Bacterial sheens on water can often look like oil. However, they are completely harmless and natural. Below is a simple activity you can do when you come across a sheen to help you identify what it is. Petroleum (oil or gas) sheens and spills need to be reported, the bacteria ones do not.
- A rock or a stick
If you come across a sheen in the water observe it.
Use a stick or a rock to poke a hole in the sheen. In a bacterial sheen, "the sheen will typically break into small platelets. In contrast, a petroleum sheen will quickly try to reform after any disturbance."
If you were unable to determine the nature of the sheen from the test above, stand close to the sheen if you can without touching it and waft the air above it towards your nose. If it smells of oil or gas, it's likely a petroleum product that needs to be reported, if it doesn't smell, it's likely a natural bacterial sheen.
If you've found that the sheen is potentially oil from the tests above it needs to be reported. To report a release or spill, contact the federal government's centralized reporting center, the National Response Center (NRC), at 1-800-424-8802.
Here is a post form the Minnesota Pollution Control Agency on this: https://www.pca.state.mn.us/sites/default/files/c-er4-07.pdf