Is that an oil slick in the stream?

Francesca "Frankie" Bigica is a sophomore at UVM studying Environmental Studies in the Rubenstein School of The Environment and Natural Resources.

As a sophomore at UVM, I’ve spent a good deal of time wandering around Centennial Woods in the past year and a half. On multiple occasions, I noticed stuff floating on top of some of the low-flowing areas of the stream. It was a shiny, oil-like substance that looks like what you might find in a puddle of rain on the street. With my naïve knowledge of the wilderness and my Environmental Studies mind-set that makes me think pollution and human-impact is everywhere, I assumed this substance was actually oil or petroleum. I figured it was probably just runoff from the nearby campus buildings and roads. It always saddened me to think about the fact that even in a protected natural area, human impact had such obvious presence. Then, during one of my Natural History of Centennial Woods class, my friend and I started discussing the oily substance that we spotted underneath a bridge. My TA, Sam Hubert, overheard the discussion and stepped in to inform us it was not actually oil but a bacterial build-up that was completely naturally occurring. He said that our professor Teage had once told him about it. He told my friend and I that it was common that people misinterpreted the oily-film for pollution including some of his Rubenstein peers who had been studying Centennial Woods.

Upon further research, I discovered the oily sheen on top of the water was iron-oxidizing bacteria. Iron is a very common element found in soil and water. These bacteria feed on the iron in the water. When the iron is dissolved it reacts with the oxygen in the air creating an oily film on the water. It can also cause a fuzzy, bright-orange deposit to build up on the stream, as well as an unpleasant rotten-egg like odor. This deposit of oxidized iron is completely harmless to fish and aquatic life as it is a natural occurrence in low-flowing streams, much like the one in Centennial Woods. It occurs commonly in areas with acidic soils that contain substantial levels of iron.

If you spot an oily substance on the surface of a stream and think it's petroleum pollution and not iron oxide, there is a simple test to find out. Just grab a stick and poke at it. If the oily sheen swirls back together after being disturbed by the stick, it could be due to runoff of petroleum products which is potentially hazardous to the water supply and aquatic animals. If the oily sheen fractures and breaks apart into pieces and does not come back together, then it is iron oxide and is no cause for concern. Eventually, the natural flow of the stream will carry off the iron oxide and clear the water.

Thankfully, my natural history mystery was solved with a happy ending. It’s comforting to know that while walking around Centennial Woods, I will no longer be confronted by the idea of petroleum pollution in the stream, but assured that the oily-sheen I see is just nature at work.