Leachate and iron bacterial mat
Leachate is formed when groundwater passes through buried organic material. The organic material decays and oxygen is used up in the decay process. Groundwater passing through the rotting organic material becomes depleted in oxygen and as a result leaches iron, magnesium and other metals from the soil. When the groundwater encounters oxygen, the iron precipitates. There are a variety of bacteria that specialize in taking advantage of the energy available in the oxidizing iron reaction and some of these same bacteria will utilize sulfur as well. Hence, along ditches and springs where low oxygen water reaches the surface these iron fixing bacterial will thrive.
Iron precipitate. Note sheen on water where iron is precipitating on the water surface.
Landfills full of buried organic waste set up good conditions for generating leachate, but naturally buried organics can do the same. If you think about, it is a great way to visualize how many ore bodies form where a sharp change in groundwater chemistry or temperature takes place precipitation will occur. Leachate in drainage ditches and streams gives a glimpse of the process. If the process observed in the pictures last long enough, a small iron rich ore will be left behind. In this particular case the leachate is the result of an old landfill.
But thinking about these bacteria and observing them gives a glimpse at life that thrives in an environment that is very alien to us humans. Dana Hunter posted a jaw dropping picture of Blood Falls in Antarctica and a link to an article on the life forms within the falls by Mikuchi and Priscu (2007) entequilaesverdad.blogspot.