how to detect oil in a water sample
You might want to ask your public health department. I asked an engineer/pilot, and he suggested the same visual test used by pilots of small airplanes. You can see an example of this at http://m.youtube.com/#/watch?v=kN_1dR-f7R4&desktop_uri=%2Fwatch%3Fv%3DkN_1dR-f7R4. Of course, not all oil products settle out this nicely. You might have a need for a more refined measurement. Again, check with your health department. It can get complicated (http://www.pdhengineer.com/courses/en/M-4023.pdf).