How to Shock a Well: Bleach Volumes, Contact Time, and When It Will Not Hold
By SubcontractorHub Editorial Team·Published September 2026
Short version: work out the standing water volume from casing diameter and water column, add roughly 3 quarts of unscented 5–6% bleach per 100 gallons, circulate it until you smell chlorine at every tap, then leave it 12–24 hours before flushing. Retest for coliform 1–2 weeks later, once the chlorine is gone. If it comes back positive twice, the well has a defect and shocking it a third time will not fix it.
Shock chlorination is a one-time disinfection: a strong dose of chlorine held in the well and the household plumbing long enough to kill what is living in them. It is the standard response to a positive coliform test, to pump or casing work, and to a well that has been flooded or left idle.
It is also routinely done wrong, in three specific ways: too little bleach because the volume was estimated from well depth rather than standing water, too little contact time because someone needed a shower, and no retest afterwards so nobody actually knows whether it worked.
Safety first. Bleach is corrosive — eye protection and gloves, and never mix it with anything acidic, which releases chlorine gas. Nobody drinks, cooks with, bathes in or waters animals from this supply while the chlorine is in it. Do not run chlorinated water into a septic system in one slug; flush to open ground away from plants, waterways and storm drains. If the water serves a public or shared supply, or if anyone in the household is immunocompromised, this is a job for a licensed well contractor and your health department, not a weekend task.
1. Work Out the Standing Water Volume
This is the step people get wrong. What matters is not how deep the well is, it is how much water is actually standing in the casing. A 300 foot well with a static water level at 250 feet holds only 50 feet of water.
Standing water column = total well depth − static water level
Gallons in the well = water column (ft) × gallons per foot for the casing diameter
Add roughly 2 gallons for the pressure tank and household plumbing. If you do not know the static water level, your well log will have it, and the drilling contractor or your state well records office usually holds a copy.
2. Bleach Volume
Use plain unscented household bleach, 5 to 6% sodium hypochlorite. No scent, no thickener, no detergent, nothing marked splash-less or colour-safe. Target a concentration of roughly 200 ppm free chlorine, which works out at about 3 quarts of bleach per 100 gallons of water.
Rounding up modestly is fine and is better than under-dosing. Grossly over-dosing is not — very high chlorine concentrations are hard on pump seals, wiring insulation and the pressure tank bladder, and the excess takes far longer to flush out.
3. The Procedure, Step by Step
- Bypass the treatment equipment. Put softeners, iron filters, carbon and UV into bypass. Carbon media will strip the chlorine straight out and be ruined doing it, and high chlorine damages softener resin. If there is no bypass, this alone is a reason to call a professional.
- Turn off the water heater and let it cool. Chlorinated water sitting in a hot tank is hard on the anode and the lining.
- Open the well cap. Kill power to the pump at the breaker first. Note the condition of the cap and the seal while it is off — a cracked or missing sanitary cap may well be the whole reason you are doing this.
- Pre-mix the bleach. Dilute the measured bleach into 5 to 10 gallons of water rather than pouring it neat down the casing. Neat bleach sinks in a slug and does not disperse.
- Pour it in, washing down the casing walls with a hose as you go. The biofilm on the inside of the casing is a large part of what you are trying to kill.
- Circulate. Restore power, run a garden hose from an outside spigot back into the well casing and let it recirculate for 15 to 30 minutes until you can clearly smell chlorine coming out of the hose.
- Pull it through the house. Open every fixture in turn — hot and cold, every bathroom, kitchen, outside spigots, washing machine, ice maker line, hose bibs — and close each one when you smell chlorine. Do not skip the seldom-used ones; a guest bathroom that never gets flushed is exactly where bacteria survive.
- Seal it up and wait 12 to 24 hours. Replace the well cap. No water use at all during this period.
- Flush outdoors first. Run an outside spigot to waste, away from plants, waterways and storm drains, until the chlorine smell is gone. This protects the septic system from a large chlorine slug, which can knock out the bacteria the tank depends on. Watch the pump — do not run the well dry.
- Then flush indoors, return the treatment equipment to service, and refill the water heater.
4. Retesting: the Step That Gets Skipped
A shock without a retest has told you nothing. Clear water with no smell is not evidence.
Wait 1 to 2 weeks until all chlorine has cleared the system before sampling. Residual chlorine in the sample suppresses bacterial growth in the lab and gives a false negative — a clean result on a well that is still contaminated is worse than no result at all. Then test for total coliform and E. coli through a certified lab, and confirm with a second test two to four weeks later, because intermittent contamination can pass one sample and fail the next.
5. When Shocking Will Not Hold
Shock chlorination treats an event. It kills what is in the well now and leaves nothing behind. If water is entering the well continuously, you are emptying a bathtub with the tap running.
Repeat positives after a properly executed shock point at a defect, not at a dosing error:
- A cracked, corroded or poorly sealed casing
- A missing, damaged or non-sanitary well cap
- Casing that does not extend at least 12 inches above grade, or ground sloping toward the wellhead
- A septic system, livestock area or agricultural runoff too close to the well
- A shallow or surface-influenced aquifer, which no amount of chlorine will change
Two failed shocks is the stopping point. At that stage the answer is a well inspection to find the entry path, plus permanent disinfection rather than a third round of bleach. For a private well that means an NSF/ANSI 55 Class A UV system at 40 mJ/cm2, sized on peak flow and UV transmittance with sediment filtration ahead of it — the UV sizing calculator works that out, and the treatment system selector places it correctly in the train.
