Shock chlorination of well water is needed after a positive bacteria test, a repair, or flooding. Chlorine is added to the well and its plumbing, left in contact for a time the written procedure sets, and then flushed out. The chlorine amount and the contact time come from the state or local health department or a certified well professional, because both depend on the well's depth, diameter and water volume.
The Environmental Protection Agency (EPA) does not regulate private wells, so the owner is responsible for testing and treatment, according to the agency's private wells guidance. That responsibility includes the lab retest that shows whether the disinfection worked.
Shock Chlorination Disinfects a Well After a Positive Bacteria Test
Shock chlorination disinfects a private well and its plumbing, after a positive total coliform result, a repair, or flooding.
The process runs in three stages. Chlorine goes into the well and through the household plumbing, stays in contact for the time the procedure sets, and is flushed out until the procedure says the flush is complete. The procedure also says where that flushed water goes.
EPA says disinfection is followed by a retest at a state-certified laboratory to confirm the bacteria are gone. Until the procedure says the water is safe and that retest is clear, general guidance from the Centers for Disease Control and Prevention (CDC) is to keep the water out of drinking and cooking.
If the same well keeps testing positive, the answer changes. Repeated bacteria point to a repair or a continuing treatment device, and treatment equipment for wells is covered in our well water filter guide.
Well Water Chlorination Follows Bacteria, Repairs or Flooding
A well needs disinfecting when a test finds coliform bacteria, after the well is repaired, and after flooding.
EPA says private well owners should test at least once a year for total coliform bacteria, nitrates, total dissolved solids and pH. EPA also calls for testing after flooding, after repairs, and after any change in taste, color or odor.
EPA says to test after a change in taste, color or odor. A result in hand shows whether coliform bacteria are present at all. Our well water testing guide covers what to order, and our page on what to test first helps set the order.
| Event | Why It Matters | First Step |
|---|---|---|
| Positive total coliform result | Bacteria could be entering the well | Inspect the well, cap, casing and nearby sources |
| Well repair | EPA lists repairs as a time to test | Follow the disinfection procedure for the repair, then retest |
| Flooding | EPA lists flooding as a time to test | Test, and disinfect if the procedure calls for it |
| Change in taste, color or odor | EPA lists it as a time to test | Test first, then decide on disinfection |
Finding the Contamination Source Comes Before the Chlorine
A positive total coliform result shows that bacteria could be entering the well, but it does not name the source. Guidance from EPA and CDC points owners toward inspecting the well itself, the cap, the casing and nearby sources.
A positive total coliform result does not name the source, so inspect the well, cap, casing and nearby sources. Chlorine disinfects the well and its plumbing.
If bacteria come back repeatedly, a repair or a continuing treatment device may be needed. Our coliform bacteria test guide explains what a total coliform result covers and what it leaves out.
Chlorine Amounts and Contact Times Come From the Health Department
The amount of chlorine and the contact time for a well come from the state or local health department or a certified well professional. Both figures depend on the well's depth, diameter and water volume. One number cannot fit every well.
At The Indoor Report, we publish no chlorine dose and no contact time on this page for that reason. A figure printed for an average well could be wrong for a deeper, wider or shallower one. Our rule on this site is to send the reader to the agency that sets the figure.
Bring these questions to the state or local health department or a certified well professional, the two sources EPA's private wells guidance and CDC direct well owners to. Get the answers in writing, because the written procedure is what you follow at the wellhead and what you read the retest against.
- How much chlorine this well needs, given its depth, diameter and water volume.
- How long the chlorine stays in the system before flushing.
- Where the flushed water should go, given the plants and the septic system.
- When the household can use the water again, and for what.
- When to take the follow-up bacteria sample, and which state-certified laboratories accept it.
Chlorine Safety Rules Cover Handling, Flushing and Water Use
Chlorine is a hazardous chemical, and CDC general safety guidance is to keep it away from other cleaning products and to ventilate the space where it is handled. Read the whole procedure before opening the chlorine.
The flush is the stage that reaches the yard and the septic system. Chlorinated water can harm plants, and the volume of chlorinated water sent into a septic system is a concern. The procedure from the health department addresses disposal, so follow its instructions for where the water goes.
Arrange another source of drinking and cooking water before the chlorine goes in. The written procedure sets when tap water is usable again.
Never mix chlorine with any other cleaning product.
Ventilate the space where chlorine is handled.
Keep well water out of drinking and cooking until the procedure says it is safe and a follow-up bacteria test is clear.
Who Should Not Rely on a One-Time Well Chlorination
This advice does not serve a well owner whose results keep coming back positive. Repeated bacteria point to a repair or a continuing treatment device.
It also does not serve a well with a damaged cap or casing that has not been fixed, or an owner without a written procedure for the well. In either case, hand the job to a certified well professional. The professional works from the well's depth, diameter and water volume, which set the chlorine amount and contact time.
Two results would change this advice. A positive retest moves the well from disinfection to repair or ongoing treatment. A report that flags nitrates, total dissolved solids or pH needs its own reading, because shock chlorination is a bacteria response, and our guide to reading a water report walks through each line.
Retesting at a Certified Lab Ends Well Water Chlorination
EPA says disinfection is followed by a retest at a state-certified laboratory to confirm the bacteria are gone. That lab result, read alongside the written procedure, decides when normal water use resumes. Work through the steps in order.
- Keep the well water out of drinking and cooking until the written procedure says it is safe.
- Take the follow-up sample when the procedure says to, and send it to a state-certified laboratory.
- Read the total coliform result with our guide to reading a water report.
- If the result is clear, return to the yearly test EPA recommends for total coliform bacteria, nitrates, total dissolved solids and pH.
- If bacteria come back repeatedly, a repair or a continuing treatment device may be needed.
Get the written procedure for this well and line up a state-certified laboratory for the retest, so chlorination of well water ends with a result you can read.
Common questions
What does shock chlorination do?
Shock chlorination disinfects a private well and its plumbing. Chlorine goes into the system, stays in contact for a time the procedure sets, and is then flushed out.
How long before I can drink well water after shock chlorination?
Wait until the written procedure says the water is safe and a follow-up bacteria test comes back clear. The health department or well professional sets the timing. Use another source for drinking and cooking water until both conditions are met.
Why did my well test come back positive again after chlorination?
A repeat positive means bacteria may still be getting into the well. A positive total coliform result does not name the source, so the well, cap, casing and nearby sources need inspecting.
Can I shock chlorinate a well myself?
Take the procedure and the chlorine amount from the state or local health department or a certified well professional. This site gives no amounts.
Will shock chlorination hurt my septic system?
The flushed water is the risk to a septic system. The volume of chlorinated water sent into a septic system is a concern, and chlorinated water can also harm plants. Follow the disposal instructions in the procedure the health department supplies.
How soon should I retest after shock chlorination?
Retest when the written disinfection procedure says to, and send the sample to a state-certified laboratory. EPA describes that retest as the step that confirms the bacteria are gone. The health department or well professional sets the timing.