Shock chlorination is a one-time disinfection of a well and the plumbing it feeds, used after a positive bacteria test, after work on the well or pump, after flooding, or when a sulfur smell suggests bacterial activity. Done properly it means calculating a dose for the actual water volume, getting the chlorine everywhere, and holding it there long enough to work.
Before you start, find out what you are treating
Private wells are not federally regulated under the Safe Drinking Water Act, so a well owner is the only person testing. Shock chlorination is a response to a result, not a routine, and it works best when you know what you found. Test for coliform bacteria first, and note the well depth, casing diameter and static water level, because those figures determine the water volume you are dosing. Your well log or driller's report has them if you do not.
Calculating and mixing the dose
The dose is based on the volume of water standing in the casing plus the plumbing, which is why the casing diameter and water depth matter. Unscented household bleach is the usual chlorine source, and it is mixed with water in a clean bucket rather than poured neat down the casing, so it disperses instead of sinking as a slug. Never mix bleach with any other cleaner. Work outside, wear eye protection and gloves, and turn off any equipment that chlorine will damage.
Circulating it everywhere it needs to go
Pouring chlorine into the casing treats the well but not the system. After adding the solution, connect a hose to an outside tap and run water back down the casing to wash the walls and mix the solution thoroughly. Then go through the house opening every fixture, hot and cold, one at a time until you smell chlorine at each, and close it. Do not forget outside taps, the hot water tank, ice makers and any rarely used bathroom, because untreated pipe recontaminates the rest.
Contact time, then flushing out
Leave the chlorine in the system without drawing water, commonly overnight and often longer, since disinfection depends on time as much as concentration. Then flush the chlorine out, starting at an outside tap so the bulk of it goes somewhere that will not harm a septic system, plants or a stream, and finish inside until the smell has gone from every fixture. Take care with a septic system, which is a biological process a large chlorine slug will disrupt.
Retesting, and what a repeat failure means
Wait until the chlorine has completely cleared, then retest for coliform bacteria, and test again some weeks later. One clean result immediately after shocking proves only that chlorine kills bacteria. If contamination returns, the well itself is the problem: a cracked casing, a failed sanitary seal, a poor cap, surface water entering, or a septic system too close. In that case the repair comes first, and continuous treatment such as ultraviolet or chlorination follows. Our well shock treatment page covers hiring it out.
Questions people ask about how to shock a well
How long should chlorine stay in the well?
Commonly overnight at minimum, and longer for a heavier contamination, because contact time matters as much as dose. Do not draw water during the hold or you pull untreated water in behind it.
Can I drink the water during shock chlorination?
No. Arrange alternative drinking and cooking water for the whole process, and do not use the water for bathing while chlorine is in the system. Flush completely before returning to normal use.
Will shocking a well fix a rotten egg smell permanently?
Sometimes, if sulfate reducing bacteria were the cause. If the smell returns within weeks, the bacteria are recolonizing or the sulfide is in the source water, and continuous treatment is needed.
Is it safe to flush chlorinated water into a septic system?
A large slug can harm the bacteria a septic system relies on, so flush the bulk of it through an outside tap onto ground away from plants and watercourses instead.