Shock Chlorinating a Well: The Step-by-Step Procedure
The lab email says "Total coliform: PRESENT" and nothing else useful. You're boiling water for coffee, the kids are brushing their teeth with bottled, and the well guy can't come until Thursday. In the garage sit a jug of bleach and a garden hose, and every forum thread says the same three words: shock the well. None of them say how much bleach, how long it sits, or which of the four things plumbed into your basement it will destroy on the way through.
Two tests that decide whether to shock at all
Chlorine is a reset button, not a diagnosis. Two cheap measurements tell you whether it's the right tool and whether it worked.
The coliform test, before and after. A state-certified lab runs total coliform and E. coli for $20–50. The EPA's maximum contaminant level for total coliform is zero positive samples, and the CDC recommends private-well owners test annually — no agency tests your well for you. A positive result after a pump replacement, a flood, or a cracked cap is what shock chlorination is for; one that returns every few months is a construction problem bleach won't fix.
Free-chlorine test strips, during the job. Buy strips or a DPD kit reading to 500 ppm — 0–10 ppm pool strips saturate instantly and tell you nothing. You use them twice: at each fixture during the fill, to confirm chlorinated water reached it, and at the tap after flushing, to confirm the residual is under 0.5 mg/L before anyone drinks. A well that won't hold a reading for an hour is consuming chlorine on iron or organic matter and needs a higher dose, not a longer wait.
The procedure, and what it costs
Start with volume. Take static water level and total depth off the well log, subtract to get the standing column, multiply by the figure for your casing diameter, then add 40–60 gallons for the pressure tank, water heater, and piping.
| Casing diameter | Water per foot | 100 ft of standing water | Bleach at 200 mg/L | Bleach at 500 mg/L |
|---|---|---|---|---|
| 4 inch | 0.65 gal | 65 gal | ~1 quart | ~2 quarts |
| 5 inch | 1.0 gal | 100 gal | ~1.5 quarts | ~3.5 quarts |
| 6 inch | 1.5 gal | 150 gal | ~2 quarts | ~1.25 gal |
| 8 inch | 2.6 gal | 260 gal | ~1 gal | ~2 gal |
| 10 inch | 4.1 gal | 410 gal | ~1.5 gal | ~3.5 gal |
Use unscented household bleach at 5.25–6% sodium hypochlorite, no thickeners or "splash-less" additives; it loses roughly half its strength in a year on a warm shelf, so a fresh jug matters more than the brand. Then the sequence:
- Bypass or pull everything sensitive (see the table below) — 15 minutes now saves a $250 membrane.
- Open the well cap. Kill power to the pump at the breaker first. Note where the wiring runs so you pour clear of it.
- Mix and pour. Gloves and eye protection, outdoors, and nothing else in the bucket — bleach mixed with acid, ammonia, or descaling cleaners releases toxic gas. Dilute the measured bleach into 5–10 gallons of water and pour it down the inside wall of the casing, not straight down the middle.
- Recirculate 15–30 minutes. Restore power and run a hose from an outside spigot back into the casing until the hose water smells of chlorine. This mixes the column and washes the casing above the waterline.
- Pull it through the house. Open every fixture — hot and cold, tubs, spigots, laundry — one at a time until each smells of chlorine, then shut it. A single untreated dead leg reseeds the system.
- Hold 12–24 hours with no water use at all.
- Flush outdoors first. Run a hose to gravel or a ditch, clear of the septic drain field, surface water, and anything planted, until the spigot reads under 0.5 mg/L; then flush indoors. Expect 1–3 hours, resting the pump every 30–45 minutes.
- Retest. Coliform sample 7–14 days after the chlorine is gone, and a second 2–3 weeks later. One clean result right after a shock proves the chlorine worked, not that the well is fixed.
