Yuck! Is It Polluted?

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“Yuck! Is it polluted?” is a frequent question when people see our ditches and waterways with fluffy orange growths, orange or red colors, or a surface sheen. Each of these conditions can occur through natural processes that depend on the presence of iron.

Photo of natural sheen in ditch.

Natural sheen in ditch; Photo: D. Rashash, Oct. 2015

NC soils contain a lot of iron, which gives our soils varying shades of yellow, orange, and red. Examples of some of the varying soil colors are shown in the photo below.

Color in soil due to iron

Red & orange mean iron!, Photo: D. Rashash, Oct. 2015

In the photo, iron content increases from left to right. When oxygen is present in the soil, the iron is in the Fe+3 form. Fe+3 is not soluble and is firmly attached to its soil particle. When conditions are right – high water table, little oxygen, organic matter present as food, and active soil microbes – the iron is reduced to Fe+2. This form of iron is colorless and soluble; it is able to move with the surrounding water through the soil.

When the soluble Fe+2 reaches a ditch or waterway, naturally occurring bacteria in the water utilize it. The microbial reaction re-oxidizes the iron to Fe+3 and combines it with oxygen in the water to form iron oxide (Fe2O3). Yep, rust! You can see this as staining on curbs, orange fluffy growths, orange or red water, or an oily sheen.

It is easy to check if the oily sheen is natural or the result of a spill. Use a stick to stir the sheen. If it breaks into irregular jagged chunks – like puzzle pieces, it is natural; if it swirls smoothly back together, it is more likely a hydrocarbon spill. With the recent rains, flooding, and addition of organic matter; expect to see a variety of colors and growths in our waterways, ditches, gutters, and sidewalks.