How to shock chlorinate a well: bleach amounts, steps and safety from state and federal procedures

To shock a well, you mix unscented household bleach with water, pour it into the well, and circulate it until chlorinated water reaches every tap in the house. You let it sit for hours, flush it out somewhere it won't harm a septic system, plants or a stream, then send a sample to a certified lab. Penn State Extension's definition is "introducing a high concentration of chlorine to the water for a short time." Every official procedure we read follows that sequence.

The numbers are another matter. Minnesota aims for 50 to 200 parts per million (ppm) of chlorine, Pennsylvania for 100 to 150, and Wisconsin starts at about 100 with stronger mixes up to 300. Contact times run from a two-hour minimum (Minnesota) to "preferably for 24 hours" (Wisconsin). The first retest comes anywhere from as soon as the chlorine is flushed out (Minnesota) to 10 to 14 days after disinfection (Penn State). This guide keeps each agency's numbers separate. Pick one procedure, ideally your state's, and follow it start to finish; a bleach amount from one agency and a contact time from another is not any agency's procedure.

When to shock a well

Shock chlorination is for a contamination event, not routine upkeep. Wisconsin's Department of Natural Resources (DNR) says "well disinfection is not intended to be used on a regular basis," and frequent chlorination can corrode metal well parts. The triggers the agencies name:

Keep your expectations modest. Penn State says that "When done properly, shock chlorination will kill all the bacteria existing in a well," but it doesn't close the gap they came in through. In a Penn State study, shock chlorination plus a sanitary well cap kept coliform out for one year in 15 percent of contaminated wells. It worked best with low counts (fewer than 10 colonies per 100 mL) and no E. coli. Pennsylvania's DEP calls its procedure "only a temporary measure," not a permanent fix for a well that keeps getting contaminated because of its location or construction.

When to call a licensed contractor instead

CDC says that after an emergency, wells "should be disinfected by a well or pump contractor, due to safety concerns." EPA's flood sheet says drilled, driven or bored wells "are best disinfected by a well or pump contractor." Wisconsin DNR recommends a licensed well professional; Minnesota offers its instructions or a licensed contractor. Call a pro, per the source named, if:

• You aren't confident around electricity, or the flow drops sharply while you run the hose (MDH).
• Electrical controls were under water, or the well or wiring looks damaged. MDH's flooded-well page sends these to a licensed well contractor; EPA says "If you notice any damage, call a professional before the disinfection process."
• The well is in a pit. MDH: "people have died from asphyxiation or electrocution in well pits." CDC: "Do not enter the well pit."
• The discharge pipe runs through a sanitary seal with no removable plug, or the well has a "packer-type" jet pump (MDH's flooded-well page).
• Floodwater covered the well or may have gotten in. MDH says a contractor should inspect it and, if necessary, clean it, because "Using your well pump to do this could ruin the pump."
• The pH has to be adjusted. Wisconsin: "The addition of pH controls can cause toxic fumes to be released."
• The job calls for acid, which sometimes happens with iron bacteria. MDH: "Only trained professionals should do an acid treatment." "Never mix acid and chlorine."
• The water smells of fuel or chemicals. CDC: that water "will not be made safe by boiling or disinfection." Call your health department.
• It's a spring. Penn State calls shocking one "difficult and rarely successful."

We found no official contractor price. Penn State calls shock chlorination "simple and inexpensive"; Wisconsin says a professional's experience "can go a long way and save you money and time."

Which bleach to use

Every agency allows plain unscented chlorine bleach with nothing added; Pennsylvania also allows stronger solutions and granules. Where they differ most is strength:

Bleach loses strength as it sits. Pennsylvania says to buy fresh "Since liquid laundry bleach weakens with time," and MDH wants it made within six months. Strength changes the dose: Pennsylvania calls for about 1½ quarts at 5 to 6%, ¾ quart at 10%, or ½ quart at 14%. Wisconsin warns that pool-section bleach "is often double the strength of laundry bleach." Match the strength your procedure assumes. None of these procedures mentions "splashless" bleach; all say unscented, no additives.

How much bleach to shock a well

The dose depends on how much water stands in the well, and that comes down to casing diameter and depth of water. MDH and Wisconsin both define the depth of water as total well depth minus the static water level. Wisconsin says to look on your well construction report; MDH points Minnesotans to the Minnesota Well Index. If you can't find the water depth, MDH says to use the well's total depth. Each table below comes from a single named source. Don't use one agency's table with another agency's steps.

Minnesota (MDH): bleach added to the 5-gallon pail

These amounts cover the well plus an average home's plumbing, including pipes, tanks and water heater. Use 1 cup less if you're doing only the well. MDH says you may need more for several buildings, lots of piping or storage, or "if you're disinfecting because your well was flooded, has nuisance bacteria, or is a larger diameter well." Test strips confirm at least 50 ppm; MDH's ceiling is 200.

