Short definition
Iron bacteria are non-pathogenic biofilm-forming microorganisms (Gallionella, Leptothrix, others) that consume dissolved iron in groundwater and produce a slimy, reddish-brown deposit. They’re not a health hazard, but they cause staining, smell, and reduced well yield. Common across iron-rich groundwater in Snohomish, Skagit, Whatcom, and parts of King and Pierce counties.
What it is
Iron bacteria live wherever there’s dissolved iron in water and a small amount of oxygen. They oxidize ferrous iron (Fe²⁺) to ferric iron (Fe³⁺), use the energy to grow, and leave behind a gelatinous mat of orange-brown slime. Common species in WA wells include Gallionella, Leptothrix, and Crenothrix.
You’ll first notice them as reddish-brown slime in the toilet tank — usually the homeowner’s first clue — or as recurring “swamp” or “petroleum” odors at fixtures, or as a slow drop in well yield as slime mats clog the well screen and pipes. Sometimes they pair with sulfate-reducing bacteria, which adds a rotten-egg (H2S) smell.
These bacteria are not on any drinking-water health regulation list. EPA’s secondary (aesthetic) MCLs limit dissolved iron to 0.3 mg/L and manganese to 0.05 mg/L, and iron-bacteria activity is the downstream consequence of dissolved iron in groundwater.
Why it matters to a homeowner
Iron bacteria aren’t going to make anyone sick. The reason to address them is operational: stained laundry, clogged plumbing, reduced well yield, and bad-smelling water. The other reason is diagnostic — a slimy well can mask other contamination behind a confusing water-quality picture, so the right move when iron bacteria appear is a full water-quality test panel rather than guessing.
Treatment is layered. Shock chlorination is the first-line response: kills the bacteria, dissolves the biofilm, restores well yield. But unless the underlying iron level is reduced (oxidation + filtration system) or continuous chlorination is added, the bacteria typically return within months to years. The cost-effective long-term answer for an iron-bacteria-prone well is usually a continuous chlorination injection or oxidizing-media iron filter.
When you’ll encounter this term
- Reddish-brown slime in the toilet tank — usually the first homeowner finding
- Recurring rust staining on shower walls, laundry, and fixtures despite a water softener
- Well yield dropping over months or years
- Persistent “swamp” or “rotten egg” smell from cold water
- New rural homeowner finding evidence after the first month
Common variants and disambiguation
- Iron bacteria vs. sulfate-reducing bacteria. Different organisms; sulfate-reducers produce H2S (rotten egg odor) rather than slime. May coexist.
- Iron bacteria vs. dissolved iron. Different problem: dissolved iron is a chemistry issue (iron filter, oxidation system); iron bacteria is a biological issue (chlorination, biofilm flush). See manganese / iron / sulfur in water.
- Iron bacteria vs. coliform. Iron bacteria are not coliform, don’t trigger health concern, and aren’t detected by coliform tests. A clean coliform test does not rule out iron bacteria, and a positive total coliform doesn’t necessarily mean iron bacteria are present.
Common failure modes (signs and situations)
- Reddish-brown slime in toilet tank — usually the first homeowner finding
- Smelly hot water when iron bacteria interact with the magnesium sacrificial anode in the water heater — replacing the anode with aluminum/zinc reduces the smell
- Decreased well yield — slime mats clog well screens; periodic chlorination required
- Stained laundry — iron-laden water with bacterial activity stains worse than dissolved iron alone
- After well repair or new construction — disturbed aquifer often shows a transient iron-bacteria spike
Cost data
| Treatment | Cost |
|---|---|
| Shock chlorination (DIY: bleach + circulation) | $30–$80 in materials |
| Pro well rehabilitation with chlorination | $300–$800 |
| Continuous chlorination injection system | $800–$2,500 install |
| Oxidizing-media iron filter (manganese greensand, KDF) | $1,500–$4,000 install |
| Filter cartridge replacement (annual) | $50–$200/year |
Washington note
WA Department of Health publishes well-disinfection guidance covering iron bacteria as part of well-rehabilitation procedures. Shock chlorination per WAC 246-290-451 and AWWA C654 is the standard procedure most WA private well owners can do themselves.
For recurring iron bacteria — the typical pattern in iron-rich Snohomish, Skagit, and Whatcom County wells — periodic shock chlorination every 1–2 years, paired with a continuous oxidation/filtration system, is the durable solution. If the well yield is dropping despite chlorination, that’s well-driller territory: brushing or swabbing the well casing during a full rehabilitation.