with a poor recovery rate.

We have reviewed our well water article on the topic (WELL CHLORINATION SHOCKING PROCEDURE) to be sure that there are warnings about running out of water during well testing or well shocking. In the main article explaining how to shock and flush out a well following a failed bacteria test you will see several WATCH OUT warnings that include discussion about problems that can arise during the bleach flush-out process. Here we'll add some more detailed suggestions that can help address the worry of running out of well water during well shocking and flushing.

Indeed while it has not come up during well shocking this problem has been discovered by building inspectors more than once during a septic loading and dye test to test for septic system failure - as that water test volume is run, typically 250 gallons or so, the inspectors discovered that the well itself was inadequate when the well flow diminished or stopped entirely. It sounds as if you have run into the same problem of a poor or inadequate well flow rate.

Low flow rate wells usually recover but watch out for pump damage

As long as no one makes the mistake of leaving a well pump running when it is dry of water, the system is not damaged, and the well will recover, usually in a few hours; but once discovering that your well has a poor flow rate, indeed you will want to take steps to address the problem, either by adding water storage capacity or by increasing the well yield, or in the worst case, drilling a better well.

Steps to protect from running out of well water during shocking and flushing out a well

  1. Recycle well casing flush-water: One of the steps in shocking a well, using a garden hose connected to the building to use chlorinated water to wash-down the well casing sides during the initial steps in the well shock procedure will reduce the water consumed, but during the flush-out period a low-yield well may still run out.
  2. Empty the water pressure tank (and water heater tank if that was shocked): A second step that may be able to reduce the water wasted trying to flush out the bleach odor from the building water supply after shocking a well (thus also reducing the chances of running out of water) is to completely empty the water pressure tank and the water heater tank, refilling them with fresh water from the well as soon as incoming well water at a close-by tap is low or has no bleach odor.
  3. Install equipment to protect the well pump: A third step to protect a well and pump from "running out of water" is the installation of special low water controls in the well that will recycle water in the pump and reduce or stop well water flowing into the structure should water level in the well drop too low. See WELL PIPING TAIL PIECE for details.
  4. Don't leave water running unattended. On occasion when a well has a limited recovery rate, an inspector, home owner, or water tester may "forget" to keep track of how long water has been run, simply leaving faucets open without watching for a diminished or stopped water flow.

    Any time you are running water to test or service a building that uses a private pump and well system for its water supply, you should continuously monitor the water running out of one or more fixtures. If you see the water flow rate significantly diminish or stop you should immediately turn off the well pump to protect it from damage.

    Then try waiting half an hour or more before turning the pump back on. That procedure will also give insight into how quickly a low-flow-rate well will recover.
  5. Increase well yield: There are several approaches to dealing with low-flow wells, including storing more water on site and increasing well yield. See WATER QUANTITY IMPROVEMENT for details.

But in all cases if the well runs "dry" during well flush-out after a well shocking procedure or during a septic loading and dye test, as long as you haven't done something unusual like just leaving water on for hours unattended, you've discovered a double fault if your water well runs dry during well-shock-bleach flushout: contaminated well water and a low yield or low flow-rate well.

Reader Report: trouble flushing chlorine bleach out of the well after shocking

Question: we left bleach in the well after installing a new pump then pumped it out after 2 hours

(July 9, 2015) Alan said:
My well company installed a new pump and poured bleach and pumped it after 2 hours. The water test (2 days later) showed. bacteria. He told me to put 3 gallon of chlorox in and repeat. He said to leave it the 4 days that I was on vacation. My well is 210 feet with a 75 foot water table (6 inch pipe).

Although I bought 'regular' unscented Walmart chlorox, I see it is 8.5% and I believe the standard is more like 5.5%. So I have a massive dose of strong bleach that has been down there 4 days before pumping and I can't get rid of the chlorine smell although it is lessened. I have emptied my hot water tank and well tank. I have run the outside tap a lot and also the inside taps.

Although there is an odor of chlorine, For the inside taps, I think the bleach is much lower than the level that would damage to septic tank bugs. I think the 3 gallons of strong bleach spread out into the rock over the 4 days I was on vacation, and it is slowly seeping back into the well.... any ideas or can it only be healed with time ?

Reply:5 mistakes to avoid when shocking & then flushing out a well

Two hours? What the heck is that. No well disinfection can be done with just a 2 hour exposure. Sounds as if someone was in a hurry.

One needs to use the right dose (based on the water volume in the well's static head), to flush out all of the piping and let all of it sit with disinfectant, wash down well casing sides, and other steps the experts have described and we summarize in the article above.

But your guess about the bleached well water is an interesting one. I don't really know but I suppose it's possible. You may need to re-flush the well several times. Take care not to run the well dry and thus burn up your pump.

