Black Gunk on Faucets: Iron and Manganese Bacteria Explained
Black slime around your faucet, shower head, or inside the
Free Shipping on Orders of $100 or More
Free Shipping on Orders of $100 or More
Black slime around your faucet, shower head, or inside the toilet tank can be frustrating, especially when it keeps coming back after cleaning.
In some homes, the buildup is linked to manganese or iron in the water. Bacteria that grow in plumbing systems can also produce slimy deposits. However, worn rubber parts, mold, and other materials can leave similar-looking residue.
The location and texture of the buildup may offer clues, but they aren't enough to confirm the cause. Here's how to narrow down the possibilities, choose the right water tests, and prevent the problem from returning.
For more information about other types of staining, see our guide to tap water discoloration.
Black residue can develop for several reasons, but two common possibilities in well water systems are manganese deposits and bacterial buildup.
Manganese is a naturally occurring metal that may be dissolved in groundwater. Under certain conditions, it can form dark particles or deposits that collect on faucets, shower heads, and other surfaces.
Iron and manganese bacteria are microorganisms that can grow in water systems containing these metals. Some produce a sticky substance called biofilm, which may trap minerals and other particles.
According to Penn State Extension, iron bacteria can create slimy deposits that clog plumbing and affect water quality. The University of Georgia also describes gelatinous deposits associated with iron bacteria in water systems.
The EPA's secondary drinking water guidelines list manganese at 0.05 mg/L and iron at 0.3 mg/L, mainly because of staining, discoloration, and other water quality issues.
However, not every dark deposit contains these metals or bacteria. The residue may also come from materials inside a fixture or from growth on damp surfaces.
Not all black buildup comes from well water minerals. Other possible causes include:
Where the residue appears can help narrow down the cause. For example, black flakes coming from one faucet may suggest a deteriorating component, while growth around the outside of a fixture may be related to moisture.
If you suspect biological contamination, ETR Laboratories offers a Mold and Fungi Water Test. However, a water test alone may not identify growth on the outside of a fixture.
The location of the buildup can help you identify where the problem may be starting.
If the buildup appears at only one faucet, inspect the fixture before assuming there's a problem with your water supply.
Remove the aerator and check for trapped particles. Look at the washers, rubber gaskets, and flexible supply lines for signs of wear.
If the black material feels rubbery or breaks into small flakes, a deteriorating component may be responsible.
If you notice similar residue in multiple faucets, shower heads, or toilet tanks, the water supply or shared plumbing may be contributing.
Iron and manganese deposits can accumulate in different parts of a plumbing system. Biofilm can also develop on surfaces that remain in contact with water.
Start with a few simple comparisons:
These observations won't confirm the cause, but they can help you decide which fixtures to inspect and what testing may be needed.
The color, texture, and smell of the residue can provide clues about its source. However, several different problems can produce similar-looking buildup.
| What you notice | Possible cause | What to check |
| Orange or reddish-brown slime | Iron deposits or iron bacteria | Look for rusty staining and buildup in toilet tanks or plumbing |
| Black or dark brown staining | Manganese deposits, sometimes mixed with biofilm | Check other fixtures and consider manganese testing |
| Dark slime with a rotten egg smell | Sulfur-related bacterial activity | Check if the odor occurs in hot water, cold water, or both |
| Oily-looking film on standing water | Iron bacteria or other surface material | Observe whether the film breaks apart when disturbed |
| Black, rubbery flakes | Deteriorating washers, gaskets, or hoses | Inspect the affected fixture and supply line |
| Dark growth around a damp fixture | Mold, mildew, or other surface growth | Inspect and clean the surface and address persistent moisture |
For more examples of water discoloration, see our guide to what different well water colors can mean.
Iron and manganese can also occur together. ETR Laboratories offers an Iron Water Test ($45) to measure iron concentrations. Our article on iron in well water explains common sources and water quality problems associated with iron.
Black residue doesn't automatically mean your water is unsafe. The potential health concerns depend on what's causing the buildup.
Iron and manganese bacteria are often considered nuisance organisms because they can produce slime, clog plumbing, and contribute to unpleasant odors. However, their presence doesn't rule out other water quality problems.
Manganese is a separate concern. Although small amounts occur naturally in drinking water, elevated exposure may affect the nervous system. Infants and young children are particularly sensitive to excessive manganese exposure.
The EPA has published health advisory values for manganese, including 0.3 mg/L for lifetime exposure and 0.1 mg/L for shorter-term exposure in infants. These are health advisory values, not enforceable federal drinking water limits.
For more information, see our guide to manganese and other metals in drinking water.
Dark growth around a faucet or shower head may be mold, mildew, or another type of biological buildup. However, its color alone cannot identify the organism or establish how hazardous it is.
If the growth keeps returning, inspect the affected area for moisture problems and consider professional evaluation if needed.
If you have concerns about manganese exposure, particularly for infants, contact your health department or healthcare professional while arranging appropriate water testing.
The right test depends on where the residue appears and what you suspect is causing it.
If black buildup is limited to one faucet, inspect the fixture and its components first. If similar deposits appear throughout the house, water testing may help identify contributing minerals.
Iron and manganese are common causes of staining and deposits in well water.
ETR Laboratories offers two individual tests:
Testing both metals can help determine if they're contributing to the problem. However, elevated metal levels don't automatically confirm that bacteria are present.
If the buildup is slimy, keeps returning, or appears throughout the plumbing system, further investigation may be useful.
ETR Laboratories offers several options:
These tests provide different information. A general bacteria count, for example, cannot confirm that iron bacteria are responsible for the black buildup.