6. Sulfur Smell Is a Different Case
Shocking is often recommended for a rotten-egg smell, which comes from sulfate-reducing bacteria producing hydrogen sulphide. It usually works, and the relief is usually temporary — those bacteria live in the aquifer and the formation, not just the casing, so they repopulate over weeks or months. If the smell keeps coming back, the durable answer is oxidation treatment such as an air injection filter rather than shocking on a schedule. That stage is covered in the iron filter guide, since the same equipment usually handles both.
For Well and Water Treatment Contractors
Shock chlorination is a low-ticket visit attached to a high-ticket conversation. The homeowner has a positive coliform result in hand, which is the most motivating document in this trade, and they are standing at an open wellhead with you.
The professional move is to be straight that a shock is an event and not protection, and to price both paths on the same visit: the shock now, and permanent UV with the pretreatment it needs. Homeowners who are told only that the shock is done come back angry in four months. Homeowners who were shown the difference between a one-off and continuous protection buy the system, often financed, on the day. Quoting both on site — with the retest already booked as a scheduled follow-up — is the difference between a service call and a system sale.
Frequently Asked Questions
How much bleach do you need to shock a well?
The volume depends on how much water is standing in the casing, not on how deep the well is. Work out the standing water column by subtracting the static water level from the total well depth, then use the casing diameter to get gallons per foot: a 4 inch casing holds 0.65 gallons per foot, a 6 inch holds 1.5, and an 8 inch holds 2.6. For a 200 ppm shock using ordinary unscented 5 to 6% household bleach, allow roughly 3 quarts of bleach per 100 gallons of standing water, and add about 2 gallons of water volume for the pressure tank and household plumbing. A typical 6 inch well with 100 feet of standing water holds about 150 gallons and needs roughly 1.2 gallons of bleach.
How long should bleach stay in a well after shocking?
A minimum of 12 hours, and 24 hours is the standard recommendation. Contact time is what actually does the disinfection, and cutting it short is the single most common reason a shock fails and the coliform test comes back positive again. During that period nobody may drink the water, cook with it, shower in it or give it to animals, so plan around a full day without the supply. Do not exceed roughly 24 hours either, because prolonged high chlorine concentration is hard on well components, pump seals and the pressure tank bladder.
How do you know if shock chlorination worked?
Only a laboratory coliform test will tell you, and the timing of that test matters. Wait until all chlorine has flushed out of the system, which typically takes one to two weeks, because residual chlorine in the sample will suppress bacterial growth and produce a false negative. Retest for total coliform and E. coli at that point. Many health departments recommend a second confirming test a further two to four weeks later, since intermittent contamination can pass one test and fail the next. Water that looks clear and has no odour is not evidence of anything.
Why does my well keep testing positive for coliform after shocking?
Repeated positives after a correctly performed shock almost always mean water is entering the well continuously, so disinfection only clears the contamination already present and the source keeps refilling it. The usual causes are a cracked or corroded casing, a failed or missing sanitary well cap, a casing that does not extend far enough above grade, surface water running toward the wellhead, a nearby septic system or a shallow aquifer under surface influence. Shock chlorination treats an event, not a defect. If two properly executed shocks fail, stop repeating it and have the well inspected, then install permanent UV disinfection as continuous protection.
When should you shock a well?
Shock chlorinate after any positive coliform test, after any work that opened the well such as a pump replacement or casing repair, after flooding covers the wellhead, on a newly drilled well before it is put into service, and on a well brought back into use after sitting idle for months. Sulfur odour caused by iron or sulfate-reducing bacteria is another common reason, though in that case the relief is usually temporary. Shocking a well on a routine annual schedule with no test result and no triggering event is not recommended, because it is hard on the equipment and treats a problem that may not exist.
Is shock chlorination a permanent fix for bacteria in well water?
No. Shock chlorination is a one-time disinfection event that kills what is in the well and plumbing at that moment and leaves nothing behind to protect it afterwards. The moment the chlorine flushes out, the well is exactly as vulnerable as it was before. If the contamination was a one-off, such as bacteria introduced during pump work, that is enough. If the cause is ongoing, permanent treatment is needed, and for a private well that normally means an NSF/ANSI 55 Class A ultraviolet system delivering 40 mJ/cm2, sized on peak flow and UV transmittance, with sediment filtration ahead of it.
Related Guides & Tools
- UV Water Purifier Sizing Calculator
- Well Water Treatment System Selector
- Iron Filter for Well Water
- How to Test Water Hardness
- Well Drilling Cost Calculator
- Water Softener Sizing Calculator
- Water Softener vs Reverse Osmosis
- → SubcontractorHub for Plumbing
- → Water Treatment Contractor Software
Book the Retest Before You Leave the Wellhead
30-minute demo. See how water treatment dealers quote a shock and a permanent UV system on the same visit, schedule the follow-up retest automatically, and put financing beside the cash price.
Book a DemoWater Treatment, Well Water, Plumbing
SubcontractorHub
See How Top Contractors Close More Jobs
AI proposals, sales pipeline, and project management — all in one platform.
- ✓AI-powered proposals in the field
- ✓CRM built for contractor sales
- ✓Project tracking from sale to install
30 minutes. No commitment.