Your annual water test just came back positive for coliform bacteria, and now you’re staring at the lab report wondering if your well is ruined. Take a breath. It almost never means that.
A positive coliform hit is unsettling, especially when your family drinks that water every day. The good news is that for most wells in the Wenatchee Valley, shock chlorination fixes the problem for about $30 in supplies and one weekend of patience. Shock chlorinating a well means dosing it with a strong chlorine solution, letting it sit, then flushing it out.
In this guide we’ll cover when your well needs it, exactly how much bleach to use, the 7 steps start to finish, how to retest, and the situations where you should put the bleach down and call a pro instead.
When Your Well Needs Shock Chlorination
Shock chlorination is a one-time disinfection, not a water treatment system. It kills bacteria that got into the well. It does not fix an ongoing contamination source, and it won’t remove nitrates, arsenic, or hardness.
Here’s when it’s the right tool:
- A positive total coliform test. This is the big one. Coliform bacteria themselves usually won’t make you sick, but they signal that surface water or debris is reaching your well.
- Any time the well is opened up. New pump, pressure tank work, a pulled drop pipe, a new well cap. If the seal was broken, shock it.
- A brand new well, before first use. Most drillers do this, but confirm it.
- After flooding or heavy snowmelt reaches the wellhead. We see this every spring in low spots around Cashmere and the creek bottoms near Leavenworth.
- A well that sat unused for months, like a vacation place up at Lake Chelan.
- Rotten egg smell or slimy red-brown buildup. Sulfur and iron bacteria respond to chlorine, at least temporarily. If your water smells like rotten eggs, shocking is usually step one.
Marlene in Cashmere is a good example of how this usually goes. She bought a 3-acre orchard property last fall, and the pre-purchase water test came back positive for total coliform, negative for E. coli. She was quoted $2,800 by an out-of-town outfit for a “well remediation package.” Instead, she spent $24 on bleach and test strips, shocked the well herself on a Saturday, and her retest two weeks later came back clean. Same well, same water, $2,776 saved.
Not every story ends that way, and we’ll get to the exceptions. If you’d rather have a licensed tech handle the whole thing, request a free estimate and we’ll give you a straight price over the phone, no remediation package required.
One more thing before you start. If you haven’t had a full water test in the last year, do that first. The EPA’s private well guidance recommends testing annually for coliform and nitrates at minimum. You need a baseline, because shocking a well right before testing can mask a real problem.
What You’ll Need Before You Start
Gather everything first. Once the chlorine goes in, you’re committed for 24 hours.
- Plain, unscented household bleach, 6% to 8.25% sodium hypochlorite. No splashless, no scented, no “advanced cleaning” formulas. Buy fresh bottles, because bleach loses strength fast on the shelf.
- Chlorine test strips rated for high range (up to 200+ ppm). About $10 at a pool supply store.
- A clean 5-gallon bucket and a garden hose that reaches from an outside spigot to the wellhead.
- Rubber gloves and eye protection. Strong chlorine solution will burn skin and eyes.
- Stored water for 24 to 48 hours. Drinking water, water for pets and livestock, and a few buckets for flushing toilets.
- Basic hand tools to remove the well cap bolts.
Also plan your bypasses. Water softeners, carbon filters, and reverse osmosis units should be bypassed or have cartridges removed, since concentrated chlorine wrecks filter media and membranes. Most people leave the water heater alone, but don’t run hot taps during the process.
How to Shock Chlorinate a Well in 7 Steps
Set aside a full weekend. The work itself takes a few hours. The waiting takes the rest.
Step 1: Kill power and open the well cap
Flip the breaker for the well pump before you touch the cap. The wiring for your pump runs right through the wellhead, and you do not want to be loosening bolts next to live 240-volt conductors. Remove the cap bolts and lift the cap, keeping the wiring connected and out of your way.
Stop here if the cap is corroded shut, the wiring looks chewed or brittle, or you see a sanitary seal you don’t understand. That’s a call to us at (509) 351-8404, not a pry bar situation.
Step 2: Inspect the wellhead
Look down the casing with a flashlight. Check for cracks in the cap, missing screens on the vent, insect nests, or debris. A damaged cap is the number one reason wells keep testing positive, and a replacement cap costs less than repeated lab fees.
Step 3: Mix and pour the chlorine
Turn the breaker back on so the pump can run. Mix your measured bleach (amounts in the table below) with 5 to 10 gallons of clean water in the bucket. Pour it down the casing slowly, swirling it around the inside walls so it washes down the casing surface, not just straight into the water.
Step 4: Recirculate
Connect a garden hose to the nearest outside spigot and run the other end down into the casing. Turn it on and let the well pump its own water back into itself for 30 to 60 minutes. This mixes the chlorine through the full water column and rinses the casing, drop pipe, and pump. You’ll smell chlorine strongly at the hose when it’s mixed. Test strips should read 100 to 200 ppm.
Step 5: Pull chlorine through the house
Go inside and open every cold tap, one at a time, until you smell chlorine at each one. Then shut it off. Hit every fixture: sinks, tubs, outside spigots, and run a short toilet flush on each toilet. Now the chlorine is sitting in every pipe, the pressure tank, and the well itself.
Step 6: Wait 12 to 24 hours
Close the well cap. Don’t use the water at all. No showers, no laundry, no pet bowls. Twelve hours is the minimum contact time, and 24 is better. This is where your stored water earns its keep.