If the water results don't explain the problem, examining the solid material may help identify its composition.
Our guide to identifying unknown residue in water explains how laboratory methods such as FTIR and SEM can help investigate unknown deposits.
If you suspect iron bacteria, contact our laboratory before ordering. We can help you determine which available analysis is appropriate and what type of sample is needed.
Cleaning can remove visible residue and improve water flow, although the buildup may return if the underlying cause remains.
Start by cleaning the affected fixture:
Avoid abrasive cleaners that could damage fixtures, and never mix household cleaning chemicals.
If the residue returns within a short period or appears throughout the home, cleaning alone may not solve the problem.
Preventing black buildup starts with identifying what's causing it. Cleaning removes existing residue, but mineral deposits or bacterial slime may return if conditions in the water system remain unchanged.
Elevated iron or manganese levels may require water treatment to reduce staining and deposits.
Depending on your water chemistry, a treatment professional may recommend a system designed to remove these metals. The appropriate method depends on their concentrations and the forms in which they occur.
Our guide to iron filtration systems explains how these systems work.
Iron and manganese bacteria can establish themselves inside wells, pipes, and other water system components.
Well disinfection may help reduce bacterial buildup, but it doesn't always eliminate the problem permanently. Biofilm can protect microorganisms, allowing growth to return after treatment.
If disinfection is recommended, follow your state or local well maintenance guidance or consult a qualified well contractor.
ETR Laboratories offers a Well Disinfection Kit, with instructions and supplies for the treatment process. Our guide to why household bleach should not be used to disinfect well water explains important precautions.
If deteriorating rubber parts or other fixture components are responsible, replacing those parts may solve the problem.
Check the faucet, supply line, washers, and gaskets for wear. If the residue disappears after replacing the damaged component and doesn't appear elsewhere, whole-house water treatment may not be necessary.
After any water treatment or well maintenance, follow the recommended testing schedule to check that the problem has been addressed.
The gunk keeps returning because the bacteria and the metals that feed them keep arriving with your water. Penn State Extension says slime layers protect bacteria from chlorine, and the University of Georgia notes they tend to rebuild within months. Here is a retest loop:
It can be mold, but in well water it is often manganese deposits with a bacterial film. A metals test helps separate the causes, and our Fungi Count in Water test ($50) looks at mold and yeast in your water.
Slime-forming bacteria are generally treated as a nuisance, but black color does not tell you the manganese level or whether another contaminant is present. Test the water and review the results with your health department or a professional first.
Our product pages say softeners can reduce lower concentrations of iron and manganese, but a softener may not address bacterial growth in a well. Ask a licensed professional to size any treatment from your results, since the right choice depends on how much iron and manganese your water carries.
Often not permanently. Penn State Extension says removing these bacteria entirely can be impossible, and they tend to build up again within months. Follow your state's well disinfection guidance, and consider a licensed well contractor for severe cases.
If black particles appear at only one faucet, the problem may be inside that fixture. Worn rubber washers, gaskets, or flexible supply lines can shed dark material, while aerators can collect mineral deposits and biofilm. Inspect and clean the fixture first, then compare it with other taps to see if the problem is isolated.
A toilet tank holds water that sits for long periods, so dark slime there can point toward manganese, iron deposits or biofilm in the water system rather than one faucet. Testing can confirm the cause.
If black residue keeps appearing around several faucets or inside your toilet tank, testing your water for iron and manganese may help identify the cause.
ETR Laboratories offers individual Iron and Manganese Water Tests, along with additional options for investigating biological growth and unknown deposits.
Not sure which test you need? Our water testing panel guide can help you compare options, or you can contact ETR Laboratories at (800) 344-9977 for assistance.
Black slime around your faucet, shower head, or inside the
Some drinking water contaminants, including arsenic, benzene, and certain PFAS,
Pesticides used on farms, lawns, and golf courses can sometimes
Related Post.
Black Gunk on Faucets: Iron and Manganese Bacteria Explained
Black slime around your faucet, shower head, or inside the toilet tank can be frustrating,
How to Test Your Water for Carcinogens: A Homeowner Guide
Some drinking water contaminants, including arsenic, benzene, and certain PFAS, have been linked to cancer.
Pesticides in Drinking Water: What to Test For and Why
Pesticides used on farms, lawns, and golf courses can sometimes make their way into nearby
1,4-Dioxane in Drinking Water: Why Filters Miss It
Most household water filters reduce common contaminants, but some chemicals are much harder to remove.
Is Your Well Water Radioactive? Radium and Gross Alpha
Radioactive substances can occur naturally in well water, especially in areas where groundwater moves through
Aluminum in Drinking Water: Sources, Limits, Testing
Aluminum occurs naturally in soil and rock, but it can also enter drinking water through
Endotoxin in Water: Why a Bacteria Test Isn’t Enough
A negative bacteria test is usually good news, but it doesn't tell you everything about
Arsenic III vs Arsenic V: What Speciation Shows in Wells
Finding arsenic in your well water is concerning, but knowing how much is present is
Baseline Water Test Before Nearby Drilling or Mining
A lease offer arrives, a permit notice goes up, or a drill rig appears down
Ammonia in Well Water: Causes, Risks and How to Test
A lab result, a sharp smell from the tap, or a chlorine residual that will
Pool Water Testing: What a Lab Scan Adds to Test Strips
Your test strip looks fine, yet the pool is cloudy, the water has a green
Detection Limits in Water Testing: Why Non-Detect Isn’t Zero
You open your water test report and find ND or a less-than sign beside the