The Question: For years I have noticed an oily substance on a stretch of Lake Huron shoreline that leads into a wetland/marsh area. The oily substance has the rainbow pattern that you see when there is a film of oil on water. Is it possible that this is a natural occurrence of some oil deposits, or should I investigate it as an environmental contaminant? There is a road about 30 yards uphill from this spot, and for years it used to be a dirt road that the city sprayed oil on to keep the dust down.
The Short Answer: Geoff Peach, Coastal Resources Manager at The Lake Huron Centre for Coastal Conservation, in Ontario, Canada,
looked at your pictures. His response:
“This is a common wetland phenomenon. You generally see it in shallow pooled water amongst the vegetation. The oil is produced by microbes and we tend to see it when it becomes concentrated, as it does in these isolated pockets.”
More Information: We often think of the natural world as separated into animals and plants. Or maybe multi-cellular life and single-celled life. Or if you are a biologist, maybe eukaryotes (animals, plants, fungi) and prokaryotes (bacteria, archea). But another interesting way to divide the natural world is into those organisms that live in habitats with free oxygen (aerobes) and those that live in habitats without free oxygen (anaerobes). In habitats with oxygen, aerobic bacteria release carbon dioxide. In habitats without oxygen, anaerobic bacteria release methane. The surface of a marsh is aerobic – it has enough free oxygen for plants, animals, and aerobic bacteria. But because there is so much biological activity by all the oxygen-loving organisms above ground, most of the oxygen in the water gets quickly used up, and the resulting low oxygen water seals off the underlying mud from atmospheric oxygen.
Just a couple of inches down, there is no free oxygen and the anaerobic bacteria rule a world of drowned muck.
Because methane is a small, easily evaporated molecule, most of the methane produced by the anaerobes in their mud world escapes into the atmosphere. But some small percentage of it gets converted into larger hydrocarbons that are less likely to evaporate. They still are lighter than water, however, so they float on the surface. There is little difference between these naturally produced compounds and hydrocarbons like gasoline or oil, so the sheen on the water looks the same as if someone had spilled gas or oil.
Another possible source for oil on the water of a marsh is oil released directly by plants, or oil released when plants and animals die. Because there is so much living and dying of plants and animals in a marsh, there is a fair amount of oil produced.
Having said all that, of course, it is always possible that a gasoline or oil spill nearby caused this sheen. Lakes with heavy boat traffic are always subject to spilled gas and oil. Or it could be oil washed off from the oiling of the road, as you suggest. It’s hard to know for sure, and impossible to tell from pictures alone, of course. You’ll just have to use your judgment as to whether what you are seeing could be the natural marsh production of hydrocarbons, or something too thick and heavy and localized to be natural.