Oily Film on Well Water: Iron Bacteria vs Contamination
How to identify and treat that mysterious sheen on your water
📋 In This Guide
Seeing an oily film or rainbow sheen floating on your well water is alarming. Your first thought is probably contamination — is there petroleum in my water? In most cases, that oily appearance isn't oil at all. It's typically caused by iron bacteria, a harmless but annoying organism that creates a biofilm resembling an oil slick.
However, actual petroleum contamination does occur and requires immediate action. This guide will help you quickly distinguish between iron bacteria and real contamination, and explain what to do about each.
The Two Main Causes of Oily Film
While there are other possible causes, the vast majority of "oily well water" cases fall into two categories:
1. Iron Bacteria (Most Common — 90%+ of Cases)
Iron bacteria are naturally occurring microorganisms that feed on dissolved iron in groundwater. As they multiply, they produce a slimy biofilm that floats on water and can look remarkably like an oil slick. This biofilm:
- Often has an iridescent or rainbow appearance
- May appear brownish, orange, or yellowish
- Creates a "swampy" or musty odor
- Leaves rusty or slimy deposits in toilet tanks
- Clogs filters, pipes, and fixtures over time
Iron bacteria are not harmful to human health but create significant aesthetic and maintenance problems.
2. Petroleum Contamination (Less Common but Serious)
True petroleum contamination comes from fuel, oil, or chemical products reaching groundwater. Sources include:
- Underground storage tank leaks (gas stations, farms, old home heating oil tanks)
- Surface spills migrating through soil
- Improper disposal of motor oil, solvents, or chemicals
- Road runoff containing automotive fluids
- Industrial contamination plumes
Petroleum contamination is dangerous and requires immediate action. Even small amounts can make water unsafe.
How to Tell the Difference
Before panicking, perform these simple tests to identify what you're dealing with:
🔬 The Stick Test (Do This First)
Fill a clear glass or white container with the affected water. Let it sit until you see the film form on top. Then stir it with a stick, spoon, or your finger.
- Iron Bacteria: The film breaks into irregular fragments that float separately and don't reconnect
- Petroleum: The film immediately flows back together, reforming a continuous sheen
👃 The Smell Test
Smell the water carefully:
- Iron Bacteria: Musty, earthy, swampy, or "rotting vegetation" odor
- Petroleum: Gasoline, diesel, fuel oil, or chemical solvent smell
👁️ The Color Test
Look at the water and any residue left in containers:
- Iron Bacteria: Often accompanied by rusty, orange, or brown coloration; may leave slimy residue
- Petroleum: Water may look clear aside from the sheen; no rusty color
🚽 The Toilet Tank Test
Look inside your toilet tank:
- Iron Bacteria: Rusty or slimy buildup on tank walls, chains, and float
- Petroleum: Clean tank, film only on water surface
Comparison Summary
| Characteristic | Iron Bacteria | Petroleum |
|---|---|---|
| Stick test | Breaks into fragments | Reforms into continuous film |
| Smell | Musty, swampy, earthy | Gasoline, diesel, chemical |
| Water color | Often rusty or orange | Usually clear |
| Residue | Slimy deposits | Greasy feel |
| Toilet tank | Rusty, slimy buildup | Clean, film on surface only |
| Health risk | Aesthetic only | Serious hazard |
What To Do If You Suspect Petroleum
If your tests suggest petroleum contamination (reforming film, fuel smell), take these steps:
- Stop drinking and cooking with the water
- Do not run dishwashers or washing machines
- Avoid bathing in the water (skin absorption is possible)
- Contact a certified water testing lab immediately
- Report to your county environmental health department
Testing for Petroleum
Request a lab test for:
- VOCs (Volatile Organic Compounds): Includes benzene, toluene, ethylbenzene, xylene (BTEX)
- TPH (Total Petroleum Hydrocarbons): Broad screen for petroleum products
- MTBE: Fuel additive that indicates gasoline contamination
Cost: $150-400 for a comprehensive petroleum panel.
Sources of Petroleum Contamination
If tests confirm petroleum, investigate these potential sources:
- On your property: Old buried fuel tanks, tractor/equipment storage areas, stored chemicals
- Nearby: Gas stations (current or historical), auto repair shops, farms with fuel storage
- Historical: Underground tanks from previous property owners (home heating oil was common decades ago)
- Regional: Industrial sites, landfills, or contamination plumes that migrated to your area
Remediation for Petroleum
Petroleum contamination requires professional remediation:
- Identify and eliminate the source
- Install activated carbon filtration (removes most petroleum compounds)
- Consider air stripping for volatile compounds
- In severe cases, the well may need to be sealed and a new one drilled
- May qualify for state cleanup funds if from a regulated source (gas station tank)
Treating Iron Bacteria (The Common Cause)
If testing confirms iron bacteria (or the stick test clearly shows biofilm), here's how to address it:
Shock Chlorination (First-Line Treatment)
Shock chlorination involves adding a high concentration of chlorine bleach to the well, circulating it through the entire system, then flushing. This kills iron bacteria and removes biofilm.
Process overview:
- Calculate chlorine needed based on well depth and casing diameter
- Pour chlorine bleach into well (typically 1-3 gallons for residential wells)
- Recirculate by running water through a hose back into the well
- Run all faucets until you smell chlorine, then stop
- Let sit 12-24 hours
- Flush thoroughly until chlorine smell is gone
Cost: $20-50 DIY, $200-400 professional.
