Shock chlorinating a well

Best for

Wells with a positive coliform result (without E. coli or with a fix underway), recent pump or well work, a new well, or iron or sulfur bacteria.

Wrong fit

Wells with an open defect such as a cracked casing or missing cap, or persistent E. coli. Fix the well first, and call your health department. Shocking alone won't hold.

Tradeoff

It's cheap, but you lose normal water use for most of a day, and bacteria often come back if the cause isn't fixed.

When to shock chlorinate

Penn State Extension lists three common reasons: after building a new well, after work on an existing well or installing a new pump, and after a positive coliform bacteria test. CDC adds wells that were flooded. The Minnesota Department of Health (MDH) also uses it for iron bacteria, which cause slime, odor and staining.

Shock chlorination is a one-time disinfection. It kills bacteria in the well and plumbing at that moment. It does not fix the way bacteria got in. Penn State found that shock chlorination plus a sanitary well cap removed bacteria for one year in only about 15% of wells studied. It worked best where counts were below 10 colonies per 100 mL and no E. coli was found.

If E. coli was found, use bottled water or boiled water for drinking and call your state health department. CDC advises a rolling boil for 1 minute until the water tests safe.

Do it yourself or hire a pro?

CDC says wells should be disinfected by a well or pump contractor because of safety concerns, and Penn State recommends a well professional. MDH says you can do it yourself, but hire a licensed well contractor if you aren’t confident working safely around electricity.

Hire a pro if any of these apply:

• You’d need to open a well pit or confined space. CDC says not to enter a well pit, because gases can build up.
• The well cap, casing or wiring looks damaged.
• The well was flooded.
• You’ve already shocked once and bacteria came back.

Safety first

From MDH and CDC instructions:

• Eye and skin protection. Wear goggles or a face shield and rubber gloves.
• Never mix bleach with other chemicals. They can form harmful gases.
• Ventilate. Gases can collect in well pits and displace oxygen.
• Electricity. Turn off power to the pump before opening the well, and keep wires dry. Water and electricity can be deadly.
• Keep children and pets away, and post signs so nobody uses the water during treatment.
• Have drinking water on hand for at least a day.

What you need

• Unscented household chlorine bleach. MDH specifies a fresh, unopened container with no additives, made within six months, labeled 6% or 8.25% sodium hypochlorite. CDC’s guidance allows 5% to 9% unscented bleach.
• Not pool or hot tub chlorine. MDH says those often contain algicides and fungicides.
• A clean 5-gallon bucket, a funnel, a garden hose, chlorine test strips, and a wrench for the well cap.
• Your well’s total depth and static water level. MDH says the water in the well equals total depth minus static water level.

How much bleach

MDH’s chart gives cups of bleach by well diameter and feet of water in the well:

MDH’s notes on the chart:

• Mix the bleach into a 5-gallon pail about three-quarters full of water.
• Use 1 cup less if you’re disinfecting only the well and not the plumbing.
• Use more for multiple buildings, large piping or storage, flooding, nuisance bacteria, or a larger-diameter well.
• Aim for 50 to 200 ppm chlorine. Don’t exceed 200 ppm. MDH gives the same 200 ppm ceiling for iron bacteria.

If your well is a large-diameter dug or bored well, or not on this chart, use your state’s chart or call a contractor. CDC publishes separate tables for drilled and bored wells.

Step-by-step

These steps follow MDH’s procedure, with CDC’s contact time noted.

1. Protect treatment equipment. Put softeners, RO and similar systems on bypass and remove filter cartridges. Penn State notes high chlorine can damage some treatment equipment, so check your manuals.
2. Turn off pump power at the breaker and confirm it’s off.
3. Open the well. Remove the cap or vent, keeping wires dry.
4. Inspect. Look for damaged wires, casing or cap. Fix defects so contaminants can’t get back in.
5. Add the bleach solution through a funnel, keeping it off wires and metal parts.
6. Recirculate. Restore power. Run a hose away from the well, septic system, plants and water bodies until you smell bleach or strips show chlorine. Then put the hose back into the well and recirculate for about 30 minutes. Strips should read at least 50 ppm. If not, add more bleach and repeat.
7. Rinse and close up. Cut power, rinse well parts with unchlorinated water, replace the cap, restore power.
8. Draw chlorine through the house. Remove faucet aerators. Run hot water until strips show at least 50 ppm, then cold, then every faucet, shower, tub, toilet, appliance and outdoor tap.
9. Wait. MDH says at least two hours, preferably six hours or overnight. CDC’s guidance says at least 12 hours. Longer contact is the safer choice when bacteria were found.
10. Flush. Run water through an outdoor hose, not near the septic system, landscaping or streams. MDH says flushing can take 30 minutes to 24 hours or longer. CDC suggests draining onto a plant-free area such as a driveway. Flush the water heater, then run every tap until strips read 0 ppm.
11. Clean and reconnect equipment. Disinfect treatment devices per their instructions, then take them off bypass.

Don’t flush into the septic system. Large amounts of chlorinated water can harm it. That’s why both MDH and CDC say to discharge outdoors.

Planning the day

Using MDH’s and CDC’s times, a typical schedule looks like this:

Start in the afternoon so the contact time runs overnight. Fill jugs of drinking water and plan meals and showers around it. Expect the first water you flush to smell strongly of chlorine and to look discolored if your water carries iron.

Retesting

This is where most people cut corners. Shock chlorination isn’t done until a clean test says so.

Test for total coliform and E. coli at a certified lab. MDH says to replace filters only after a lab confirms the water is coliform-free. See how to test well water for sampling steps.

If bacteria come back

• Repeat once, after checking the cap, casing and grading.
• Have a licensed well contractor inspect the well. MDH says to call one if repeated disinfection fails.
• Plan continuous disinfection. Penn State recommends it when bacteria return. Options are a UV system or chemical injection.
• Iron bacteria may need physical cleaning of the well and pump by a contractor, according to MDH.
• Sulfur smell back within weeks? Penn State says sulfur-reducing bacteria and the rotten-egg odor often return quickly after shocking. Continuous treatment may be needed.

More on bacteria in well water.