Oily Water: What Causes it & is it Dangerous?

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An oily sheen or film on your drinking water could be caused by several natural sources or human causes and may have damaging health effects.

Here, we’ve discussed everything you need to know about water with an oily film, including what it is, what it’s caused by, and how to remove the contaminants responsible.

📌 Key Takeaways:

  • An oily film or sheen on water may be caused by iron bacteria, decaying plant matter, oil spills or improper oil disposal, or various contaminants from agricultural, industrial, and urban runoff.
  • You can determine the source of an oil sheen on your water by considering your water source, stirring and smelling your water, and conducting a water test.
  • Some of the methods to fix an oily film on water are chlorine injection, reverse osmosis, and shock chlorination.

🤔 What Is Oily Water?

Oily tap water has an oily film floating on the water surface. Certain contaminants can cause oil sheens on water, giving water a glazed, filmy, or discolored appearance.

An oily sheen on faucet water is rare, and may be caused by natural sources or human activities.

🔎 What Causes An Oily Film On Water?

There are several different causes of an oily film on water.

Causses and effects of an oily film on water

These include:

  • Decaying organic plant matter – A common cause of natural sheens in well water is decaying plant matter in the well, producing hydrogen sulfide.
  • Iron bacteria – Water with an oily bacterial sheen is usually caused by iron bacteria. You’ll also notice orange or reddish-brown stains on your fixtures and water-using appliances and metallic-tasting water.
  • Oil spills – A local oil spill could cause oil to be carried via runoff into a water source that is eventually used for drinking.
  • Industrial discharge – Some industrial processes can cause oily substances as byproducts, which may end up in water bodies due to accidental leaks or improper waste disposal.
  • Agricultural & urban runoff – Certain agricultural chemicals and motor oil/grease from roads may be carried by rainwater into a water source that’s used for drinking.
  • Improper cooking oil disposal – Restaurants may improperly dispose of cooking oil, causing it to contaminate drinking water supplies.

You’re more at risk of drinking water with an oily sheen if you have a private well. Unlike municipal water, well water isn’t regulated, and it’s the responsibility of the well owner to test and treat the water for safety.

While public water systems routinely test their tap water supplies, most well owners only test their water once a year. That means your well might suddenly become contaminated by a pollutant – and it’s your job to remove this pollutant with a suitable water treatment method.

⚠️ Is An Oily Film On Water Dangerous?

In most cases, the oily substance in your water is caused by a natural source (such as iron bacteria or decaying organic matter/hydrogen sulfide). These naturally occurring contaminants don’t usually pose a health risk, though they do affect water quality and taste.

However, if your water has an unmistakable petroleum sheen or you have reason to believe that it’s caused by manmade pollutants, especially if these are present in high levels, your health might be at risk.

Some chemicals, fuels, and oils can have serious long-term health effects if they’re ingested even in low concentrations. That’s why it’s important to get your water tested if you notice an oily film and can’t be certain of the cause.

rainbow film on water

🔬 How To Determine The Cause Of Oily Film On Water

There are a few different methods that we recommend when determining the cause of the oily sheen in your water:

  1. Consider your water source. If you get your water from a well, natural sources are likely the cause of the oily film. However, if the well is located near a factory, agricultural site, or an oil/fuel storage facility, there’s an increased risk of contamination from harmful pollutants. If you have city water, the iridescent film could be caused by other sources, which might have more dangerous health effects.
  2. Smell your water. Smell your water to get an idea of what you’re dealing with. A rotten egg smell indicates the presence of hydrogen sulfide, while a diesel or gasoline smell is more concerning, indicating petroleum contamination.
  3. Stir your water. You can also narrow down the cause of the oily film by stirring your water. Fill a glass, then stir the water with a spoon. If the sheen breaks apart but quickly reforms after you stop stirring, the issue is likely caused by oil pollution. If the sheen breaks down and doesn’t reform, it likely has a natural cause.
  4. Test your water. A water test will tell you exactly what your water contains. We recommend reaching out to a certified laboratory to discuss your water quality issues and as for advice on the best water test to detect the contaminants to blame.
water testing with tap score

🚫 How To Prevent Oily Film On Water

Here are the best ways to respond to an oily film problem in your drinking water.

Get Your Well Professionally Inspected

If you have a private well and you’re not completely sure about the cause of the oily sheen on your water and whether or not it can be prevented, contact a well contractor to arrange for a professional in-home inspection of your well.

A well inspector can assess your well for structural integrity and determine whether there are any issues with the well design that may be causing your water supply to be contaminated. They can also test your water to help you clarify the cause of the contamination.

Use A Water Treatment System

Once you know the cause of the oily film on your water, you can look at water treatment methods to remove the problem contaminant.

Here are some of the water treatment systems to consider, depending on the cause of the issue:

Chlorine injection systemRemoves & prevents the formation of iron bacteria.
Reverse osmosis systemRemoves up to 99.99% of all total dissolved solids, including most causes of an oily film.
Water distillerRemoves the majority of impurities, including many that cause an oily film.

If you discover that iron bacteria is the cause of the oily film, you may also need to shock chlorinate your water.

Shock chlorination is when you treat your well with chlorine, which kills iron bacteria and clears out your well, preventing regrowth. Read our full guide on shock chlorinating a well here.

Pouring chlorine to Springwell chemical injection system

📑 Final Word

The best solution to your oily film water problem depends on the cause.

Because of the risk of petroleum contamination, we strongly recommend that you report the issue to your local water utility or health department immediately (if you’re on a city water supply) or stop drinking your water while you get it tested and arrange for a well inspection (if you use well water).

Thankfully, the most common causes of an oily sheen are natural and harmless, but if you’re unsure, switch to bottled water while you work on getting to the bottom of the problem.

  • Brian Campbell
    President & CEO, CWS, CWR

    Brian Campbell, a WQA Certified Water Specialist (CWS) and Certified Water Treatment Representative (CWR) with 5+ years of experience, helps homeowners navigate the world of water treatment. After honing his skills at Hach Company, he founded his business to empower homeowners with the knowledge and tools to achieve safe, healthy water. Brian's tested countless devices, from simple pitchers to complex systems, helping his readers find the perfect fit for their unique needs.

2 thoughts on “Oily Water: What Causes it & is it Dangerous?”

  1. Avatar for Brian Campbell

    Hi, just wandering how Reverse Osmosis System works.
    I came across this while surfing the Internet for answer to the oily surface of water in our private residence.

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