Oil pollution to a watercourse is often obvious. A rainbow sheen (as seen below) with an associated hydrocarbon smell could indicate a pollution. More concentrated spills of oil can be less obvious as they do not produce a rainbow sheen but will have a stronger smell. Red diesel however shows up as a red colouration (see examples below).Natural sheens can also be formed. They don’t have any associated smell. The best way to tell the difference is to throw a stone or stick in the sheen. If the sheen instantly comes back together it could be oil. If it breaks up into small segments and doesn’t instantly reform it could be natural. If in doubt report.