How to shock chlorinate a well

If you buy something through our links, we may earn a commission. We'd recommend the same thing either way, and we'll tell you when a free option exists. How we make money

Shocking kills bacteria in the well and pipes.

Shock chlorination puts a strong dose of chlorine into the well and plumbing for a few hours, then flushes it out. Penn State Extension lists 3 times to do it: after a new well is built, after the well or pump is worked on, and after a positive coliform bacteria test. The Minnesota Department of Health adds a 4th: any time a well floods.

It won't fix the reason bacteria got in. Penn State found it worked best in wells with low counts (fewer than 10 colonies per 100 mL) and no E. coli. In one Penn State study, shock chlorination plus a new sanitary well cap kept bacteria out for a full year in only 15 percent of wells. If the well tests positive again, Pennsylvania DEP says the well's construction or location needs a closer look.

You can hire it out or do it yourself.

The National Ground Water Association recommends a licensed well professional who uses chlorine products certified for drinking water. It doesn't recommend household bleach, partly because bleach loses strength on the shelf.

State health agencies in Minnesota and Pennsylvania publish bleach instructions for owners who want to do it themselves. The steps below follow theirs. If your state has its own, use it. Call a contractor if the well is in a pit, if wiring looks damaged, or if you can't get the cap off safely.

Gather what you need.

• A new, unopened jug of plain unscented bleach, 6% or 8.25% sodium hypochlorite, made within the last 6 months. Minnesota says no additives and no pool or hot tub chemicals.
• Rubber gloves and eye protection
• A clean bucket and a funnel
• A garden hose that reaches from an outside tap to the well
• Chlorine test strips, if you want to check the dose
• Bottled water for drinking for a day or 2

Work out how much water is in the well.

Multiply the feet of water standing in the well by the gallons per foot for your casing size. Feet of water is the total well depth minus the depth to water. Both are often on the well record.

These come from the volume of a round pipe. They match Pennsylvania DEP's figure that 100 feet of water in a 6-inch well is about 150 gallons.

Use enough bleach to reach 50 to 200 mg/L.

Minnesota's target is 50 to 200 parts per million (the same as mg/L) of chlorine, and it says not to go over 200. One gallon of 6% bleach in 1,000 gallons of water gives about 60 mg/L.

Pennsylvania DEP says about 1½ quarts of 5% to 6% bleach treats about 150 gallons of water at 100 to 150 mg/L.

Example: a 6-inch well 200 feet deep with water at 80 feet has 120 feet of water. 120 × 1.5 = 180 gallons. About 1½ to 2 quarts (6 to 8 cups) of 6% bleach puts that between roughly 125 and 165 mg/L.

Our shock calculator does this math for you.

Follow these steps in order.

1. Switch any water softener, filter or UV system to bypass. The EPA says to do this before chlorine goes through the house.
2. Turn off power to the pump, then remove the well cap or the vent plug.
3. Mix the bleach into a bucket of water, then pour it into the casing with a funnel. Pennsylvania DEP says diluting it first spreads the chlorine through the well and keeps strong bleach off the pump's metal parts. Keep it off the wires, too.
4. Turn the power back on. Run a hose from an outside tap back down into the well until you smell chlorine at the hose. Minnesota keeps it running about 30 more minutes after that. This mixes the chlorine and rinses the inside of the casing.
5. Open every cold water faucet, inside and outside, one at a time, until you smell chlorine. Then close it. Minnesota also runs each hot tap until chlorine shows up, so the water heater gets disinfected too.
6. Put the cap back on. Leave the chlorine in the system at least 6 hours, and overnight is better. The EPA says not to drink, cook, bathe or wash with the water during this time.
7. Flush it out through an outside hose until the chlorine smell is gone. Minnesota says this can take 30 minutes to 24 hours or more, and a test strip that reads down to 0 tells you when it's done. Run it somewhere away from lawns, gardens, streams and the septic system.
8. Retest for bacteria. Penn State says about 10 to 14 days after disinfecting. If that test is clean, test again in 2 to 3 months.

Keep yourself and the septic system out of harm's way.

Never mix bleach with other chemicals. Minnesota warns they can form harmful gases. Don't send the flushed water into the septic tank: Minnesota says chlorinated water harms septic systems, landscaping and water bodies. Its handbook says the small amount left in the house plumbing after the well is flushed can go down the drain.

If the well keeps testing positive after several tries, have a contractor inspect it. The Minnesota Department of Health says the next step is to find and remove the source and check the well for defects. Penn State says that if bacteria keep coming back, you'll need continuous treatment such as UV.

Questions people ask