Well shock chlorination calculator: bleach for a drilled well

A positive coliform test, a flood that reached the wellhead, or a new pump dropped down the casing: any of these usually ends with the same job, a shock chlorination. The first question is always how much bleach. This gauge works it out from the size of the well and the strength of the bleach on your shelf.

Enter the Well casing inside diameter in inches; the driller's records usually give it, or you can measure across the inside of the casing with the cap off. For Depth of standing water in the well, take the well's total depth and subtract the depth down to the water, both in feet. The driller's log or well completion report usually lists them. Pick a Target chlorine of 50, 100 or 200 ppm to match the guide you are following, and type the Bleach strength from the label, the percent of sodium hypochlorite.

How it works

A drilled well is a tall cylinder of water. Each foot of casing holds about 0.0408 times the diameter squared, in gallons: a cylinder one foot tall, divided by the 231 cubic inches in a US gallon. Multiply by the feet of standing water and you have the gallons in the well. Then the bleach: 1% is 10,000 parts per million, so dividing the target ppm by the bleach strength times 10,000 tells you what share of the well's water must be bleach.

bleach gallons = well gallons × ppm ÷ (bleach % × 10,000)

The readout, titled "Bleach for the water in the well", gives cups of bleach as the big figure. A banded scale from 0 to 2 gallons of bleach runs under it, with three bands: under a quart of bleach, between a quart and a gallon, and more than a gallon. The lines below give the water standing in the well with the gallons per foot, the bleach in gallons, and the bleach in fluid ounces.

Worked example

The gauge opens on a 6 inch casing with 100 feet of standing water, a 100 ppm target and 6% bleach. Each foot of 6 inch casing holds 0.0408 × 36, about 1.47 gallons, so 100 feet holds about 147 gallons. Then 147 × 100 ÷ (6 × 10,000) is about 0.24 gallons of bleach. The readout shows:

• Bleach for the water in the well: 3.9 cups of bleach, in the band under a quart of bleach
• Water standing in the well: 147 gal (1.47 gal per foot)
• Bleach in gallons: 0.24
• Bleach in fluid ounces: 31

Try the same well with 8.25% bleach and the dose drops to about 2.8 cups. Double the target to 200 ppm and it doubles too. The water in the well is the one number that never changes, so it is worth getting right.

Doing it safely

The number is the easy part. The steps around it, as extension service well guides commonly lay them out, matter more.

1. Use plain, unscented household bleach. Labels typically run about 5 to 8.25% sodium hypochlorite. Never use scented or splash-less bleach, and never mix bleach with other cleaners; bleach and ammonia or acids give off toxic gas.
2. Protect yourself and your equipment. Wear eye protection and gloves. Many guides suggest bypassing water softeners and carbon filters during the treatment.
3. Add the bleach and mix it. Pour it into the well, then run a garden hose from an outside tap back down the casing until you smell chlorine at the hose, which mixes the water and wets the casing walls.
4. Draw it through the house. Open each tap, hot and cold, one at a time, and run it until it smells of chlorine. The gauge doses only the water in the well; running the taps pulls chlorinated water into the tank, the heater and the pipes.
5. Wait. Leave the system unused for 12 to 24 hours.
6. Flush it out. Run an outside hose until the chlorine smell is gone, sending the water onto the ground away from the septic field, gardens and streams. Don't drain it into the septic system.
7. Retest. Have the water tested for coliform bacteria about 7 to 14 days later, once no chlorine remains.

When to test, and what shock chlorination won't fix

Test a private well for coliform bacteria at least once a year, along with nitrate, and test again after floods, repairs or any change in taste, color or smell. When storms dump heavy rain on steep ground, small streams can rise over a low wellhead in hours, as our explainer on flash floods in steep terrain describes. If coliform keeps coming back after a shock treatment, bacteria are probably getting in somewhere, through a loose cap, a cracked casing or surface water draining toward the well, and that needs a well contractor's eye rather than more bleach.

Chlorine does nothing for nitrate. The EPA's drinking water standard is 10 mg/L as nitrate-nitrogen, about 45 mg/L as nitrate, and the main risk is to infants under 6 months. Boiling concentrates nitrate instead of removing it; treatment is reverse osmosis, anion exchange or distillation.

A rotten-egg smell is hydrogen sulfide, often produced by sulfate-reducing bacteria. If you smell it only in hot water, the water heater's magnesium anode is the usual suspect; in hot and cold alike, it points to the well or the groundwater. Radon can travel in well water too, and our radon unit converter sorts out the units on a radon report. For more on reading lab results against standards, see how to read a water quality report.

Sources

• Casing volume from the geometry of a cylinder and the US gallon of 231 cubic inches, which gives 0.0408 × diameter squared gallons per foot.
• Parts per million: 1% equals 10,000 ppm.
• Target doses, contact time, flushing and retesting from university extension service well disinfection guides.
• Testing schedules and the nitrate standard from the U.S. EPA's private well pages.

Questions

When can I drink the water again?

Not until the chlorine has been flushed out and a coliform test taken about 7 to 14 days later, once no chlorine remains, comes back clean. Until then, use another source of drinking water. If the retest still shows coliform, find out how the bacteria are getting in before shocking again.

Is a stronger dose better?

Not necessarily. Extension guides commonly name a target between 50 and 200 ppm, and the gauge offers those three settings. A heavier dose takes longer to flush out and does nothing about a cracked casing or a loose cap.

Can I use scented, splash-less or pool chlorine products?

Scented and splash-less bleach are out: their additives don't belong in a drinking water well. The gauge is set up for plain liquid household bleach, so any other chlorine product needs its own instructions and its own arithmetic.

Does the gauge count the water in my pressure tank and pipes?

No. It counts only the water standing in the well. Running each tap until it smells of chlorine is what carries the treatment through the tank, the water heater and the plumbing.