Have you ever noticed orange slime or a rainbow sheen in puddles on the dirt roads and ditches? This may look like someone’s car has leaked fuel or someone’s dumped something they shouldn’t have. The good news is you’re likely seeing the result of a very natural process that is not harmful to people or the environment.

Some species of bacteria that live in water and soil eat dissolved iron. They cause a chemical reaction that turns the dissolved iron (ferrous iron) into an insoluble form of iron (ferric iron) that can appear as reddish/orange staining on the water and soil surface.

The bacteria form a “biofilm” that is just the bacteria floating together on the water’s surface. The biofilm appears as a rainbow sheen that can look very similar to an oil sheen.

How do you tell the difference between biofilm and an oil sheen? It’s easy—just poke the sheen with a stick. If it breaks apart, it’s likely bacteria; if it goes back together, it’s likely a petroleum product (see photo above).

Image of natural sheen created by bacteria – it breaks into tiny pieces and doesn’t reform after being disturbed (source: Michigan Government).

Information for this article was gathered from the following sources:

Michigan Government. Bacteria: A naturally-occurring phenomenon. Department of Environmental Quality. Available from: https://www.michigan.gov/documents/deq/deq-oea-nop-bacteria_378414_7.pdf.

Government of British Columbia. Other aquatic phenomena. Available from: https://www2.gov.bc.ca/gov/content/environment/air-land-water/water/water-quality/algae-watch/recognize-algae/other-aquatic-phenomena.

Redoubt Reporter. Science of the seasons: Yellow boy bacteria has people seeing red. Dr. David Wartinbee. Available from: https://redoubtreporter.wordpress.com/2010/03/24/science-of-the-seasons-yellow-boy-bacteria-has-people-seeing-red/