Shock chlorinate for sulfur bacteria

Best for

Wells where the smell fades after running, started suddenly, came after repairs or a period of low use, or comes with slime.

Wrong fit

A steady smell in hot and cold water that never fades means H2S in the groundwater, which comes straight back after shocking. Hot-water-only smell is the water heater.

Tradeoff

It costs little more than bleach and a day without water, but sulfur bacteria can be hard to kill for good, so you may need to repeat it or move to continuous treatment.

When shock chlorination is the right first step

Sulfur-reducing bacteria live in low-oxygen water in wells, pressure tanks, pipes and softeners, and they make hydrogen sulfide from sulfate. Shock chlorination kills them. The Minnesota Department of Health recommends it when the smell comes from the well, plumbing or softener rather than the groundwater.

Signs that bacteria are the cause:

• The smell is strong when a tap first opens and fades after a few minutes of running. Minnesota links this pattern to sulfur bacteria in the well or plumbing.
• The smell started suddenly, or after the water sat unused. The Wisconsin DNR notes odors can be more noticeable after low water use.
• It began after well or pump work. UGA Extension lists opening the well for repair or maintenance as a reason to shock chlorinate.
• There’s slime. UGA lists “slime (biofilm) or odor” as signs of iron or sulfur-reducing bacteria, and Minnesota says sulfur bacteria slime can be white, grey, black or reddish brown.

If the smell is steady in hot and cold water and never fades, the gas is probably in the groundwater. Shocking won’t hurt, but the smell will return fast, and you’ll need treatment sized to a hydrogen sulfide test. If only the hot water smells, see hot water smells like rotten eggs.

Before you start

Get the right bleach. Minnesota says to use fresh, unscented household bleach with no additives, made within the last six months, with 6 or 8.25 percent sodium hypochlorite on the label. UGA says at least 5 percent. Minnesota warns not to use pool or hot tub chemicals, which usually contain algicides and fungicides.

Get chlorine test strips that read up to 200 ppm and down to zero.

Bypass treatment equipment. Minnesota says to bypass softeners, reverse osmosis units and other devices that can’t tolerate bleach, and to remove their filters. UGA says to disconnect carbon filters and RO units, and to check with the maker before chlorinating through a softener, iron filter or sand filter. Penn State Extension notes high chlorine can damage or exhaust some treatment equipment.

Plan for no water. You can’t use the water during the contact time, which may be overnight or a full day.

Plan the flush. You’ll be running a lot of chlorinated water. It should go outdoors, away from the septic system, landscaping, wells and water bodies.

Stay safe. Minnesota says to ventilate the area, wear goggles or a face shield and rubber gloves, and never mix bleach with other chemicals. Turning the pump breaker on and off and opening the well cap involves live wiring. If you aren’t confident around electricity, Minnesota says to hire a licensed well contractor.

How much bleach

Two well-documented methods use different approaches. Use one, not both.

UGA method (by well volume). Find how many feet of water stand in the casing (well depth minus the static water level, from your well log). Multiply by gallons per foot:

Then use 3 pints of bleach per 100 gallons of water in the well, plus 3 pints for the plumbing. Example: a 6-inch well with 150 feet of standing water holds about 220 gallons, so roughly 6.6 pints for the well plus 3 for the plumbing, or about 10 pints (1.25 gallons). If you don’t know the water depth, UGA says to use twice the amount for 150 feet of water in the casing.

Minnesota method (by concentration). Mix bleach in a 5-gallon pail about three-quarters full of water, pour it in, and recirculate until strips read at least 50 ppm. Minnesota’s starting doses in cups:

Minnesota says to increase the amount when nuisance bacteria are present, aiming for 50 to 200 ppm, and not to go above 200 ppm because stronger solutions reduce disinfection effectiveness. Its iron bacteria guidance says close to, but not above, 200 ppm. For sulfur bacteria, aim for the upper part of that range.

Step by step

1. Pump out sediment. If the water is cloudy or the well hasn’t been used, run water outdoors until it clears.
2. Open the well. Turn off the pump breaker, remove the cap, and scrub accessible parts of the casing with a strong bleach solution. UGA suggests 1/2 gallon of bleach in 5 gallons of water for this.
3. Add the bleach using one of the methods above.
4. Recirculate. Turn the pump back on. Run a hose from an outdoor spigot back into the well to mix and wash down the casing. UGA suggests about 15 minutes; Minnesota says about 30 minutes after you first smell bleach, checking that strips read at least 50 ppm.
5. Close the well. Turn off the pump, rinse the well parts, reattach the cap, and turn the pump back on.
6. Pull chlorine through the house. Remove faucet aerators. Run every hot and cold tap, shower, tub and toilet until you smell strong chlorine or a strip reads at least 50 ppm. Hot water takes longer, because the whole heater tank has to fill with chlorinated water. Don’t forget outdoor spigots and outbuildings.
7. Wait. See contact time below.

Contact time: sources differ

For sulfur bacteria, which are hard to kill once established, use the longer time.

Flushing

• Run water first from an outdoor hose, away from the septic system, landscaping and water bodies, until strips show no chlorine. Minnesota says flushing can take 30 minutes to 24 hours or more.
• UGA says not to run more than 100 gallons of chlorinated water into a septic system.
• Then run each indoor tap until the chlorine smell is gone, and flush the water heater.
• Reconnect any equipment you bypassed. Disinfect softeners and filters following the maker’s instructions.

Afterward: retest and watch the smell

Test for total coliform bacteria once the chlorine is gone. Sources differ on timing:

• Minnesota: after the chlorine is fully gone, and again two to four weeks later.
• Penn State: about 10 to 14 days after disinfection, then again in two to three months.
• UGA: after 5 to 7 days of normal use.

Then pay attention to the smell. A smell that’s gone for months means the bacteria were the cause. A smell that’s back within days points to H2S in the groundwater, or to the water heater.

If the smell comes back in a few weeks

Sulfur bacteria are persistent. Minnesota says they can be hard to remove once established, and that scrubbing the casing, special chemicals or agitating the water may be needed first, especially when iron bacteria are also present. That work is for a licensed well contractor. Virginia Cooperative Extension says control may be temporary and routine shock chlorination may be needed. The Wisconsin DNR lists periodic shock chlorination as the fix for sulfur bacteria.

If you’re shocking every few months, look at continuous treatment, usually chlorine injection. If iron is also present, read sulfur and iron in well water.