How to shock chlorinate a well with household bleach

Shock chlorination is a one-time, high-strength dose of chlorine that disinfects the well, the pressure tank and every pipe in the house. It is the standard fix for a well that tests positive for coliform bacteria. It is not a water treatment system, and it does nothing for chemical problems: University of Florida Extension notes that it won’t lower nitrate or lead at all¹.

Done carefully, it takes an afternoon plus a night without tap water. Done carelessly, it can damage the pump, kill the grass or overload a septic system. This guide follows the procedures published by state health departments and university extension services, and points out where they differ.

When to shock chlorinate

Extension services recommend it in a few specific situations¹⁶:

• After a positive test for coliform bacteria. This is the most common reason; our guide to a positive coliform test covers what to do before and after.
• After a new well is drilled, or after work on the well, the pump or the plumbing that could let bacteria in.
• After flooding. Minnesota’s Department of Health says that if flood water reached your well or covered the top of the casing, you should assume the well is contaminated⁷.

It’s also the usual first step against nuisance bacteria, such as the sulfur bacteria behind a rotten-egg smell; Minnesota notes these may need extra bleach².

What you need

• Plain, unscented household bleach labeled with its sodium hypochlorite percentage, usually 6% or 8.25%. Minnesota recommends an unopened bottle made within the last six months, and no pool or hot tub products². Avoid scented bleach and bleach with added cleaners⁴⁶.
• Rubber gloves and goggles or a face shield².
• A clean bucket (or several) and a garden hose that reaches from an outdoor faucet to the well.
• Your well’s casing diameter and depth, from the well log or drilling record.
• Drinking water for a day. You can’t use the tap until the system is flushed.
• Chlorine test strips that read down to 0 ppm, if you can get them².

Before you begin, bypass or disconnect carbon filters and reverse osmosis units³. Water softeners and other filters should be disinfected the way their manufacturer describes⁵.

How much bleach your well needs

The dose depends on how much water is in the system, which depends on two numbers from your well log: the casing diameter and the depth of water standing in the well. The depth of water is not the total depth of the well. It’s the total depth minus the static water level, the distance from the ground down to the water when the pump is off⁴.

Agencies don’t agree on one target strength. Oregon State aims for 50 to 100 ppm⁴, Montana DEQ for 200 ppm³, and Minnesota keeps it between 50 and 200 ppm, warning that going above 200 ppm makes the disinfection less effective². The calculator starts at 100 ppm; if your state or county gives a number, use theirs.

Interactive tool

Shock chlorination calculator

Enter the numbers from your well log. The calculator includes Montana DEQ's 100-gallon allowance for the house plumbing, pressure tank and water heater.

Units

Feet, gallons Meters, liters

Well casing diameter 2 in (5 cm) · drilled 3 in (8 cm) · drilled 4 in (10 cm) · drilled 5 in (13 cm) · drilled 6 in (15 cm) · drilled 8 in (20 cm) · drilled 10 in (25 cm) · drilled 12 in (30 cm) · drilled 24 in (61 cm) · bored 30 in (76 cm) · dug 36 in (91 cm) · dug 48 in (122 cm) · dug

Measure across the casing pipe or check the well log. Most drilled wells are 6 inches.

Total well depth (ft)

Static water level (ft)

Both are on the well log or drilling record. Leave the water level empty if you don't know it: the tool then uses the full depth, which gives a larger dose.

Bleach strength on the label 6% sodium hypochlorite (regular) 7.5% sodium hypochlorite 8.25% sodium hypochlorite (concentrated) 5.25% sodium hypochlorite (older bottles)

Plain, unscented bleach only: no scented bleach or bleach with added cleaners.

Target chlorine strength

50 ppm 100 ppm 200 ppm

Agencies use 50 to 200 ppm. If your state or county gives a number, use theirs.

