How to shock chlorinate a well: chlorine amount, contact time, and when it is the wrong fix

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Short answer

Shock chlorination is a one-time dose of plain, unscented household bleach, 6% or 8.25% sodium hypochlorite, sized to bring the water in the well and the house plumbing to 50 to 200 ppm chlorine. The Minnesota Department of Health caps it at 200 ppm because a stronger solution disinfects less well. MDH’s dose runs from 2 cups for 10 feet of water in the casing to 10 cups for 300 feet in a 6-inch casing. The solution sits at least 2 hours, preferably 6 or overnight (MDH), or at least 4 hours and preferably overnight (Pennsylvania DEP), then is flushed until a chlorine test strip reads 0 ppm. Retest for total coliform 2 to 4 weeks later (MDH), or at 10 to 14 days and again in 2 to 3 months (Penn State Extension). The reference limit is EPA’s Total Coliform Rule: a goal of zero and no more than 5.0% of a public system’s monthly samples positive. Private wells are not covered by that rule.

If coliform has come back after two full shockings done by the numbers below, stop reading for a bigger dose: Pennsylvania DEP and Penn State Extension both say a well that keeps testing positive has a construction or source problem, and this page cannot fix that.

How much bleach goes in?

The dose is set by the water standing in the casing, not by the depth of the well. The Minnesota Department of Health (MDH) gives it in cups of household bleach for an average home system, by feet of water and casing diameter.

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

MDH’s table covers the well plus the house plumbing; reduce it by 1 cup if only the well is being disinfected. The target is 50 to 200 ppm in the recirculating water, and MDH says not to exceed 200 ppm because a stronger solution reduces disinfection effectiveness.

The bleach is specified. MDH calls for unopened, unscented household bleach with no additives, manufactured within six months of use, labelled 6% or 8.25% sodium hypochlorite. Pool and hot tub disinfectants are ruled out because they typically contain algicides and fungicides.

Pennsylvania DEP sizes the dose by product strength. Each amount below disinfects about 150 gallons to 100 to 150 ppm, which PA DEP equates to 100 feet of water in a 6-inch well.

Product	Available chlorine	Amount (PA DEP)
Liquid bleach	5 to 6%	about 1.5 quarts
Liquid chlorine	10%	about 0.75 quart
Liquid chlorine	14%	about 0.5 quart
Granular calcium hypochlorite	65 to 70%	about 4 ounces (8 tablespoons)

How long does it sit, and how do you know it is flushed out?

MDH says the solution sits in the water system for a minimum of 2 hours, preferably 6, or overnight. PA DEP says at least 4 hours and preferably overnight. Overnight satisfies both.

MDH says running the bleach out takes 30 minutes to 24 hours or more, and the end point is a chlorine test strip that reads down to 0 ppm showing no bleach present.

Step	MDH	PA DEP	Penn State Extension
Target concentration	50 to 200 ppm, never above 200	100 to 150 ppm	Defers to the PA DEP procedure
Contact time	2 hours minimum, 6 preferred, or overnight	At least 4 hours, preferably overnight	Defers to the PA DEP procedure
First retest	2 to 4 weeks after disinfection	2 to 5 days after the water is chlorine-free	10 to 14 days after disinfection
Second retest	Repeat disinfection if positive	At least annually	2 to 3 months later, if the first was clear

When do you retest, and what does a second positive mean?

Total coliform is the indicator group a laboratory reports on a bacteria test, and it is what the retest looks for.

Penn State Extension sets expectations plainly. In its study of contaminated wells, shock chlorination plus a sanitary well cap removed the bacteria for one year in 15% of wells, and worked best where counts were under 10 colonies per 100 mL with no E. coli.

A second positive splits the sources. MDH says multiple disinfections are not unusual when coliform has been growing in a system for a long time, and that if attempts fail the well may need cleaning as well as disinfecting.

Penn State says bacteria returning in either follow-up test means a continuous disinfection system. PA DEP says re-evaluate the well’s construction or location.

Before you buy anything

Do not buy a continuous chlorinator on the strength of one repeat positive: MDH does not usually recommend continuous injection, and PA DEP says recurring contamination is a construction question first. What you do need before the first dose is a pack of chlorine test strips that read down to 0 ppm . They are the only listed way to confirm the flush is finished.

Why does coliform or iron bacteria keep coming back?

Two failure modes account for most repeat positives, and neither is a dosing problem.