CDC: bleach for a drilled or driven well

CDC calls these amounts "approximate," aimed at "a chlorine concentration of >100 mg/L," and its table also has columns for 10-, 24- and 36-inch casings. Bored and dug wells have their own CDC table. Note that CDC's table references are swapped: the drilled-well steps say "Use Table 2" and the dug-well steps say "Use Table 1," but the drilled-well table is marked Table 1 and the dug-well table Table 2.

Agencies that give a rule instead of a table

• Pennsylvania DEP: about 1½ quarts of 5 to 6% bleach in 6 to 10 gallons of water. That brings about 150 gallons to 100 to 150 ppm, which "corresponds to 100 feet of water in a six-inch diameter well." Scale it proportionately for other wells.
• Wisconsin DNR: for up to a 70-foot water column, 100 gallons of solution, commonly three new 33-gallon trash cans with 1 cup of 5.25% bleach in each. The mix's pH "should not exceed 7.5." For deeper 6-inch wells or other diameters, calculate the volume.
• Vermont: the state's online calculator, or "about ½ cup of bleach for every 10 feet of depth for a standard 6-inch diameter drilled well," aiming for 100 to 200 ppm.
• MDH, flooded wells: in 1 gallon of clean water, ¼ cup for casings 2 inches or less, 1 cup for 3 to 4 inches, 2 cups for 5 to 6 inches; "up to 4 times the amount" if the well was submerged.
• EPA, after a flood: one gallon of bleach poured into the casing.

Wisconsin's factors for how much water each foot of well holds:

One well, four answers

Take a 6-inch well with 100 feet of water. MDH's table gives 4 cups, and that covers the house plumbing too. CDC gives 5 cups. Pennsylvania gives about 1½ quarts. Wisconsin gives no figure, because the water column is past its 70-foot cutoff, so you calculate. They differ because each targets its own strength (50 to 200 ppm, over 100 mg/L, 100 to 150 ppm) with its own bleach (6 or 8.25%, 5 to 9%, 5 to 6%). Each recipe is matched to its own assumptions, and mixing them breaks the match.

More bleach is not better. MDH: "Do not use a bleach solution greater than 200 ppm as it will reduce the disinfection effectiveness." Its iron bacteria page gives the same ceiling. Wisconsin disagrees. If 100 ppm fails, it lists mixes of about 200 ppm (1½ quarts per 100 gallons) and about 300 ppm (2 quarts), but "do NOT exceed 100 ppm in arsenic areas." Wisconsin also says chlorine works best below pH 6 and stops working as a disinfectant above pH 8, and that chlorine products can push pH up, especially in hard water.

Minnesota's procedure, step by step

We walk through MDH's procedure because it's the most detailed one we found, and it's the one our coliform page summarizes. Read the full MDH page, or your state's, before you start. You'll need a long garden hose, a clean 5-gallon bucket, a funnel, a tarp, eye protection and rubber gloves, five gallons of water known to be bacteria-free, chlorine test strips and the bleach.

1. Isolate equipment. Put softeners, reverse osmosis units and anything else that can't take bleach on bypass. Remove all filters.
2. Cut the power at the breaker, lock it out if you can, and confirm with a voltmeter.
3. Open the well. With a standard cap, move the wires outside the casing so the connectors stay dry. With a sanitary seal, take out only the vent and leave the compression bolts in place.
4. Inspect the wire insulation and connectors, the casing and the cap. MDH says it's important to fix defects "so that surface water, insects, vermin, or other contaminants cannot enter the well." Our well inspection guide covers the rest.
5. Mix. "Add water from the water system into a clean 5-gallon pail until it is about three-fourths full," then add the bleach from the table.
6. Pour it in through a funnel, keeping it off the metal cap parts and wires, which it corrodes.
7. Recirculate. Turn the breaker on. "CAUTION: The wires in the well casing are now energized." Run the hose away from the well, septic system, landscaping and water until you smell bleach and the water runs clear. If flow drops sharply, cut the power and call a licensed well contractor. Then put the hose in the well and recirculate for about 30 minutes after you first smell bleach. If a strip reads under 50 ppm, add more solution and repeat. Turn the power off, rinse the well parts with fresh, unchlorinated water, replace the wires and cap, and turn the power back on.
8. Reach every fixture. Remove aerators. At each faucet, run the hot water until a strip shows at least 50 ppm (the whole water heater has to fill first), then the cold. Include toilets, appliances, outdoor faucets, yard hydrants and outbuildings.
9. Wait. Turn the pump off, put up signs so nobody uses the water, and wait "a minimum of two hours, preferably six hours, or overnight."
10. Flush. Turn the pump breaker back on, then run an outside hose. "Do not discharge chlorinated water into or near your septic system, landscaping, or any water bodies, because bleach solution will harm them." This can take "30 minutes to 24 hours or more." Flush the water heater and every faucet until a strip that reads down to 0 ppm shows no chlorine.
11. Test. Disinfect and reconnect treatment equipment following the manufacturer's directions. Then confirm with an accredited lab that the water is negative for total coliform before you drink it or cook with it. Install new filters only once the lab confirms it.

