How to Shock Chlorinate a Well, Step by Step

Before you start: is this a job for you?

Shock chlorination kills bacteria in the well and the pipes. It does not fix how they got in. British Columbia lists the usual entry points to check first: a casing that sticks up less than 30 cm (1 ft) above ground, a missing surface seal, a cracked cap¹. Penn State found that shocking plus a sanitary cap kept bacteria away for a year in only 15% of the wells it studied².

Do it when a lab test finds E. coli or coliform bacteria, after a flood, after the pump or well has been opened for repairs, or when the well is new^1,3,4. After a flood, Québec says to wait at least 10 days after the water goes down before disinfecting⁵.

Hire a licensed well contractor if:

• it is your first time (Health Canada suggests it)³;
• the pump was under water: the US EPA says not to turn it on until an electrician or pump contractor checks the wiring⁶;
• the well is in a pit. Harmful gases can build up in well pits and push out the oxygen⁷.

What you need

• Unscented household bleach, freshly bought. Minnesota asks for a bottle made within the last 6 months⁷.
• Safety glasses and rubber gloves³, a funnel, a wrench, and a garden hose that reaches the well³.
• Drinking water for at least 12 hours⁴, and the lab bottle for the retest.
• Optional: chlorine test strips. Health Canada and Minnesota both suggest them to confirm the dose^3,7.

Read the label percentage. Health Canada’s table assumes 5.25% bleach³, but Minnesota says bottles now read 6 or 8.25%⁷. Stronger bleach means less product for the same dose. No pool or hot tub chlorine: it contains other chemicals you don’t want in the well^3,7. Ontario accepts fresh unscented bleach or a product certified to NSF/ANSI 60⁸.

Our take: plain unscented bleach is what the agencies’ tables are built for, so it is the simplest choice. If you prefer a product sold for wells, the kit below is on NSF’s drinking-water chemical listing, but it comes with its own dosing chart: follow it, not the bleach tables.

Well sanitizing kit (calcium hypochlorite)

Well Safe Well Sanitizer Kit

NSF/ANSI 60

Contents

8 oz granules + 8 oz sinking pellets

Target

100 ppm (maker's brochure)

Contact time

8–24 hours (maker)

NSF max. use level

14 mg/L

Strengths

• Listed by NSF for drinking-water treatment chemicals
• Pellets sink to the bottom of the well; granules treat the upper casing
• Dosing chart by well diameter and water depth in the box

Drawbacks

• The shock dose (100 ppm) is far above the NSF use level (14 mg/L): flush completely before drinking
• You still need to know your water depth

Good fit: a homeowner who wants a product made for wells rather than laundry bleach.
Skip it if: you need ongoing disinfection, or you haven't found how the bacteria got in.

High-range test strips (checks the dose)

Hach Free Chlorine Test Strips, 0–600 mg/L (2890200)

No NSF/ANSI listing found

Range

0–600 mg/L

Colour steps

0, 25, 50, 100, 200, 400, 600

Tests

100

Result

under 1 minute

Strengths

• Reads shock levels that ordinary 0–10 mg/L strips cannot
• No liquid reagents

Drawbacks

• Coarse steps: it confirms the dose is in range, not an exact number
• Useless for checking that the chlorine is gone

Good fit: checking that the recirculating water reached 50 mg/L or more.
Skip it if: you only need to confirm the flush. Use a 0–10 mg/L strip.

Low-range test strips (checks the flush)

Hach Free & Total Chlorine Test Strips, 0–10 mg/L (2745050)

No NSF/ANSI listing found

Range

0–10 mg/L

Colour steps

0, 0.5, 1, 2, 4, 10

Tests

50

Reads

free and total chlorine

Strengths

• Low range made for checking that chlorine is flushed out
• Pairs with the high-range strip from the same maker

Drawbacks

• Tops out at 10 mg/L: it cannot read the shock dose
• Colour matching only

Good fit: confirming the chlorine is down to a low level after flushing.
Skip it if: you want to know if the bacteria are gone. Only a lab test tells you that.

How much bleach

You need the depth of water in the well: total well depth minus the static water level. Both are on the driller’s report or in your provincial well database³. The two tables below come from two methods. Health Canada doses the water in the well at 50 ppm50 mg/L³. Minnesota’s table also covers an average home’s plumbing and aims for 50–200 ppm50–200 mg/L⁷.

