Oily Film on Well Water: Iron Bacteria vs. Petroleum Contamination
That rainbow-like sheen floating on your well water is alarming—but don't panic yet. While it looks like oil, it's usually caused by harmless iron bacteria. Here's how to tell the difference and what to do either way.
📋 In This Guide
Concerned about contamination? We test wells and diagnose water quality issues throughout San Diego County.
Call (760) 440-8520What Causes an Oily Film on Well Water?
Two main causes produce an oily appearance on water surfaces:
1. Iron Bacteria (Most Common)
Iron bacteria are naturally occurring microorganisms that feed on dissolved iron in groundwater. They're found in soil and aquifers throughout San Diego County.
What they produce:
- Oily, rainbow-colored sheen on water surface
- Reddish-brown or yellowish slime
- Gelatinous deposits in pipes and toilet tanks
- Musty, swampy, or "earthy" odor
Where you'll see it:
- Toilet tank (most obvious)
- Standing water in glasses or containers
- Pet water bowls
- Water left in bathtub
2. Petroleum Contamination (Rare but Serious)
Actual petroleum products can contaminate groundwater from:
- Leaking underground fuel storage tanks
- Old oil/gas spills that reached the aquifer
- Industrial contamination
- Improper disposal of petroleum products
- Nearby gas stations or industrial sites
This is a serious health hazard requiring immediate action.
The Stir Test: How to Tell the Difference
This simple test distinguishes iron bacteria from petroleum:
How to Perform the Test
- Fill a clear glass or bowl with tap water
- Let it sit until film appears on surface
- Use a stick, straw, or utensil to stir/disturb the surface
- Observe what happens to the film
Results
| Observation | Likely Cause | Urgency |
|---|---|---|
| Film breaks into fragments/flakes that don't reunite | Iron bacteria | Low—nuisance, not hazard |
| Film swirls and flows back together | Petroleum/oil | HIGH—stop using water, test immediately |
Additional Clues
| Characteristic | Iron Bacteria | Petroleum |
|---|---|---|
| Smell | Musty, swampy, earthy | Gasoline, chemical, fuel-like |
| Color | Rainbow sheen, reddish-brown | Rainbow sheen, may be darker |
| Slime/deposits | Yes—gelatinous, in toilet tank | No slime |
| Nearby sources | Natural (soil/aquifer) | Gas stations, tanks, spills |
If It's Iron Bacteria
Iron bacteria is a nuisance, not a health hazard. The organisms themselves aren't toxic, but they create problems:
Problems Caused by Iron Bacteria
- Aesthetic issues: Unpleasant appearance, taste, and smell
- Staining: Orange/brown stains on fixtures
- Clogging: Slime clogs pipes, reduces flow
- Well damage: Can clog well screens, reducing yield
- Equipment fouling: Clogs filters, water treatment systems
Treatment for Iron Bacteria
Step 1: Shock Chlorination
The first line of treatment is shocking the well with high chlorine concentration:
- Introduces chlorine (50-200 ppm) into well
- Kills bacteria throughout well and plumbing
- Must be done correctly to reach all affected areas
- Professional service recommended
Step 2: Ongoing Treatment
Iron bacteria often returns after shocking. Ongoing treatment options:
- Continuous chlorination: Chlorine feed system with contact tank
- UV disinfection: Kills bacteria without chemicals (pre-filter required)
- Iron filtration: Removes iron that bacteria feed on
- Regular shocking: Annual or semi-annual chlorination
Important: Complete elimination of iron bacteria is often impossible—the goal is management to acceptable levels.
Cost of Treatment
- Professional shock chlorination: $150-$400
- Continuous chlorination system: $800-$2,000
- UV disinfection system: $500-$1,500
- Iron filtration system: $1,500-$4,000
If It's Petroleum Contamination
This is a serious situation requiring immediate action.
Immediate Steps
- Stop using the water – Don't drink, cook, or bathe
- Get professional testing – Lab analysis for VOCs (volatile organic compounds)
- Contact your county health department
- Document everything – Photos, dates, observations
Professional Testing
Request testing for:
- BTEX (benzene, toluene, ethylbenzene, xylene)
- TPH (total petroleum hydrocarbons)
- MTBE (fuel additive)
- VOCs (volatile organic compounds)
Cost: $100-$300 for comprehensive VOC panel
Who to Contact
- San Diego County DEH: (858) 505-6700
- California State Water Resources Control Board
- Your local water district (if applicable)
- EPA Region 9: For serious contamination
Treatment Options for Petroleum
If contamination is confirmed:
- Activated carbon filtration: Removes many petroleum compounds
- Air stripping: Removes volatile compounds
- Alternative water source: May be necessary during remediation
- New well: In severe cases, drilling a new well in uncontaminated zone
Note: Contamination source must be identified and addressed, or treatment is futile.
Preventing Iron Bacteria
Iron bacteria can be introduced during well work. Prevention tips:
- Use sanitized equipment – Insist contractors disinfect tools before entering well
- Chlorinate after any well work – Pump repairs, inspections, deepening
- Maintain proper well seal – Prevents surface bacteria from entering
- Test annually – Catch problems early
Other Causes of Water Surface Film
Less common causes of surface films:
- Manganese bacteria: Similar to iron bacteria, produces black deposits
- Sulfur bacteria: Creates white or grayish film, rotten egg smell
- Algae: In surface water or poorly sealed wells
- Minerals: Some mineral combinations create iridescent films
Need Water Testing?
We diagnose water quality issues and recommend appropriate treatment. Don't guess—get tested.
See our iron bacteria guide or water testing guide.
Call (760) 440-8520Serving San Diego, Riverside & San Bernardino Counties
Frequently Asked Questions
Why does my well water have an oily film on top?
The most common cause is iron bacteria—harmless organisms that feed on iron in your water and produce an oily-looking film. Less commonly, it could indicate petroleum contamination from leaking fuel tanks or industrial sources, which requires immediate action.
How can I tell if it's iron bacteria or oil contamination?
Do the "stir test": stir the water surface with a stick. If the film breaks into sheets or flakes that don't reunite, it's iron bacteria. If the film swirls back together like actual oil, it may be petroleum contamination. Also check for gasoline/chemical smell vs. musty/swampy odor.
Is iron bacteria in well water dangerous?
Iron bacteria itself isn't harmful to human health—it's naturally occurring and non-toxic. However, it creates unpleasant taste and odor, stains fixtures, and can clog pipes and well screens over time. It's a nuisance rather than a health hazard.
How do I treat iron bacteria in my well?
Treatment involves shock chlorination of the well (high chlorine concentration kills bacteria), followed by ongoing treatment like continuous chlorination, UV disinfection, or iron filtration to prevent regrowth. Complete elimination is difficult; management is the realistic goal.
Why does the oily film keep coming back?
Iron bacteria are extremely persistent. They live in biofilms throughout your well and plumbing, making complete elimination nearly impossible. Regular shocking and ongoing treatment (chlorination or UV) keeps populations manageable.
Can I drink water with iron bacteria?
While not directly harmful, we don't recommend drinking water with active iron bacteria due to unpleasant taste, odor, and potential for other bacteria to colonize the biofilm. Treatment is recommended before consumption.
Get Expert Help
Contact Southern California Well Service for water diagnosis and treatment.
Call (760) 440-8520Serving San Diego, Riverside & San Bernardino Counties
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