Iron Bacteria in a Well: Slime, Sheen and How to Kill It
You lift the lid off the toilet tank and the walls are coated in something that isn't a stain. It's rust-colored jelly, and it smears when you drag a finger through it. The standing water carries an oily rainbow film. Downstairs, the sediment filter you changed six weeks ago is packed with reddish sludge, and the first water after a weekend away smells swampy. This isn't rusty water — it's a living version of it.
Three tests that tell you what you actually have
Iron bacteria, plain dissolved iron, and sulfur bacteria all leave orange or dark deposits, and each needs a different fix. Ten minutes and no equipment separate them.
The toilet tank check. Touch the film on the porcelain above the waterline. Iron bacteria leave a soft, greasy coating that wipes into a smear; chemical iron leaves a gritty ring that needs scrubbing. That texture difference is the most reliable home indicator there is.
The sheen test. Let a white bucket of water stand undisturbed for an hour. If a rainbow film forms, poke it with a toothpick. A bacterial film breaks into rigid plates and stays broken; a petroleum film — from a leaking pump or fuel contamination — flows back together within seconds. Several state health departments publish this exact test.
The clear-glass test. Water that's orange the instant it hits the glass carries ferric iron; water that pours clear and turns orange over 10–30 minutes is ferrous, still dissolved. Iron bacteria show up with either, so this test narrows the filtration question but never settles the bacteria one.
| What you see | Iron bacteria | Dissolved iron only | Sulfur bacteria |
|---|---|---|---|
| Deposit texture | Slimy, gelatinous, smears | Gritty, hard, scrubbable | Slimy, often black or gray |
| Color | Rust, orange-brown, sometimes yellow | Orange to red-brown | Black, dark gray |
| Smell | Swampy, musty, cucumber, sewage-like | Metallic, or none | Rotten egg (hydrogen sulfide) |
| Surface film | Oily sheen that shatters when poked | None | Usually none |
| Timing | Worst after the well sits idle | Steady, every draw | Worst on hot water first |
| Confirmed by | BART or iron-bacteria lab test, $30–80 | Total iron panel, $20–60 | Sulfide/sulfate panel, $30–90 |
Iron sits in the EPA's secondary drinking water standards at 0.3 mg/L — the aesthetic tier, covering color, taste, and staining rather than health effects — with manganese at 0.05 mg/L. There is no federal limit for iron bacteria, since they aren't classified as pathogens. Still run a coliform test ($20–50): they usually enter during drilling or pump service, and whatever carried them in could have carried something else.
What each treatment does, and what it costs
The options split into "kill what's in the well" and "keep killing it forever."
| Treatment | When it applies | How it works | Typical cost |
|---|---|---|---|
| DIY shock chlorination | First occurrence, accessible well cap, mild slime | Bleach solution circulated down the casing, held 12–24 hours, then pumped out | $50–150 in materials |
| Contractor shock + mechanical cleaning | Repeat cases, dropping well yield, screened wells | Wire brushing and surging break the biofilm off the casing so chlorine can reach it | $300–1,500 |
| Acid or dispersant pretreatment | Heavy mineral-and-slime buildup on screen | Sulfamic or phosphate chemistry loosens the deposit before disinfection | $400–1,200, professional only |
| Continuous chlorine injection + contact tank + carbon filter | Regrowth inside 12 months; iron above 10 mg/L | Metered dosing oxidizes iron and bacteria; carbon strips residual chlorine | $1,500–3,000 installed |
| Hydrogen peroxide injection + catalytic carbon | Same as above, no chlorine taste wanted | Peroxide oxidizes, breaks down to water and oxygen, no salt or chloride added | $1,800–3,200 installed |
| UV disinfection | Polishing step only, after iron is removed | Kills organisms passing the lamp; does nothing to biofilm already downstream | $300–900 |
Dosing the shock is arithmetic: work out the water standing in your casing, multiply for the target concentration, use unscented household bleach at 5.25–8.25% sodium hypochlorite. Routine disinfection targets about 200 mg/L free chlorine; state health and natural-resources agencies typically call for more against iron bacteria — often 500–1,000 mg/L — plus agitation, because the colony hides inside a protective slime layer. Follow your state's published procedure; rules and disposal limits vary.
