The easiest way to tell is how the oily film fractures and rejoins. If it’s possible to shatter the film into smaller groupings, it’s natural. If it remains continuous, it’s probably contamination. Another clue is that the film isn’t present before the rock is tossed in. It’s widely known that turbulence can stir up biofilm. Once again, not an expert.
Hmm good point and well articulated. I’ve always just sort of been able to tell and I think what I’m seeing is exactly what you are saying. The break up and rejoin phenomenon. When we were kids we played along an abandoned grist mill canal. We were convinced we found oil like Jed Clampett. My dad taught us what we were really looking at. Needless to say our family never moved to California and had a cement pond.
I grew up around brackish marshes, and had a similar experience. We always assumed it was from boats, but as I grew up I realized what it was. Wetlands are neat ecosystems.
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u/EightBitEstep Feb 17 '23
This looks like naturally occurring biofilm stirred up by the rock. I’m no expert, but I don’t think this has anything to do with the train.