Re: What can cause an orange deposit in a freshwater stream (?algae)

Hi Peter,

The orange stuff you are seeing is a form of iron oxide known as ferric oxide. The other form, ferrous oxide is soluble, but tends to be more stable in an environment without oxgyen such as in the groundwater supply. Ferric oxide is not soluble but tends to form in environments where oxygen is present. Thus, ferrous oxide occurs as a clear solution while ferric oxide exists as orange/red-brown sediment.

Ferrous oxide + Oxygen --> Ferric oxide

When the water seeps into a stream, ferric oxide may spontaneously form from ferrous oxide on exposure to oxygen, especially if the water is acidic. Alternatively, the ferrous oxide may be oxidised by sheathed bacteria to form ferric oxide. As the particles of ferric oxide get bigger they settle out on the stream bed. Either way, the end result is the same. It is true that you typically observe this in streams draining from spoil heaps, but I have also seen this phenomenon in wilderness areas as well. In this case, the iron comes from the rocks and sediments through which the water drains.

Richard Kingsley