Iridescent Swamp Water

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I was walking on the boardwalk at Phinizy Swamp the other day (near the Visitor Center), and I noticed water with a reflective, fascinating iridescence.

Why is there iridescence on the surface of the water under the boardwalk? Is this pollution?

So I did some reading. The answer is no, this is supposed to be a natural phenomenon and NOT evidence of spilled oil. It’s true that an oil spill can create a pretty rainbow of color in a thin layer across the surface of a natural body of water. But this effect can also be created by a natural swamp process, and that is what seems to be happening here.

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Kelsey Laymon explained the iron reduction process in wetland soils in a recent Research Blog post: https://phinizycenter.org/dirt-is-just-dirt-right/

Because the soil under the water surface stays wet most of the time, there is very little oxygen in it. This means that most of the biological processes that are most familiar to us, the ones that use oxygen, just aren’t happening. Instead, special bacteria that use non-oxygen processes (anaerobic) are at work. Some of these bacteria live by reducing iron that occurs naturally in the soil from one form to another, and the iridescence you are seeing is a result of those processes. The very thin, iridescent film on the surface of the water is a form of iron.

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In a review of internet resources, not all knowledgeable people tend to agree about whether a given patch of iridescence in swamp water is due to iron bacteria interacting with iron compounds, or is an actual petroleum spill. After all, swamps have been used as waste dumps. If the iridescent layer is thick and tarry, and certainly if it smells like petroleum – it probably is.

One quick field test sometimes suggested for determining whether an iridescent layer on water is from iron bacteria or petroleum is to stir it a bit with a stick. If the surface sheen gloms back together smoothly, it’s petroleum. If it breaks into jagged-edged shapes, it’s a natural iron-breakdown phenomenon caused by iron bacteria. Here’s a blowup of the photo above (taken right off the Phinizy Swamp boardwalk), and you can see there are jagged edges on the iridescent film:

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And here’s another photo:

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Iron bacteria contamination happens a lot to people with wells in areas where the soil has a high iron content. We think of it as contamination because iron bacteria working in an enclosed area produce some really unpleasant smells. They can create slime that clogs plumbing, and often a bright orange form of iron precipitate that stains porcelain. Have you encountered this? You don’t want it in your plumbing, but in the swamp, it’s a valuable kind of natural anaerobic process.

-Priscilla Hollingsworth