| Casing diameter | Water per foot of depth | 100 ft of standing water | Rough bleach volume at ~200 mg/L |
|---|---|---|---|
| 4 inch | 0.65 gal | 65 gal | About 1 quart |
| 6 inch | 1.5 gal | 150 gal | About 2 quarts |
| 8 inch | 2.6 gal | 260 gal | About 1 gallon |
After the bleach goes in, recirculate with a hose from an outside spigot back down the casing for 15–30 minutes to wash the upper walls, then open every fixture until you smell chlorine. Hold 12–24 hours, then pump to waste away from the septic field and any surface water — concentrated chlorine kills a septic tank's bacteria. Retest for coliform 7–14 days later, once the residual is gone.
Why buying a bigger iron filter doesn't fix this
The common sequence: someone sees orange, buys a $1,500 air-injection or greensand tank, and watches it work beautifully for four months before the slime returns — now inside the new tank.
The reason is position. The filter sits in the basement; the bacteria live in the casing, on the pump, and along the drop pipe, all upstream of it. Every gallon carries fresh cells into the media bed, which is close to ideal habitat — wet, dark, iron-rich, enormous surface area. Within months the bed fouls with biofilm rather than iron, channels form, backwash no longer scours it clean, and the tank passes what it was bought to stop. Most oxidizing-filter spec sheets say so outright: iron bacteria are an application the media does not cover.
UV fails for a related reason plus a physical one. The lamp disinfects only the water inside its chamber and does nothing about colonies already living in the water heater, pressure tank, and pipes. It also needs clear water — typical manufacturer limits are iron below 0.3 mg/L, manganese below 0.05 mg/L, hardness under 7 gpg, turbidity under 1 NTU. Iron-bacteria water fails all four, coats the quartz sleeve, and the lamp keeps glowing while its real dose collapses.
Common mistakes
- Treating it as a filter problem. No media bed removes a colony living upstream of it. Disinfect the well first, then decide what filtration the remaining iron needs.
- Under-dosing the shock. A cup of bleach down the casing does almost nothing to slime. Calculate from casing diameter and standing depth, recirculate, hold 12–24 hours.
- Dumping the chlorinated water into the septic system. A few hundred gallons at 200–1,000 mg/L can wipe out the bacteria a septic tank runs on. Pump clear of the drain field and any surface water.
- Assuming one treatment is permanent. Regrowth in 6–24 months is normal. Plan an annual shock, or price continuous injection if you're re-treating twice a year.
FAQ
Are iron bacteria in well water dangerous to drink?
State health departments consistently describe iron bacteria as a nuisance organism rather than a health hazard — they are not known to cause disease, and no EPA limit exists for them. What they do cause is slime, staining, odor, plugged screens, and lost well yield. Since they typically enter during drilling or pump work, run a coliform test ($20–50) to confirm nothing harmful came in with them.
How do I know it's iron bacteria and not just rusty water?
Texture and smell. Iron bacteria leave a soft, slimy, rust-colored coating that smears under a finger, an oily sheen that shatters into flakes when poked, and a swampy or cucumber-like odor. Plain dissolved iron leaves a hard, gritty orange stain and a metallic taste, with no slime. A BART or iron-bacteria lab test runs $30–80 if you want it confirmed.
How much bleach do I need for shock chlorination?
Start from casing volume: a 6-inch casing holds about 1.5 gallons per foot, so 100 feet of standing water is roughly 150 gallons and needs about 2 quarts of 5.25–8.25% unscented bleach for 200 mg/L. State guidance for iron bacteria often calls for 500–1,000 mg/L, several times that. Hold 12–24 hours, then pump out clear of the septic field.
Will iron bacteria come back after shock chlorination?
Usually, yes. Typical regrowth runs 6–24 months, and a heavily colonized well can be back within a few months, because chlorine rarely reaches every pocket of biofilm on the casing and screen. That's why contractors add wire brushing and surging at $300–1,500, and why households re-treating twice a year move to continuous chlorine or peroxide injection at $1,500–3,000.
Related:
General information about home water quality and equipment, not medical advice. Iron and manganese are aesthetic issues under EPA secondary standards, and iron bacteria are a nuisance organism rather than a regulated contaminant. 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. Dosing, disposal rules, costs, and regrowth intervals vary by state and by well.