What keeps returning	Likely cause	What the sources say fixes it
Total coliform, clear water	Contamination reaching the well through its location or construction	PA DEP: shock chlorination is a temporary measure, not a permanent correction; re-evaluate construction and location. Penn State: the procedure that worked in 15% of wells paired chlorine with a sanitary well cap
Total coliform, regrowing after a clear test	Bacteria established in the plumbing or well	MDH: repeat the disinfection; if that fails, clean the well as well as disinfect it
Slime, sheen, swampy odor	Iron bacteria	MDH: pump the well until clear or physically clean it first, then chlorinate at close to but not above 200 ppm, and expect to treat more than once

Iron bacteria show as sticky rusty, brown or grey slime, an oil-like sheen or a swampy odor, per MDH, and a lab test confirms them; the iron in well water guide covers what remains afterwards. MDH’s position is that chlorine may not always get rid of iron bacteria, and that both concentration and contact time matter.

The sources disagree on the last resort. Penn State recommends a continuous disinfection system after two failed retests.

MDH does not usually recommend continuous chlorine injection, because it may hide other bacterial contamination and cause corrosion. PA DEP’s reading reconciles them: find out why the well is exposed before chlorinating it forever.

What is the actual limit, and does it apply to a private well?

EPA’s National Primary Drinking Water Regulations set the Total Coliform Rule for public systems: a maximum contaminant level goal of zero, and a maximum contaminant level of no more than 5.0% of samples total coliform-positive in a month. Two consecutive positives with one also positive for E. coli or fecal coliform is an acute violation.

That rule does not bind a private well: EPA states it does not regulate private drinking water wells and that the homeowner is responsible for maintaining the water.

EPA and CDC both recommend testing a private well at least once a year for total coliform bacteria, nitrates, total dissolved solids and pH. Beyond the limit, contact the local health department.

What should you do first?

Measure the water column in the casing and pick the row from MDH’s table, reducing by 1 cup if only the well is being treated.
Buy unopened, unscented 6% or 8.25% bleach and test strips that read to 0 ppm; if there is slime, pump the well clear or clean it first.
Mix, recirculate to 50 to 200 ppm, and let it sit overnight, which satisfies both the MDH and PA DEP minimums.
Flush for 30 minutes to 24 hours or more until the strips read 0 ppm, then retest at 10 to 14 days (Penn State) or 2 to 4 weeks (MDH).
If the retest is positive again, stop dosing and have the well’s construction and location evaluated.

Frequently asked questions

What kind of bleach do you use to shock chlorinate a well?

The Minnesota Department of Health specifies unopened, unscented household chlorine bleach with no additives, manufactured within six months of use, with a label listing 6% or 8.25% sodium hypochlorite as the active ingredient. Swimming pool and hot tub disinfectants are ruled out because they typically contain algicides and fungicides. Pennsylvania DEP also allows 65 to 70% granular calcium hypochlorite, at about 4 ounces.

How much bleach does a 100-foot well need?

Under the Minnesota Department of Health table, 100 feet of water in the casing takes 2 cups of bleach in a 2-inch well, 3 cups in a 4-inch well and 4 cups in a 6-inch well, well plus house system; reduce by 1 cup for the well alone. Pennsylvania DEP treats 100 feet of water in a 6-inch well with about 1.5 quarts of 5 to 6% bleach.

How long before the water is chlorine-free and can be retested?

The Minnesota Department of Health says the solution sits at least 2 hours, preferably 6 or overnight, then flushing takes 30 minutes to 24 hours or more, verified with a chlorine test strip reading down to 0 ppm. Pennsylvania DEP resamples 2 to 5 days after the water is chlorine-free; MDH retests 2 to 4 weeks after disinfection; Penn State Extension retests at 10 to 14 days.

Why did coliform come back after shock chlorinating the well?

Pennsylvania DEP calls shock chlorination a temporary measure that does not correct a well continuously exposed to contamination through its location or construction, and says a repeat positive means re-evaluating both. Penn State Extension found the procedure plus a sanitary cap kept water clear for a year in only 15% of contaminated wells. MDH adds that regrowth is common and may need a repeat dose or cleaning.

Does shock chlorination get rid of iron bacteria for good?

Not reliably. The Minnesota Department of Health says chlorine may not always get rid of iron bacteria, that more than one treatment is often needed, and that the well should be pumped until clear or physically cleaned before the chlorine goes in, at close to but not above 200 ppm. MDH does not usually recommend continuous chlorine injection, because it can hide other bacterial contamination and cause corrosion.