A Note on Sheens (not Charlie or Martin)

I recently read a Phase I ESA report where the inspector noted a sheen on water in a stormwater inlet (also referred to as a catch basin). The Phase I ESA report identified this sheen as a recognized environmental condition (REC), and recommended performing a Phase II investigation in the vicinity of the inlet. There are often grey areas and disagreement on what constitutes a REC, and in this case I had to disagree with the consultant’s findings. Here’s why.

In urban areas, stormwater inlets are frequently fitted with devices on the exiting pipe to prevent floating materials from entering the pipe.  The pipe is typically connected to a combined sewer system (both wastewater and stormwater). Cities require these traps to be installed to minimize the amount of trash that needs to be screened at the treatment plant. Most of us have noted sheens on stormwater in streets and parking lots at the start of a rain event. These are an inevitable result of automobile use. The oil stains that one sees in parking stalls are normally considered “de minimis” observations that do not meet the definition of an REC.  My conclusion is that the minor sheens resulting from these minor surficial releases of oil, whether observed on the surface or inside a stormwater inlet, would also be considered de minimis. This example further emphasizes the point that was made in my last blog, that knowledge of the ASTM Phase I ESA standard is only part of what is really necessary to draw conclusions that are based on sound science.

And while we’re on the subject of sheens, it’s interesting to note that not all sheens are the same. In addition to oil sheens, there are biological sheens that can result from degrading leaves and other organic matter. How do you tell the difference? An oil sheen, when disturbed, moves in a swirly manner but generally stays intact. On the other hand, a biological sheen, when disturbed, breaks apart into sharp-edged pieces, much like a broken piece of glass. Below are photos depicting the two.

Non-Petroleum Sheen (L)                 Petroleum Sheen (R)