Mystery Pond Film

Question
Hello,

I am currently in charge of maintaining a storm water run off pond for work. I was out there today and noticed a strange film. First, some background on our set up.

We have a large scrap yard, with some underground and above ground systems to channel all of our ground water to our pond. Before entering and leaving the pond, the water passes through some hydrocarbon filters as well as a general filter. Our intake manhole contains hydrocarbon pads, after which water goes through two boom filters. Our Outlet contains the general filter (not sure of composition, new to the position). We also have a fountain set up outside the secondary boom for water circulation.

The Film- It is inside the manhole, only on the suurface, and is being stopped, but not absorbed, by the pads. Where the water is moving, right below the pipe, it is still fairly fluid. On the outside edges, however, where the water moves less, it is turning to a floating solid. When pushed with a stick, it moves and breaks up like a thin sheet of ice would on a puddle. It is fractions of a millimetere thick, and when I attempted to pick up a piece, it crumbled. Once seperated, it doesn't reform like a hydrocarbon sheen does, but stays in seperated pieces. There is also a strong, unpleasant but natural smelling odour which I could not identify.

Any help would be greatly appreciated,

D.

Answer
Hi Daniel:  There are only a few substances that are lighter then water or that would float.  My biggest fear is Mercury or Mercury mud.  Another source would of course be oil or an oil byproduct but that would leave a sheen on the water regardless.  I would take a sample to your local county department of toxic substances or even the HAZMAT unit.  It is probable that something has seeped into or collected in your run-off channels and the larger particles are being stopped by the smaller filter pads.  Whatever it is may be harmless but I would err on the side of caution.  Do not handle it until you know what it is.  Please keep me posted... dave