Is it Oil, or Is It Iron?

The Pennsylvania Department of  Environmental Protection would like to remind you all that not all scum is pollution. Oil Sheens - All That Glitters ... Isn't Necessarily Pollution

One of the most visible and obvious signs of water pollution is an oil sheen. Seeing a shiny, translucent reflection on a pond or lake just looks wrong - but did you know that not all sheens are the result of pollution? Here is what you should know about sheens, how to tell the difference between naturally occurring sheens and those that are the result of pollution, and what you should do if you discover a sheen caused by pollution.

A sheen is "an iridescent appearance on the surface of the water." The iridescent, lustrous appearance could be caused by a petroleum spill finding its way into the water - or the sheen could be a naturally occurring result of iron bacteria. Iron bacteria are small living organisms which naturally occur in soil, shallow groundwater, and surface waters and are harmless to the environment. Petroleum sheens on the other hand are harmful to the environment and should be reported to the Pennsylvania Department of Environmental Protection (DEP). But how can you tell the difference between the two and what should you do if you discover a sheen caused by pollution?

A very simple, yet very effective method to tell if an oil sheen is organic or the result of pollution, is to throw a rock into the sheen, or to break it apart with a stick. A bacterial sheen will typically break into small platelets when disturbed while a petroleum sheen will quickly reform. Another way to tell them apart is by smell. Natural sheens don't smell like petroleum.
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Please remember, the next time that you see an oily sheen on surface water, do the rock test to determine if the sheen is organic or the result of pollution and report all pollution sheens to DEP. 

I take from this that Pennsylvania has a substantial number of false reports of oil pollution caused by iron bacteria. The effect can be exacerbated by mines, like, say coal mines that Pennsylvania has in abundance, that can bleed acid mine waste loaded with the reduced iron that iron bacteria grow on, and cause the film, thus being a sign of pollution, albeit a different type of pollution than oil.

Several different pieces of ironstone on the beach today

However, the process is, in fact natural in many places, and if it goes on long enough can produce a well known form of iron ore, bog iron, which Vikings exploited as iron ore.

The process is quite common around here, usually in drainage ditches. Moreover, we have lots of evidence for the process from the past, in the form of ironstone, a form of sandstone commonly found in the ground, and washed up on our beaches.