Step 7: Flush it all out
Run a hose from an outside spigot to a gravel area or unused corner of the property. Let it run until the chlorine smell is gone and test strips read zero, which can take 2 to 8 hours depending on your well. Then flush the inside taps. Two rules here: don’t dump chlorinated water into your septic system in volume, and never let it drain to a creek, pond, or irrigation ditch. Chlorine kills fish and beneficial soil life, and the Washington Department of Health specifically warns against discharging it to surface water or landscaping.
How Much Bleach? Dosage by Well Depth
The target is 100 to 200 ppm of chlorine in the well. To get there, you need to know roughly how much water is standing in your casing. Most residential wells in Chelan, Douglas, and Grant counties have 6-inch casings, which hold about 1.5 gallons per foot of water depth.
Your well log lists total depth and static water level. Water depth is the difference between the two. If you can’t find your log, Washington’s Department of Ecology keeps well reports on file, or just estimate on the high side.
| Water depth in well (6” casing) | Approx. gallons standing | Regular 6% bleach needed |
|---|---|---|
| 50 ft | 75 gal | 2.5 pints |
| 100 ft | 150 gal | 4.5 pints (about half a gallon) |
| 150 ft | 220 gal | 7 pints (about 0.9 gallon) |
| 200 ft | 295 gal | 9 pints (about 1.1 gallons) |
| 300 ft | 440 gal | 13.5 pints (about 1.7 gallons) |
For a 4-inch casing, use about half these amounts. For an 8-inch casing, nearly double them. If you’re using 8.25% concentrated bleach, reduce the dose by about a quarter.
A little extra won’t hurt anything except your flushing time. A lot extra, like dumping in 5 gallons “to be safe,” makes the water corrosive, hard on pump seals, and miserable to flush. More is not better here.
Retesting After You Shock the Well
Flushing isn’t the finish line. The retest is.
Wait until all chlorine is out of the system, then wait another 7 to 14 days before collecting your coliform sample. Test too soon and leftover chlorine gives you a false clean result. Use a state-certified lab, follow their sampling instructions exactly, and test from an inside tap with the aerator removed.
If the retest is clean, test again in 3 to 6 months to confirm it stays clean, then go back to annual water testing like clockwork.
If it comes back positive again, you don’t have a bacteria problem. You have a pathway problem. Something is letting contamination in, and no amount of bleach fixes a route.
The Hendersons near Chelan learned that one the hard way. Their well tested positive three times over two summers, and they shocked it after each result. Worked for a few weeks, then the bacteria came back. When we finally got a look at the wellhead, the cap had a hairline crack and a missing vent screen, and we found mouse nesting material six feet down the casing. A $260 cap replacement and one final shock ended a two-year problem that three rounds of bleach never could.
When Shock Chlorination Isn’t Enough
Knowing when to stop DIYing matters as much as knowing the steps. Call a professional when:
- E. coli shows up on any test. That’s fecal contamination, and it’s a health risk right now. Stop drinking the water, switch to bottled, and get help the same day. Our emergency well service runs 24/7 for exactly this kind of thing.
- The well tests positive twice after proper shocking. There’s a defect somewhere: a cracked casing, a bad surface seal, a flooded pit, or a septic system too close to the wellhead.
- Iron or sulfur bacteria keep coming back. Shocking knocks them down for a month or two, but heavy infestations need stronger acid-based well cleaners and sometimes mechanical scrubbing of the casing, which is pull-the-pump territory.
- The wellhead is buried or in a well pit. Older properties around the valley still have these, and they’re contamination magnets that need a proper pitless adapter conversion.
- Anything electrical beyond flipping the breaker. Period.
Rick in Quincy hit two of these at once. His 280-foot irrigation-and-house well had a sulfur smell strong enough that his wife refused to do laundry in it. He shocked it twice in one summer, and both times the smell came back inside six weeks. Turned out he had a heavy iron bacteria colony coating the drop pipe and pump intake. We pulled the pump, found the intake screen half blocked with slime, cleaned the well with a stronger treatment, and his flow rate jumped from 9 gallons per minute back up to 14. The smell hasn’t returned in over a year. While the pump is out of the hole is also the cheapest time to fix worn parts, which is why we always inspect during a well pump repair or cleaning.
If your well keeps fighting you, get a free estimate and we’ll tell you honestly whether it needs a $30 fix or a real repair. We’re licensed and insured in Washington, and we work wells from Leavenworth to Moses Lake every week.
Frequently Asked Questions
How long after shock chlorination can I drink the well water?
Once the chlorine smell is gone and test strips read zero at every tap, the water is safe to use for washing and flushing. For drinking, the careful answer is to wait for a clean coliform retest, taken 7 to 14 days after the chlorine clears. Many homeowners drink it sooner once chlorine reads zero, but the retest is what actually confirms the disinfection worked.
Will shock chlorination damage my pump, pipes, or septic system?
At proper doses of 100 to 200 ppm, a single shock treatment won’t harm a pump or plumbing. Massive overdosing can corrode pump seals and metal fittings, so measure instead of guessing. Protect your septic by flushing the bulk of the chlorinated water outside through a hose, not down the drains.
How often should I shock chlorinate my well?
There’s no schedule for it. Shock only when there’s a reason: a positive test, opened well components, flooding at the wellhead, or bacteria symptoms like sulfur smell and slime. A healthy, sealed well can go many years without ever needing it, which is why annual testing matters more than routine shocking.
Why does my well still test positive after shock chlorination?
Repeated positives almost always mean contamination has a pathway into the well, like a cracked cap, missing vent screen, bad surface seal, damaged casing, or runoff pooling at the wellhead. Bleach kills what’s in the water today but can’t block what arrives tomorrow. Find and fix the entry point, shock once more, and the positives usually stop.