Also disinfect the house plumbing, pressure tank and water heater (adds 100 gallons, as Montana DEQ does)

Bleach to use

4¼ cups

of 6% unscented bleach

Water standing in the well

147 gal

Plumbing, tanks, heater

100 gal

Total water to treat

247 gal

Dilute in

3 buckets

• Dilute first: no more than 2 cups of bleach per 2-gallon bucket of water, so it doesn't corrode the pump or casing.
• Leave the chlorinated water in the system at least 2 hours; overnight is better.
• Retest for coliform bacteria only after the chlorine is gone. Don't drink the water until a lab says it's clear.

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The math is simple if you’d rather do it by hand. Multiply the depth of water by the gallons per foot for your casing (table below), add 100 gallons for the house plumbing³, and use this rule: at 200 ppm, you need about 3 pints of 5.25% bleach for every 100 gallons of water³. Montana DEQ’s own example, a 6-inch well with 100 feet of water plus the plumbing, comes to 7½ pints (15 cups). The calculator, which rounds up, gives 15¼ cups for the same well.

Casing diameter	2 in	3 in	4 in	5 in	6 in	8 in	10 in	12 in
Gallons per foot	0.16	0.37	0.65	1.02	1.47	2.61	4.08	5.87

Source: Montana DEQ Fact Sheet SWP-101, Table 1.

Step by step

1. Turn off the power to the pump at the breaker. Minnesota recommends confirming it’s off with a voltmeter².
2. Open the well. On many wells you remove the cap; on some you can pour through a vent or plug hole instead. Oregon State warns not to remove the bolts on caps where pipes stick up through a seal plate, because the plate and pump can drop into the well. If you’re unsure, call a well professional⁴.
3. Dilute the bleach in buckets of clean water, using no more than 2 cups of bleach per 2-gallon bucket. Straight bleach can corrode metal parts of the well⁴.
4. Pour the solution into the well, avoiding the wiring, then turn the power back on³.
5. Recirculate. Run a hose from the outdoor faucet closest to the well back down into the casing for 15 to 20 minutes³, rinsing the inside of the casing as you go, until you can smell chlorine at the hose⁴. If the flow drops sharply, turn the pump off and call a licensed contractor².
6. Draw the chlorine through the house. Open each faucet, hot and cold, one at a time, until it smells of chlorine, and flush each toilet³. If a tap never smells of chlorine, add more solution and recirculate again⁴. Keep this indoor flushing short: Montana DEQ says no more than 100 gallons of chlorinated water should reach the septic system³.
7. Wait. Leave the chlorinated water in the system for at least 2 hours; overnight is better³². Don’t use the water during this time. Montana DEQ also suggests turning off the water heater while you wait³.
8. Flush it out. Run an outdoor hose onto gravel or bare ground, away from the septic system, landscaping and any stream or pond, until the chlorine smell is gone³⁵. Then run the indoor taps until they’re clear too. This can take anywhere from 30 minutes to a day². Clean the faucet aerators afterwards: the chlorine can loosen debris³.
9. Test. Once there’s no chlorine left (a test strip reading 0 ppm is the surest check²), collect a sample for a total coliform test at a certified lab. Don’t drink the water until the result is clear².

When to retest, and what if the bacteria come back

Sources give different waiting times before that test: 5 to 10 days (University of Florida)¹, about a week (Montana DEQ)³, about two weeks (New Mexico State)⁶ or 2 to 4 weeks (Minnesota)². The point is the same: test only when the chlorine is completely gone, or the result can be falsely clean. Ask the lab which window it prefers. New Mexico State also suggests testing again 2 to 3 months later to make sure the well hasn’t been recontaminated⁶.

If coliform shows up again, Minnesota recommends repeating the disinfection once². If it keeps coming back, something is letting bacteria in, and more bleach won’t fix that:

• A cracked or loose well cap, or one that isn’t sanitary. Montana DEQ recommends replacing non-sanitary caps³.
• Bacteria in the groundwater itself. New Mexico State notes that in that case recontamination is likely and continuous disinfection may be needed⁶.
• A well that needs cleaning as well as disinfection, which Minnesota says is a job for a licensed contractor².

If your well and septic system share a yard, our guide to how often to pump a septic tank explains the warning signs of a septic problem, which can show up in well tests. And once the well is clean, keep it on a routine testing schedule.