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.