See our detailed iron bacteria treatment guide for step-by-step instructions.
Maintenance Chlorination
Iron bacteria often returns after shock treatment. Options for ongoing control:
- Annual shock chlorination: Repeat the process yearly
- Chlorine injection system: Continuous low-level chlorination ($800-1,500)
- Ozone treatment: Chemical-free oxidation ($1,500-3,000)
Physical Cleaning
For severe iron bacteria infestations:
- Pull and clean the pump (biofilm accumulates on pump and pipes)
- Brush or jet the well casing to remove buildup
- Replace severely affected drop pipe sections
- Clean or replace pressure tank bladder
Filtration
Filters that help with iron bacteria:
- Oxidizing filters (Birm, Greensand): Remove iron that bacteria feed on
- Sediment filters: Catch biofilm particles (require frequent changes)
- UV treatment: Kills bacteria passing through (doesn't remove biofilm)
Other Causes of Oily Appearance
Less common causes of oily-looking well water:
Decomposing Organic Matter
Decaying leaves, insects, or animals that entered the well can produce oily-looking compounds. Signs include:
- Sudden onset after a storm or seasonal change
- Decaying or rotting smell
- Visible debris in water
Solution: Clean the well, ensure the cap is properly sealed, and shock chlorinate.
Hydrogen Sulfide
High levels of hydrogen sulfide can create a slight oily sheen. More commonly noticed as rotten egg smell. See our sulfur smell guide.
Tannins
Organic compounds from decomposing vegetation create tea-colored water that can appear to have a film. More common in areas with organic soils. See our tannins guide.
Manganese
High manganese levels can create iridescent films and black staining. Often accompanies iron issues.
| Cause | Appearance | Smell | Treatment |
|---|---|---|---|
| Iron bacteria | Iridescent film, breaks apart | Musty, swampy | Shock chlorination |
| Petroleum | Rainbow sheen, reforms | Fuel, chemical | Carbon filtration, remediation |
| Organic decay | Slight sheen with debris | Rotting, decaying | Clean well, seal cap |
| Hydrogen sulfide | Faint film | Rotten eggs | Aeration, oxidation |
| Tannins | Tea-colored, possible film | Earthy | Anion exchange |
Prevention: Keeping Your Well Clean
Prevent iron bacteria and organic contamination with proper well maintenance:
Well Cap and Seal
- Use a sanitary well cap (vermin-proof design)
- Ensure the seal is intact with no gaps
- Check and replace damaged seals annually
- Keep area around wellhead clear of debris
Annual Maintenance
- Test water quality annually
- Inspect well cap and visible components
- Flush water heater to remove sediment
- Consider preventive shock chlorination if iron bacteria is recurring
Environmental Protection
- Store fuel and chemicals away from the well (minimum 50 feet)
- Direct surface runoff away from the wellhead
- Maintain proper septic system (minimum 100 feet from well)
- Report any spills near your well immediately
When to Call a Professional
While iron bacteria can often be treated DIY, call a professional for:
- Suspected petroleum contamination
- Iron bacteria that returns repeatedly despite treatment
- Severe biofilm requiring pump pulling and cleaning
- Unknown contamination source
- Any situation where you're uncertain about safety
Oily Water? Get Answers Today
From testing to treatment, SCWS can diagnose your water quality issue and provide solutions.
Call SCWS: (760) 440-8520
Water testing • Iron bacteria treatment • Contamination assessment • San Diego County
Frequently Asked Questions
Why does my well water have an oily film on top?
Oily film on well water is usually caused by iron bacteria, which create a biofilm that looks like oil. Less commonly, it can indicate petroleum contamination or decomposing organic matter. The key difference: iron bacteria film breaks apart when disturbed, while true oil reforms into a continuous layer.
How can I tell if it's iron bacteria or real oil?
Stir the water with a stick. Iron bacteria biofilm breaks into fragments that don't reconnect. Real petroleum oil immediately flows back together into a continuous sheen. Iron bacteria also typically produces a rusty color and swampy smell, while petroleum has a distinct fuel odor.
Is oily well water safe to drink?
If the cause is iron bacteria, the water is safe but unpleasant — iron bacteria are not harmful to human health. If the cause is petroleum contamination, DO NOT drink or use the water until it's been tested and properly treated. When in doubt, stop using the water and get it professionally tested.
How do I get rid of iron bacteria film?
Shock chlorination (super-chlorinating the well) kills iron bacteria and removes biofilm. The treatment involves adding chlorine bleach to the well, circulating through the system, letting it sit 12-24 hours, then flushing thoroughly. This may need to be repeated annually, or you can install continuous treatment.
What causes petroleum contamination in wells?
Common sources include: underground storage tank leaks (current or historical), fuel spills near the wellhead, improper disposal of motor oil, nearby gas stations or industrial sites, and road runoff containing automotive fluids. Even old contamination can migrate through groundwater to reach wells years later.
Can I filter out the oily film?
For iron bacteria, oxidizing filters that remove iron can help reduce biofilm by eliminating the bacteria's food source. For petroleum, activated carbon filtration removes most petroleum compounds. In either case, addressing the source is important — filters alone won't solve the underlying problem.
Why does the oily film keep coming back?
Iron bacteria are present in most groundwater and can recolonize your well after treatment. If biofilm returns quickly, you may need more aggressive initial treatment, ongoing chlorination, or investigation into whether the bacteria are entering from the surface. Ensuring a proper sanitary well cap helps prevent reinfection.
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