(July 10, 2015) alan said:
Thanks Dan..I am draining my tanks again today and running the outside tap for 30 mins at a time with 30 minute gaps. I will do one gallon of 5.5% next time--or a half gallon of this 8.5% Walmart chlorox. I first flushed on Sunday and still wifey is going to the laundromat as she can't stand the bleach smell and her Ma is highly allergic to it.



I'm assuming the bleach was poured in as liquid. If some dope (like me) dumped pool tablets into the well they sit at the bottom and it can take much longer to get the chlorine out.

Also if concentrated bleach was poured into the well it may reside on dry areas of piping and casing - better hook up an outdoor hose and wash down the well casing sides and top recirculting diluted bleach-water as part of the cleanup.

11 July 2015 alan said:
yes Dan I used liquid and then poured 5 gallons of clean water down after it. I didn't run the hose back down the well to recirculate to mix it up and that could have been a mistake as some bleach could have stuck to the inside of the pipe and slowly dripped down day by day. I drained the hot tank again today and then back flushed it by turning on the pressure and leaving the bottom of tank hose-to-drain connected.

Back flushing under pressure threw out a lot of black debris (rust) and I speculate that the chlorine could have been reacting with these metal fragments to cause bleach smell. I am sure this helped a lot but women's sense of smell is TEN times that of men (scientific fact) so I will get the verdict when she gets home..

So my 4 mistakes were too highly concentrated bleach (8.5%), too much bleach (3 gallons,)not washing down the inside of the well pipe with the hose (used 5 gallon bucket), not back flushing the two tanks under pressure to remove rusty particles which later combined with the bleach.


That's a nice inventory of snafus. Don't feel bad, you're not the snafu leader.

I'd try well casing washdown and then more flushout.

Alan said:
ps I should add number 5 mistake: Leaving it 4 days while I was on vacation - giving the bleach a chance to permeate the rock. (and eat the well casing no doubt). I will leave it 12 hours max next time. Good news Saturday - wife sniffed my towel wash and declared that she will try her wash today - that is progress ! I flushed this well last Sunday when we got back from the 4 day vacation, so we have lived in a faint mist of chlorine ever since then. 'Back flushing' the hot tank under 60psi pressure yesterday made the biggest difference.

Chlorine & Hydrogen sulfide (H2S) Safety Warnings for Water Treatment Equipment & Well Disinfection

Watch out: when working with chlorine, people should be in a well-ventilated place. The powder or strong liquid should not come in contact with skin or clothing. Solutions are best handled in wood or crockery containers because metals are corroded by strong chlorine solutions.

Watch out: if using chlorine disinfectants in equipment that heats water such water heaters, be sure your heater has been turned OFF and has cooled down to at least room temperature before trying to run chlorine through it. Heating water that contains a high level of chlorine might produce potentially dangerous chlorine gas coming out of a hot water faucet.

Watch out: Leaving too much chlorine in any water system can be dangerous: drinking concentrated chlorinated water could be sickening or even fatal, and less seriously, doing laundry with chlorinated water may bleach clothing by accident.

Watch out: for difficult well or equipment sterilization cases such as after area flooding or where there is a persistent bacteria source, pH adjustment of the disinfection solution may be necessary for the disinfectant to be effective, (WELL DISINFECTANT pH ADJUSTMENT )

Watch out: if the well source water contains hydrogen sulfide (H2S), when preparing a well disinfection solution you must allow the water to aerate so the H2S dissipates, otherwise the acid formed will cause an immediate release of H2S gas from the water, which poses a serious health risk.

For details about exposure to various toxic and irritating gases see

Common Well Water Disinfectants: Chloramine, Chlorine and Chlorine Dioxide

This topic has moved to a separate article found at WELL DISINFECTANT CHOICES.

Well Chlorination Procedure for Water Filters, Water Softeners, and Water Heaters


Well Cleaning & Well Disinfection Procedures after Area Flooding

This topic has been moved to a separate article found at WELL DISINFECTANT TABLE, POST FLOODING

Article series contents

This article series on well water contamination, testing, & cures explains many common water contamination tests for bacteria and other contaminants in water samples. We describe what to do about contaminated water, listing common corrective measures when water test results are unsatisfactory.

We include water testing and water correction measures warnings for home owners and especially for home buyers when certain conditions are encountered, with advice about what to do when these circumstances are encountered. Various treatment methods for contaminated water are reviewed and the pros and cons of each are discussed.

Well Contaminant & Disinfection Articles


Continue reading at WELL DISINFECTION FAQs or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.



Or see STATIC HEAD, WELL DEFINITION - table of well water volumes per foot or per meter of depth




Or see WELL CLEANING PROCEDURES to address clogged well screens, blocked foot valves, or debris in the well bore.


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WELL CHLORINATION & DISINFECTION at - online encyclopedia of building & environmental inspection, testing, diagnosis, repair, & problem prevention advice.


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