How to Shock Chlorinate a Water Well

Quick Answer

Shock chlorination kills bacteria in a well using household bleach. Most homeowners can do it themselves — it's the first step after a failed bacteria test.

Shock chlorination is the standard first response to a failed bacteria test, a rotten egg smell, or any event that may have introduced contamination to your well. Most homeowners can do it themselves with materials from a grocery or hardware store.

What You’ll Need

Regular household bleach (5–8% sodium hypochlorite, unscented — no splash-free or color-safe formulas)
A clean bucket or garden hose
Work gloves and eye protection
A flashlight
Someone to help (not strictly required, but useful)
A garden hose long enough to reach the wellhead

Safety First

Kill power to the pump at the breaker before opening the wellhead — you’ll be reaching around the electrical connections at the top of the casing
Wear gloves and eye protection — undiluted bleach is irritating
Don’t mix bleach with ammonia or other cleaners
Keep children and pets away from the wellhead during the process

Step-by-Step Shock Chlorination

Step 1: Calculate Your Bleach Dose

Estimate the volume of water in your well:

4-inch casing: 0.65 gallons per foot of water column
6-inch casing: 1.5 gallons per foot of water column

Water column = total well depth minus static water level (depth to water)

Example: 300-ft well, water level at 80 ft → 220 ft of water column in a 6-inch casing = ~330 gallons → use about 3.5 cups of bleach (1 cup per 100 gallons, or 2 cups per 100 gallons for a thorough disinfection).

If you don’t know your static water level or well diameter, use 2 quarts of bleach for a typical residential well under 300 ft deep with a 6-inch casing.

Step 2: Prepare the Bleach Solution

Mix the bleach into 5 gallons of water in a clean bucket. This dilution makes it easier to distribute throughout the well casing and reduces the risk of the concentrated bleach damaging the pump rubber components.

Step 3: Open the Wellhead and Add Bleach

With power to the pump OFF:

Remove the well cap (usually threaded or bolted)
Pour the bleach-water solution into the casing
If possible, use a garden hose connected to an outdoor spigot (bypassing the pump) to wash down the inside walls of the casing — this distributes chlorine throughout the water column and disinfects the casing walls

Step 4: Circulate the Chlorinated Water

Restore power to the pump. Open every faucet, hose bib, and fixture in the house one at a time until you smell chlorine at each one, then close them. This ensures the chlorinated water has reached all parts of the plumbing. Don’t forget:

Ice maker (turn off during the chlorination period)
Water heater (run the hot water until you smell chlorine at a hot faucet)
Any outdoor spigots or irrigation connected to the well

Once every fixture shows chlorine, shut them all off. Kill power to the pump again.

Step 5: Wait 12–24 Hours

Let the chlorinated water sit in the well and plumbing. Do not use any water during this period. Do not run water to the septic system.

Step 6: Flush the System

After the contact period, restore pump power and run water to a safe area — outside, away from the lawn, garden, and septic drain field — until the chlorine smell is gone. This can take several hundred gallons and 30–60 minutes of continuous flushing. Use a garden hose to direct the water away from vegetation.

Then flush each fixture inside the house until the chlorine smell is gone at each one.

Step 7: Retest

Wait 3–5 days before retesting. Residual chlorine in the system can suppress bacteria and produce a falsely safe result if you test immediately. Submit a sample to an accredited lab for total coliform and E. coli.

When to Call a Professional

If the well has a history of recurring bacteria problems
If the wellhead is damaged or the sanitary seal is compromised
If you test positive again after a second shock chlorination
If the well has not been professionally inspected in several years

A contractor can perform a camera inspection to find physical contamination pathways — cracked casing, improper grouting, a missing or damaged sanitary seal — and fix the source rather than treating the symptom repeatedly.

Frequently Asked Questions

When should I shock chlorinate my water well?

Shock chlorinate when: a bacteria or coliform test comes back positive, the water suddenly develops an odor (especially rotten eggs or musty smell), after a flood or heavy rain where surface water may have entered the wellhead, after any work on the well or pump system (the work may introduce bacteria from tools or equipment), after the well has been out of service for an extended period, or when moving into a home with a well of unknown maintenance history.

How much bleach do I use to shock a water well?

The standard dose is approximately 1 cup (8 oz) of regular household bleach (5–8% sodium hypochlorite, unscented) per 100 gallons of water standing in the well. To estimate well volume: a 6-inch diameter well holds about 1.5 gallons per foot of water column; a 4-inch well holds about 0.65 gallons per foot. If your static water level is 80 ft below grade in a 300-ft 6-inch well, you have about 220 ft of water column = 330 gallons, requiring roughly 3.5 cups of bleach. For a thorough disinfection, double this dose (2 cups per 100 gallons) is often recommended.

How long do I leave bleach in the well?

Leave the chlorinated water in the system for at least 12 hours, and up to 24 hours for a thorough disinfection. During this time, do not use the water (especially do not run it to a septic system — chlorine at disinfection concentrations can upset septic bacteria). After the contact time, flush the system by running water to a safe drain or outdoor area (away from lawn and garden) until the chlorine odor is gone. Retest 3–5 days after flushing — not immediately, as residual chlorine can mask a bacteria result.

Can shock chlorination damage my well or septic system?

At proper dosages, shock chlorination does not damage the well casing, pump, or pressure system. However: do not run highly chlorinated water to your septic tank during the contact period — high chlorine concentrations kill the beneficial bacteria that make a septic system work. Route flushing water away from the septic drain field. If your property has an irrigation system or soaker hoses fed by the well, don't run chlorinated water through them — it can harm plants and soil microorganisms. The pump and drop pipe are not affected by standard disinfection doses.

What if my well tests positive for bacteria again after shock chlorination?

One failed bacteria test after shock chlorination can mean the flushing wasn't complete (residual chlorine falsely indicated safe, then bacteria rebounded), the contamination source is persistent (a cracked wellhead, damaged sanitary seal, or recurring surface water entry), or the biofilm of iron or sulfur bacteria in the casing is too thick for a single chlorination to penetrate. If a second test (3–5 days after a second shock treatment) also fails, call a contractor — the well needs a physical inspection to find and eliminate the contamination source.

Request Water Testing or Service

Tell us what you are seeing — cloudiness, odor, staining, failed test — and we will follow up.