A coliform positive means something is getting into the well or the plumbing that should not be. Shock chlorination floods the whole system with strong chlorine, holds it for hours, then flushes it back out. It only sticks if you find and close the way in first, and only a clean retest afterwards proves the job worked.
Before you mix anything, find the way in
Chlorine treats the symptom. Look at the well cap and its gasket, the casing above grade, the sanitary seal, any pitless adapter, and whether surface water pools around the head after rain. Recent pump or pipe work is a classic cause. Nobody does this for you: the EPA notes that private wells are not federally regulated under the Safe Drinking Water Act, and that around 15 percent of the US population, over 43 million people, depend on them. The owner is the water system.
The shock itself, step by step
Work out the water volume in the casing, mix unscented household bleach with water in a clean bucket, and bypass or remove anything that chlorine will damage, including carbon filters, reverse osmosis units and softener resin. Pour the solution down the casing, then run a hose from an outside spigot back into the well and circulate until you can smell chlorine coming out of it. Open every fixture indoors, hot and cold, until each one smells of chlorine, then shut them and let the system stand overnight.
Flush it, retest, and know what a repeat positive means
Flush outdoors first, away from grass, planting and the septic field, then flush the indoor lines until the smell is gone; remember the water heater holds a tankful that has to be turned over too. Wait until no chlorine remains before sampling, and take a follow-up sample weeks later rather than trusting a single clean result. Repeat positives point at a defect you have not found or at a well that needs continuous disinfection equipment, and our well water testing guide covers what to sample for.
Questions people ask about how to shock a well for coliform
How long should the chlorine stay in the well?
Most guidance is to hold it overnight, several hours at minimum, so the solution reaches every part of the casing, the pressure tank and the household plumbing.
Can I drink the water while the well is shocked?
No. Use an alternative supply for drinking, cooking and brushing teeth until the chlorine has been flushed out and, ideally, until a follow-up sample comes back clean.
Will shocking the well cure a rotten egg smell for good?
It often clears sulfur odor for a while by knocking back the bacteria that create it, but the smell usually returns unless a permanent treatment system is fitted.