Where the other procedures differ

Follow one row, not a mix of rows.

Why keep it out of the septic system? Wisconsin says chlorine "may kill the bacteria that biologically break down the waste," and the volume "can also hydraulically overload the system." Pennsylvania notes fish are "very sensitive to very low levels of chlorine."

How long after shocking a well can you drink the water?

No agency gives a number of hours. Most of them say to wait for a lab result. MDH wants a negative total coliform test "before you use it for drinking or cooking," and its flood page says "You must continue to boil your water until the laboratory reports that the water is safe." Pennsylvania: "Until tested and found potable, bring the water to a rolling boil for at least one minute." CDC adds: "At elevations above 6,500 feet, boil for 3 minutes." CDC and Vermont also say to boil or use another safe source until the test comes back clean. If your water also has high nitrate or arsenic, don't boil it: MDH warns boiling "may not be the safest option," and Vermont says boiling may concentrate contaminants like nitrate. Use bottled water instead. EPA's flood sheet is the outlier. After flushing it says "you can now use the water," and only then says to test in 7 to 10 days.

While chlorine is in the pipes, EPA says not to "drink, cook, bathe or wash with this water." If someone in the house is pregnant, an infant or ill, ask your doctor or health department what applies. When it's time to test, the lab finder filtered for bacteria lists certified labs by state, and our testing guide shows how to take a sample you can rely on.

Shocking a well after a flood

After a flood, safety and damage checks come before any bleach. EPA's flood sheet (updated October 2024) starts with "DO NOT TURN ON THE PUMP," because of the danger of electrical shock, and "DO NOT WASH WITH WELL WATER." Once the pump and wiring have dried, EPA says to leave them off until a qualified electrician, well contractor or pump contractor has checked the wiring, and that "Rubber boots and gloves are not adequate protection from electric shock." EPA adds that wells "more than 10 years old or less than 50 feet deep are likely to be contaminated, even if there is no apparent damage."

MDH sorts flooded wells three ways. If floodwater reached the well or covered the casing top, "assume your well is contaminated." If it covered the well or may have gotten in, a licensed contractor should inspect it, clean it if needed, and disinfect it. If it reached the well but you're confident it didn't get in, you can disinfect it yourself or hire a contractor. If floodwater came within 50 feet but never reached the well, "you may still want to have your water tested as a precaution," and you don't need to disinfect first.

Bleach only deals with germs. EPA: "Well disinfection will not provide protection from pesticides, heavy metals and other types of non-biological contamination." It also warns that a well "may not be a safe source of water for many months after the flood," and that septic systems "should not be used immediately after floods." Our well maintenance guide covers steps to take before a flood.

Iron bacteria and sulfur smells

MDH says chlorine "may not always get rid of iron bacteria. You may have to treat with chlorine more than once." The well "should be pumped until clear or physically cleaned" before the chlorine goes in, and the strength should be "close to but not greater than 200 parts per million." Wisconsin says salt and a chimney brush "may help dislodge" bacterial slime layers.

On what comes next, the two states disagree. MDH "does not usually recommend" continuous chlorine injection, because it "may hide other bacterial contamination and cause corrosion problems." Wisconsin's iron brochure says to "consider following with continuous chlorination." Our iron guide covers both views.

If the retest comes back positive

• Repeat. MDH: "If total coliform is detected, repeat the disinfection procedure." It is "not unusual" to disinfect several times. CDC says to repeat, then contact your health department if bacteria persist.
• Clean the well. MDH: "the well may need to be cleaned as well as disinfected." Contact a licensed well contractor.
• Look at the well itself. Pennsylvania: "the construction or location of the water supply should be re-evaluated." MDH's handbook: "If multiple thorough well disinfections do not solve the problem, the well probably needs to be repaired, upgraded, or replaced."
• Plan for continuous treatment. Penn State: if bacteria return, "a continuous disinfection treatment system will be necessary." Our treatment guide compares the options.

When "all disinfection attempts fail," MDH lists the likely causes: small animals or debris in the well, cracks in the upper casing, or soil and rock that aren't filtering the water well enough. The results checker lines up your numbers, and the lab finder lists certified labs for the retest.

Questions people ask