Iron Bacteria vs Dissolved Iron - Decision Tree

Why this matters

"Brown stains" is a homeowner symptom; what causes the stains determines the treatment. Dissolved ferrous iron (Fe2+) responds to oxidation and ion exchange. Iron bacteria (Gallionella, Leptothrix, Crenothrix species) responds only to shock chlorination and ongoing disinfection. Tannin-bound iron and ferric iron from a corroded steel casing each have their own treatment paths. Selling a softener to treat iron bacteria fails in 30 days. Selling a chlorination system to treat ferric iron is overkill and the homeowner pays for a maintenance contract they do not need. This decision tree gets the diagnosis right before the quote goes out.

Step 1 - test inlet water at multiple points

The first move is sample collection at three points:

1. Outside hose bib closest to the well - this is the rawest sample, before any pre-treatment.
2. Inside kitchen cold tap - this is what the homeowner experiences.
3. Toilet tank reservoir - this gives you a static sample that has sat for hours, which reveals slow-growing iron bacteria mats.

Pull 1 liter samples in clear glass. Compare immediately and again after 1 hour sitting.

Step 2 - read the sample appearance

Dissolved ferrous iron (Fe2+):

• Clear at the tap.
• Develops a yellow-brown color in 5-30 minutes as oxygen oxidizes Fe2+ to Fe3+.
• Forms a fine reddish-brown precipitate at the bottom.
• No slime or biofilm.
• Toilet tank: orange / red staining on porcelain, no slime ring.

Iron bacteria:

• Often slightly cloudy at the tap.
• Develops a darker brown / black color.
• Stringy, gelatinous slime that floats or hangs in the water.
• Toilet tank: rust-colored fluffy biofilm on the tank walls, particularly under the waterline, with a black "shadow" inside the porcelain trap.
• Often a swampy, oily, or sewer-like smell, distinct from the metallic taste of dissolved iron.

Ferric iron (Fe3+) from corroded casing:

• Already brown / red at the tap (no waiting period).
• Fine particulate that settles immediately.
• Often accompanied by visible rust in the aerator screen.

Tannin-bound iron:

• Tea-colored to brown at the tap.
• Does NOT settle when left to sit.
• Most common in shallow wells with organic-rich aquifers (Florida, Carolina coastal plain, Wisconsin peat areas).

Step 3 - run a quantitative iron test

Use a Hach IR-18C iron test kit (resolves 0-10 ppm total iron) or a Hanna HI 721 iron tester. Get THREE numbers from a single sample:

1. Total iron - sample acidified to pH 2 with HCl, then tested. Captures all iron forms.
2. Dissolved iron - sample filtered through 0.45 micron, then tested. Captures only ferrous.
3. Particulate iron - total minus dissolved. Reveals ferric and bacterial-bound iron.

A high particulate fraction with low dissolved is a fingerprint for bacterial iron or corroded casing. A high dissolved fraction with low particulate is a fingerprint for clean ferrous iron from the formation.

Step 4 - look at the well cap and casing

Pull the well cap (vented sanitary cap on most modern wells). Look for:

• Brown / black slime on the underside of the cap or inside the casing - high probability of iron bacteria.
• Cracked or corroded steel casing - probable source of ferric iron.
• Vermin entry (mice, snakes) - common pathway for surface bacteria including iron bacteria.

A well cap installed by a contractor 30 years ago and never inspected since is the most common iron bacteria entry point.

Step 5 - culture for iron bacteria if visual is ambiguous

When the sample is borderline (slight slime but not definitive), use a Biological Activity Reaction Test (BART):

• BART iron-related bacteria test (Droycon Bioconcepts B05-IRB)
• 8-day incubation at room temperature
• Color change pattern in the vial identifies the species and approximate population

A positive BART confirms iron bacteria; a negative confirms dissolved or particulate iron without biological involvement. The test is approximately $35 per vial - cheap insurance before quoting.

Step 6 - rule out the corrosion cause

If the diagnosis is ferric iron from corroded casing:

• Confirm casing material (steel vs PVC). PVC casings do not produce ferric iron.
• Measure water pH; pH below 6.8 accelerates steel corrosion. Acid water plus steel casing creates ferric iron at all times.
• Verify with a manganese test - corroded steel casings often release both iron and manganese.

If the casing is the source, the long-term fix is a casing repair or replacement (out of scope for the water treatment contractor; refer to a well driller). Interim treatment is a backwashing sediment filter to capture particulate iron at the point of entry.

Step 7 - match treatment to the cause

Dissolved ferrous iron, under 3 ppm, no bacteria:

• Water softener with manganese greensand or a Fleck 5600 SXT with iron-cleaning resin (Purolite SST-60).
• Or an oxidation-filtration unit (Filox, BIRM, or air-injection with Pyrolox).

Dissolved ferrous iron, 3-10 ppm, no bacteria:

• Air-injection iron filter (AIO with Pyrolox media) is the most cost-effective at this range.
• Pre-filter with a 50 micron sediment cartridge to catch any oxidized fines.

Ferric iron from casing corrosion:

• 5 micron backwashing sediment filter.
• pH neutralizer (calcite media) if pH is below 6.8.
• Long-term: refer to well contractor for casing inspection.

Iron bacteria, any concentration:

• Shock chlorination of the well and entire plumbing system per the well-decontamination procedure (50-200 ppm chlorine, 12+ hour contact, full flush).
• Ongoing chlorination via a Stenner pump or other chemical feeder, followed by a contact tank and carbon filter for chlorine removal.
• Annual re-shock as maintenance.

Tannin-bound iron:

• Tannin-removal resin (anion exchange) before any iron filter.
• Carbon polish for residual color and organics.

Step 8 - confirm post-treatment

After installation, re-sample at the same three locations:

• Outside hose bib (raw): same as before, confirms the source.
• Treated water tap (after treatment train): should be under 0.3 ppm total iron per the EPA secondary MCL.
• Toilet tank after 30 days: no new staining or slime.

Quick triage table

References

• US EPA Secondary Drinking Water Standards, 40 CFR Part 143
• NSF/ANSI 44-2023 - Residential Cation Exchange Water Softeners
• NSF/ANSI 42-2023 - Drinking Water Treatment Units (Aesthetic Effects)
• Droycon Bioconcepts BART Test for Iron-Related Bacteria, technical guidance
• AWWA M58 - Iron and Manganese Removal Handbook, Second Edition (2017)
• WQA Technical Application Bulletin - Iron Bacteria Identification and Treatment (2024)