Quick Answer:
A rainbow film on filled water usually comes from a thin oily layer or dissolved minerals that create a rainbow effect in light. Do a simple visual check by filling a clear glass and holding it under bright light to confirm the pattern. Treat it as a contamination concern until proven otherwise; avoid drinking the water and investigate.
Why This Happens
- Oily residue: Small amounts of oil or hydrocarbons on the water surface create thin-film interference, which looks like a rainbow sheen.
- Plumbing work or pressure events: Disturbances in the distribution system can dislodge pipe deposits or introduce lubricants from repairs, producing a slick film. See **Slick feel after city repair** for related situations.
- Air and thermal effects: In some cases a film can form from natural organic matter or soap residues concentrated at the surface.
- Localized fixtures: A problem at one tap (rubber washers, sealants, or recent maintenance) can cause a film only at that outlet.
Step-by-Step What to Do
Step 1 — Don’t drink it
Stop using the water for drinking, cooking, or brushing teeth until you know more.
Step 2 — Visual test under light
Fill a clear glass with cold water from the affected tap. Hold it under a bright light or sunlight and tilt the glass slowly. Look for thin rainbow patterns moving on the surface — this confirms a surface film rather than a dissolved color or turbidity.
Step 3 — Note smell and feel
Smell the water from the glass cautiously; a petroleum or solvent odor is a serious sign. If it looks oily, you may notice a slippery feel on your fingers when you touch the surface.
Step 4 — Check other taps and neighbors
- Test other faucets, both hot and cold. If only one outlet shows the film, it’s likely a local fixture issue.
- Ask a neighbor if they see the same problem. If multiple homes are affected, the supply or street work may be involved.
Step 5 — Flush the lines
Run the cold water for several minutes at the affected tap and at a nearby tap to see if the film clears. If you recently experienced pressure changes, the film may wash out — refer to **Slick feel after pressure change** for similar scenarios.
Step 6 — Collect a sample if it persists
If the film remains after flushing, collect a sample in a clean glass or bottle, seal it, and keep it in a cool place. This can be useful if you later need to show it to your water supplier, municipality, or a lab.
Step 7 — Notify the water utility or a plumber
Report the issue and the tests you ran (visual check, which taps, duration of flushing). Follow their guidance on next steps or testing.
What Not to Do
- Do not assume rainbow film is harmless. Treat it as a potential contamination until tested.
- Do not light candles, run heaters, or use open flames near suspect water if you detect a petroleum smell.
- Do not try unapproved chemical treatments or DIY remedies that could make the water unsafe or void warranties.
- Do not dispose of large volumes of contaminated water down floor drains without checking local guidance, especially if you suspect hydrocarbons.
When to Call a Professional
- Call your water utility immediately if multiple homes are affected, if you detect a fuel/solvent odor, or if the sheen is heavy and persistent.
- Call a licensed plumber if the problem is isolated to one tap, if you suspect fixture materials or grease, or if flushing and simple checks don’t clear it.
- Consider professional testing (municipal or certified lab) when you detect odor, skin irritation, or if you need confirmation for health or legal reasons.
Safety Notes
- Avoid tasting or swallowing water you suspect is contaminated.
- Wear gloves if you must handle contaminated water or samples; wash hands thoroughly afterward.
- If you smell a strong petroleum or chemical odor, ventilate the area and contact your utility; prolonged exposure can be harmful.
- Keep children and pets away from the affected water until the source is identified and resolved.
Common Homeowner Questions
- What should I do right now?
Stop drinking the water, fill a clear glass and check it under bright light, then flush the tap and test other outlets. - Is it likely petrol in the pipes?
Not usually, but a petroleum smell or oily sheen is serious—report it to your water utility for testing. - How long before I can use the water again?
Use is safe once the film is gone, no odor exists, and your utility or plumber confirms the source is resolved or testing clears the water.
Related Articles
If you’re troubleshooting a similar symptom, these guides may help:
For the full directory, see Oily Sheen, Slick Feel, or Soapy-Like Water.