How to Shock Chlorinate a Well — Step-by-Step Guide

Shock chlorination is the standard treatment for bacterial contamination in private wells. Done correctly, it kills coliform bacteria, E. coli, iron bacteria, and sulfur bacteria. Here is the complete process.

When to Shock Chlorinate

• Positive coliform or E. coli test result
• Rotten egg odor in both hot and cold water
• After well service, repair, or a new pump installation
• After flooding near the well
• As part of annual well maintenance (optional but recommended)
• When moving into a home and the last treatment date is unknown

What You Need

• Unscented liquid household bleach (5.25% or 8.25% sodium hypochlorite — NOT pool shock or other concentrations)
• Garden hose
• Funnel (if well cap opening is small)
• Measuring cup

How Much Bleach to Use

Well Diameter	Well Depth	Bleach (5.25%)
4 inch	50 feet	1/3 cup
4 inch	100 feet	2/3 cup
6 inch	100 feet	1.5 cups
6 inch	200 feet	3 cups
8 inch	200 feet	5 cups

For 8.25% bleach, use approximately 60% of the amounts above. For severely contaminated wells or post-flooding, double the standard amount.

Step-by-Step Process

1. Turn off the water softener or iron filter bypass — chlorine will damage resin and filter media. Bypass all treatment equipment before proceeding.
2. Remove the well cap and pour measured bleach directly into the well casing. Use a funnel if needed.
3. Connect a garden hose to a nearby outdoor spigot. Run the hose into the well casing and recirculate water for 15–20 minutes to mix the bleach throughout the water column.
4. Open every faucet — indoor and outdoor — until you smell chlorine at each one. Then close all faucets.
5. Let sit for 12–24 hours. Do not use water during this period.
6. Flush the system — run water outdoors (not into septic system if possible — large chlorine volumes can harm tank bacteria) until the chlorine smell is completely gone. This may take 1–4 hours of continuous flushing.
7. Return all treatment equipment to service mode after flushing is complete.
8. Wait 2 weeks, then collect a certified lab sample for bacteria testing.

If Bacteria Return After Treatment

Persistent bacterial contamination despite shock chlorination indicates a structural problem with the well — cracked casing, missing seal, surface water infiltration, or inadequate depth. Have a licensed well contractor inspect the well before attempting a second treatment.

Frequently Asked Questions

What kind of bleach should I use to shock chlorinate a well?

Use unscented liquid household bleach at 5.25% or 8.25% sodium hypochlorite concentration only — never pool shock or other concentrations, which can be unsafe or ineffective for this purpose.

Should I bypass my water softener before shock chlorinating?

Yes — turn off the water softener or iron filter bypass before starting. Chlorine will damage resin and filter media, so all treatment equipment needs to be bypassed during the process.

How long should bleach sit in the well before flushing?

Let it sit for 12-24 hours without using any water, after first opening every indoor and outdoor faucet until you smell chlorine at each one, then closing them again.

How much bleach do I need for shock chlorination?

It depends on well diameter and depth — for example, a 4-inch well 50 feet deep needs about 1/3 cup of 5.25% bleach, while a 6-inch well 200 feet deep needs about 3 cups. For 8.25% bleach, use about 60% of those amounts. Double the standard amount for severely contaminated wells or after flooding.

When should I shock chlorinate my well?

Do it after a positive coliform or E. coli test, if you notice a rotten egg odor in both hot and cold water, after well service or a new pump installation, after flooding near the well, or when moving into a home where the last treatment date is unknown. It's also fine as optional annual maintenance.