Health Canada · 5.25% bleach, existing well, 50 mg/L

Water in the well	15 cm (6 in) drilled well	90 cm (36 in) dug well
1 m (3.3 ft)	20 mL	0.6 L
3 m (9.8 ft)	60 mL	1.8 L
5 m (16.4 ft)	100 mL	3.0 L
10 m (32.8 ft)	200 mL	6.0 L

Minnesota Department of Health · 6 or 8.25% bleach, well + home plumbing, 50–200 ppm

Water in the well	2-inch casing	4-inch casing	6-inch casing
10 ft	2 cups	2 cups	2 cups
50 ft	2 cups	2 cups	3 cups
100 ft	2 cups	3 cups	4 cups
300 ft	3 cups	4 cups	10 cups

More is not better. Minnesota warns that a solution above 200 ppm200 mg/L reduces how well the disinfection works⁷. Across Canada, provincial targets for an existing well range from 50–250 ppm50–250 mg/L⁹.

The steps

These follow Health Canada’s procedure³, with the details other agencies add.

1. Turn off the power to the pump before you open the well^3,7.
2. Bypass your treatment equipment. Health Canada names softeners, filters and reverse osmosis: chlorinated water can damage them³. Ontario adds that carbon filters must come out, or the pipes after them won’t be disinfected; put in new ones afterwards⁴. The guides we read don’t name UV systems, so check your UV manual.
3. Decide about the water heater. Agencies disagree. Ontario says to drain it and turn off its heat source⁴; Québec says purging it is not necessary⁵. Minnesota runs the hot taps until a strip reads at least 50 ppm50 mg/L, then flushes the heater at the end⁷.
4. Pour the bleach in. Ontario dilutes it in a bucket of water first⁴. Health Canada uses a funnel, then rinses the well cap and parts with fresh water to prevent corrosion³. If your well has a sanitary seal with bolts, don’t loosen them: use the vent or plug^1,7.
5. Recirculate. Turn the pump back on, run the hose away from the well until you smell chlorine, then put the hose inside the casing. Let it run for 30 minutes^3,7.
6. Run every tap, hot and cold, plus showers and toilets, until you smell chlorine, then close them^3,4. Remove faucet aerators first: loosened scale can clog them⁷. No smell? Add half the first amount of bleach and repeat¹.
7. Wait. Health Canada says 12 to 24 hours³; Québec says 24 hours⁵; Minnesota accepts 2 hours, preferably 6 or overnight⁷. Don’t drink, cook, bathe or wash with the water during this time³.
8. Flush outside. Run a hose to a spot away from wells, your septic system, plants and any lake or stream³. Don’t send it into the septic system^1,7. Keep going until the smell is gone; British Columbia says this can take from half an hour to four days¹. With a low-yield well, don’t overpump¹.
9. Flush inside by opening every tap until the water no longer smells of chlorine³. Ontario’s contractor rules use a free chlorine reading below 1 ppm1 mg/L as the end point⁸.

Safety

• Glasses and rubber gloves the whole time³.
• Never mix bleach with other chemicals: they can form harmful gases⁷. Ontario adds that chlorine gas can be released if the water was made acidic (below pH 5) before the chlorine went in⁸.
• The wires at the wellhead are live once the pump is back on³. Rubber boots and gloves do not protect you from electric shock⁶.
• Don’t climb into a well pit⁷.
• With a baby at home while E. coli is present, Québec advises boiled water for drinks, baby food, ice and toothbrushing, and sponge baths for young children⁵.

When the water is safe again

Only a lab test tells you the bacteria are gone. Until then, boil water for 1 minute before drinking it, or use another source³. Agencies set different schedules:

Retesting after shock chlorination · by agency

Agency	First sample	Then
Health Canada	48 hours after treatment	2 safe results in a row within 1 to 3 weeks, then 1 more after 3 to 4 months3
Québec	1 week	4 weeks after disinfection5
British Columbia	1 week	1 month1
Penn State Extension	10 to 14 days	2 to 3 months later2
US EPA (after a flood)	7 to 10 days	not specified6

In Ontario, bacteria testing for private wells is free: bottles come from Public Health Ontario or your local public health unit⁴. In Québec, use a lab accredited by the environment ministry⁵. Our testing labs guide lists the options by province and state.

If the tests don’t come back safe twice in a row, repeat the treatment and inspect the well; if results stay positive, call public health³.