Tag: Soil

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

Dirt is Just Dirt, Right?

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Figure 1: Area of Pendleton King Park classified as a wetland.

By Kelsey Laymon

Wrong! Soil is so much more than what people typically think. It is the basis for sustaining life on earth by supporting food and biomass production, regulating water quality, storing carbon and maintaining the balance of gases in the air, providing habitats, and sustaining biodiversity. The quality of soil is an important factor in the quality of the ecosystem. Its texture, water-holding capacity, porosity, permeability and organic matter content all affect the surrounding ecosystem.

Wetland soils, are formed under conditions of saturation – when flooding or ponding lasts long enough to cause anaerobic, or oxygen-poor, conditions in the upper soil layer. Wetlands themselves have three defining characteristics: (1) the presence of standing water, either seasonally or permanently, (2), unique wetland (hydric) soil, and (3) vegetation tolerant of saturation.

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Figure 2: Soil Profile

So, if a wetland soil must have wetland plants, water, and anaerobic conditions, why was a section of the soil at Pendleton King Park in Augusta, GA regarded as a wetland based on the U.S. Army Core of Engineers’ parameters when it looks like a pine forest (Figure 1)?

Looking at the soil from Pendleton King Park, it is technically a wetland. Oddly, the soil does have hydric properties. The soil has a gray color from a process known as gleying. When soil is wet for an extended period of time, anaerobic conditions will result because oxygen diffusion through the soil is extremely slow. There is only a thin surface layer that will support normal aerobic, or oxygen loving, root respiration and the deeper layers remain reduced. Reduction is a chemical process where electrons or hydrogen ions are gained and an oxygen molecule is given up. Nitrogen, manganese, iron, sulfur, and carbon are all reduced in a wetland environment. Iron reduction was seen in the orange mottles, or speckles, in the layer labeled Ba in Figure 2. Mottles that are orange/reddish-brown are from the reduction of iron, and mottles that are dark red or black are from the reduction of manganese (observed in the layer labeled Bc in Figure 2). The reduction of manganese can also be observed in the thick black layer, layer Bb in Figure 3. Another observation that the research team noticed was a rotten egg odor at about a 2.5 ft depth. This odor is only observed when sulfate is being reduced. These chemical processes are necessary for the break down of nutrients so the plants can use them to grow.

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Figure 3: Aerial view of Pendleton King Park

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Researcher attempting to understand soil.

If the soil was classified as wetland, then why were there pine trees growing in it? One theory is that the area was a forest next to a natural wetland, but during Augusta’s development the natural hydrology changed. A railroad and a paved road were both built alongside this wetland seen in Figure 3. The railroad and the road were both built up several feet on a berm. During high periods of rainfall, these features kept the water from running off naturally. The water had nowhere to go, so it just sat for long periods of time. Perhaps if the conditions stayed the same for another 50-100 years, all of the pine trees would die because they wouldn’t be able to transport oxygen, and water-loving plants, like cattails and water lilies, would move in. The pine trees would not survive because, unlike the water loving plants, they do not have the aerenchyma, or air spaces, in their roots to allow the plants to get oxygen. Other plants like reeds, and sedges have hollow stems to transport oxygen.

So to answer the question “Is dirt just dirt?” No! It is the basis to sustain life and can be used to predict changes in the environment. It influences the distribution of plant and animal species and provides nutrients for those animals. It is important to understand the aspects of soil in order to understand how the ecosystem around it will function.