| Equipment | What 200–500 mg/L chlorine does to it | What to do first |
|---|---|---|
| Water softener | Oxidizes the resin bed; heavy exposure shortens a 10–15 year bed to a few years | Set the bypass valve; sanitize it separately per the manual |
| Reverse osmosis | Destroys a thin-film composite membrane in hours, $80–250 to replace | Close the feed valve; change the carbon prefilter and membrane after |
| UV system | No damage to the lamp, but the sleeve fouls and the unit does nothing here | Bypass, or leave in place and replace the sleeve/lamp afterward |
| Cartridge and sediment filters | Hold contaminated media that reseeds the clean system | Remove the cartridges, run empty housings, install new cartridges after flushing |
| Iron/greensand/carbon tanks | Carbon strips the chlorine before it reaches the house; oxidizing media varies by type | Bypass, then backwash and check the manufacturer's chlorine tolerance |
| Septic system | A few hundred gallons at 200+ mg/L can knock out the bacteria the tank runs on | Discharge the flush outdoors, never into a drain that feeds the tank |
| Water heater | Chlorinated water must reach it, but the tank holds 40–80 gallons of it | Run hot taps during fill; drain the tank during the flush stage |
Why the coliform comes back two months later
The common story: the shock works, the retest is clean, and by the next test there's coliform again. That means the well is being recontaminated, and disinfecting it only treats the symptom.
The usual sources are physical. A cracked or unsealed cap lets insects and surface runoff in — vermin-proof caps with a screened vent are standard for a reason. Casing corroded or separated near the top lets shallow groundwater bypass the grout seal, and wells finished below grade in a pit flood every spring. State setback rules commonly require 50–100 feet between a well and a septic drain field, more in sandy soil, and older properties often miss them.
The second reason is reach: biofilm on the casing, screen, and drop pipe is a protective layer built for exactly this. That's what a contractor's $300–1,500 adds — wire brushing and surging to break the film mechanically, plus proper disposal.
If two properly executed shocks fail within a year, the choice is repair or continuous treatment. NSF/ANSI 55 Class A UV systems are validated at 40 mJ/cm² for water of unknown microbiological quality, run $500–1,200 installed, and need clear water ahead of them — typically iron under 0.3 mg/L, hardness under 7 gpg, turbidity under 1 NTU. Continuous chlorine injection with a contact tank and carbon filter runs $1,500–3,000 and tolerates dirtier water.
Common mistakes
- Pouring bleach in and calling it done. Chlorine sitting at the bottom of the casing treats the bottom of the casing. Recirculate, then pull it through every fixture — untreated water in the heater or a spare bathroom reseeds everything.
- Shocking with the softener and RO still online. A bypass valve takes 15 seconds to turn. An oxidized resin bed costs $300–700 and an RO membrane $80–250, and neither failure shows up for weeks.
- Flushing into the septic tank or onto the lawn. Several hundred gallons at 200–500 mg/L kills septic bacteria and scorches plants. Discharge to gravel or a ditch, clear of the drain field and surface water.
- Retesting too soon. Residual chlorine suppresses growth in the sample and returns a false clean. Wait 7–14 days after the chlorine clears, then confirm 2–3 weeks later.
FAQ
How much bleach do I need to shock chlorinate a well?
Dose by volume, not by guess. A 6-inch casing holds about 1.5 gallons per foot, so 100 feet of standing water is roughly 150 gallons and takes about 2 quarts of unscented 5.25–6% bleach to reach 200 mg/L. Add 40–60 gallons of household volume for the pressure tank, heater, and piping, and use 500 mg/L if you're fighting iron bacteria or heavy biofilm.
How long does chlorine need to sit in the well?
Most state health department procedures call for 12–24 hours of contact with no water use, with 8 hours a common minimum for light routine disinfection. Longer isn't automatically better: the residual drops as chlorine reacts with iron and organic matter, and a well that loses its reading within an hour needs a higher dose, not more time.
Is the water safe to drink right after shock chlorination?
No. Flush until free chlorine at the tap reads under 0.5 mg/L, then keep using bottled or boiled water until a state-certified lab returns a clean total coliform result — sampled 7–14 days after the chlorine clears, since residual chlorine can mask bacteria in the sample. The CDC advises confirming with a second test a few weeks later before trusting the well again.
How often should a well be shock chlorinated?
There's no fixed calendar. Disinfect after any pump or plumbing work that opens the well, after floodwater reaches the wellhead, and after a positive coliform test; wells with iron bacteria commonly need it every 6–24 months. Repeating it more than twice a year means the well needs inspection or continuous treatment at $500–3,000, not another jug of bleach.
Related:
General information about home water quality and equipment, not medical advice. Bacteria, nitrate, lead, and arsenic can only be measured by a state-certified laboratory, and questions about health effects belong with your doctor or state health department. Chlorine dosing rates, discharge rules, well setback distances, and licensing requirements vary by state and by well; follow your state health department's published procedure.