Sentence:  Take the floating scum or oil <WSD>sheen</WSD> prohibitions. A discharger or an inspector simply can look to see if scum, or an oil <WSD>sheen</WSD>, is coming from a particular  discharge. Assume an oil <WSD>sheen</WSD> begins at a discharge—is the <WSD>sheen</WSD> caused by that ... 
Meaning of the word marked with <WSD> tags:  A thin layer of a substance (such as oil) spread on a solid or liquid surface.