Most folks think shock chlorinating a well is like shocking a swimming pool—dump in chlorine until you hit some magic number, then you’re done. That idea comes from pool chemistry, where you add enough chlorine to reach ten times the chloramine level in parts per million. That works for a pool. It does not work for a well.
A well is not a pool. A pool holds a known volume in a concrete box. A well draws from an aquifer through rock and soil, and the water sitting in the casing is only a small part of what you’re treating. The fix is not a single dose calculated off a chart. The fix is contact time and concentration, held long enough to kill what’s growing where you can’t see it.
Getting Chlorine Into The System
The label will say what percentage it is, and that matters because you’re calculating how much to add based on the volume of water sitting in your well casing and your plumbing. The depth of the well, the diameter of the casing, and the static water level all factor in, so check gallons of water per foot of well casing by casing diameter before you work out the dose. If you don’t have those figures, contact the well driller before mixing anything. He drilled it, and he has the log.
| Water in well (feet) | 2-inch diameter (cups) | 4-inch diameter (cups) | 6-inch diameter (cups) |
|---|---|---|---|
| 10 | 2 | 2 | 2 |
| 50 | 2 | 2 | 3 |
| 100 | 2 | 3 | 4 |
| 300 | 3 | 4 | 10 |
The process is simple but it takes most of a day. You combine the chlorine with water in a bucket, pour it into the well, then run a hose from an exterior spigot back into the well to recirculate it. You want the chlorinated water moving through the whole column, not just sitting on top. After you’ve circulated for a while, you open every tap in the house, one at a time, until you smell chlorine at each one. That tells you the treated water has reached every fixture. Then you turn everything off and let it rest.
Contact time is where people cut corners. The chlorine needs to stay in the system long enough to do the work. I leave it overnight when I can. The water is not usable during this time—not for drinking, not for cooking, not for bathing. Plan ahead. Fill pots and the bathtub before you start, or do it when you can stay somewhere else.
Flushing And Testing After Treatment
After the contact time is up, you flush. Connect a hose to an outside spigot and run it until you no longer smell chlorine. That can take a while. You’re removing all the treated water from the well and replacing it with fresh water from the aquifer. Once the outside tap runs clear, go inside and flush each fixture until the chlorine odor disappears there as well.
Then you wait. Bacteria tests need time because you’re testing for living organisms. I wait a week. My wife maintains the household log, and she records the date we treated, the date we flushed, and the date we tested.
I talked to Len last spring after his test came back positive. She said, “I shocked it last year, why is it back?” I said, “Did you find out how it got in?” She said no. I said, “Then you’re just treating the symptom.” She called the driller. He found a gap in the casing seal about ten feet down, patched it, and she shocked the well again. That was eight months ago. Her last two tests came back clean.
Go watch the full video before you pour anything down your well.
What Changed Over Time
The first time I shock chlorinated a well was at the county utility yard, back when I was still learning the pumps. We had a small monitoring well that tested positive after a flood. The senior operator showed me how to calculate the volume, mix the solution, and circulate it with a hose. We let it sit for twelve hours, flushed it, and tested a week later. It came back clean. That was the standard method then, and it’s the standard method now.
What changed is the testing. Thirty years ago, you sent a sample to the state lab and waited two weeks for a postcard. Now the county lab turns it around in a couple of days, and the bottles are easier to get. The strips for chlorine and pH are cheaper and more accurate. The process itself hasn’t changed because the chemistry hasn’t changed. Chlorine kills bacteria. Bacteria need time to die. You flush, you wait, you test.
The other thing that changed is how I think about prevention. Early on, I treated shock chlorination as a fix. Now I treat it as a last resort. If you’re testing regularly and you know what a residential well looks like when it’s sealed and maintained, you catch problems before they turn into positive tests. A cracked cap, a missing vent screen, a casing that’s settled and opened a gap—those are the entry points. Fix those first. Shock chlorination is for when bacteria have already gotten in and set up in the plumbing or the formation.
The Cost Of Doing It Right
I keep track of what I spend on water treatment, not because I’m cheap but because I want to know if it’s worth it. I already had the bucket, the hose, and the test strips. My time was most of a Saturday, plus a few minutes each day for a week checking the smell and then collecting the sample. Call it forty in materials and lab costs, plus my labor.
Compare that to the cost of not doing it. If the test comes back positive and you ignore it, you’re drinking water with coliform bacteria. If you’re on a regular testing schedule, you catch it early. If you’re not, you find out when someone gets sick or when you try to pass the place along and the next owner’s inspector flags it. Then you’re paying for treatment anyway, plus whatever else has gone wrong in the meantime.
The cheapest option is prevention. Checking it twice a year costs nothing but time.
The Steps I Follow
Test the water. If coliform is present, inspect the wellhead and fix any obvious entry points.
Calculate the volume of water in the well casing and plumbing. If you don’t know how, call the driller.
Mix unscented sodium hypochlorite with water in a clean bucket.
Pour the mixture into the well. Connect a hose to an outside spigot and run water back into the well to circulate.
Open each tap in the house until you smell chlorine, then shut it off. Then let the system sit overnight before flushing.
Flush from an outside spigot until the chlorine odor is gone, then flush indoor fixtures.
Wait a week, then collect a bacteria sample and send it to the lab.
If the test is still positive, repeat the process and look harder for the source.
Watch the full video now if you’re about to do this yourself.
The chlorine bottle in my utility room has the date written on it in marker.