Shock chlorinating a well: when it helps and when it won't

Shock chlorination means putting a strong dose of chlorine into a well and the household plumbing to kill bacteria, then flushing it out. It's cheap and often effective after a one-time contamination event. It isn't a cure for a well that keeps getting contaminated.

How common bacteria problems are

Penn State Extension calls bacterial contamination one of the most common water quality problems in private wells and springs. It cites a survey of 450 private wells in Pennsylvania in which about 35% contained coliform bacteria, and notes that springs are even more susceptible.

When it's recommended

Penn State lists three situations:

1. after a new well is built (many drillers do this as standard practice);
2. after work on an existing well, such as installing a new submersible pump;
3. after a positive test for coliform bacteria.

It also points out that some positive tests come from a one-time event, such as a new pump or runoff from an unusually heavy rain. Shock chlorination is designed for exactly those cases.

The basic process

Detailed dosing depends on the well's depth and diameter and the chlorine product, so Penn State refers owners to state instructions (in Pennsylvania, DEP's Disinfection of Home Wells and Springs) and to the National Ground Water Association's well disinfection guidance. The general steps are:

1. Bypass or turn off water treatment equipment that high chlorine could damage, such as some softeners, filters, and carbon units. Check the manuals or ask the installer.
2. Add the calculated amount of chlorine to the well and recirculate so it mixes and coats the casing.
3. Run each tap, hot and cold, until you smell chlorine, then shut it off.
4. Let the chlorinated water sit for the time the instructions specify, often several hours to overnight.
5. Flush the well until the chlorine smell is gone. Run it somewhere that won't harm plants, streams, or ponds, and don't send large amounts into the septic system.

Many owners hire a well contractor to do this. The NGWA offers a contractor search.

Retest, retest

Penn State advises retesting for total coliform about 10 to 14 days after disinfection. If that test is clean, test again two to three months later. If bacteria come back in either test, the well needs continuous disinfection, such as an ultraviolet or chlorination system, and you should look for how the contamination is getting in.

Penn State's own research found that shock chlorination plus a sanitary well cap cleared bacteria for a full year in only 15% of contaminated wells. It worked best where coliform counts were low (fewer than 10 colonies per 100 mL) and E. coli wasn't present. So treat it as a first step, not a guaranteed fix.

Find the entry point

Check for a loose or unvented cap, a cracked casing, a wellhead in a low spot where water pools, or a nearby septic problem. EPA notes that surfacing wastewater can reach a well through an unsecured cap or casing cracks. See protecting your well from your septic system.

Meanwhile, safe water

Until tests come back clean, use bottled water or disinfect drinking water. EPA's emergency guidance says to bring water to a rolling boil for at least one minute, or three minutes above 5,000 feet. If you can't boil, add unscented household bleach: about 8 drops (1/8 teaspoon) of 6% bleach, or 6 drops of 8.25% bleach, per gallon. Stir, and wait 30 minutes before using.

Before you start: safety

Chlorine at shock-treatment strength is hazardous. Take precautions:

• Wear eye protection, rubber gloves and old clothes. Concentrated chlorine splashes can burn skin and eyes.
• Never mix chlorine with ammonia or acids, including some cleaning products. The combination releases toxic gas.
• Ventilate well pits and enclosed spaces, and never climb into a well pit without proper training and equipment.
• Turn off power to the pump before opening an electrical well cap.
• Use plain, unscented household bleach or a chlorine product intended for water disinfection. Scented, splash-less or "color-safe" bleaches aren't suitable.

Protecting your septic system during flushing

Flushing heavily chlorinated water into a septic system can harm the tank's bacteria and overload the drainfield with water. To limit the effect:

• Run the outside spigot or a hose to flush most of the chlorinated water, directed away from the drainfield, gardens, streams and ponds.
• Flush indoor taps only briefly, just until the chlorine smell clears in each line.
• Spread the flushing over time if possible, rather than running thousands of gallons at once.

When shock chlorination isn't enough

If bacteria keep coming back after one or two treatments, look for a persistent source:

• Cracked or short casing, or a casing that doesn't extend above the ground.
• A missing or loose cap, or a cap without a screened vent.
• Surface water pooling around the wellhead.
• A nearby failing septic system, yours or a neighbor's.
• A shallow dug or driven well that draws from near-surface water. EPA notes these wells are more easily contaminated.

Options include repairing the wellhead, extending the casing, regrading around the well, drilling a new well, or installing continuous disinfection such as ultraviolet treatment or a chlorinator.

Test schedule after treatment

CDC recommends testing well water at least once a year for total coliform bacteria, nitrate, total dissolved solids and pH, and after any repair or replacement of part of the well system. After shock chlorination:

1. Wait until the chlorine is fully flushed before collecting a sample. Residual chlorine can produce a false negative.
2. Use a state-certified lab and its sterile bottle.
3. Retest again later to confirm the result holds.

Key takeaways

• Shock chlorination is for one-time contamination, not a chronic problem.
• Handle chlorine carefully, and never mix it with ammonia or acids.
• Keep most of the flushed water out of the septic system and away from surface water.
• Retest after treatment, and find the entry point if bacteria return.