"Many springs or seepages from dolerite display what appears to be an oil scum. FeIII oxide phases (haematite, limonite) dissolve in near neutral water under reducing conditions produced by organic environments (soils). When these waters interact with the atmosphere FeII iron is also oxidised at the surface and the films are a mixture of iron types. Fe bacteria may form part of these processes and the films can thicken to look like rust if nutrient resources are sufficient...Oil has been observed to seep through dolerite...and it is important to identify the nature of any films. Metal oxide films break into irregular plates if stirred with a stick, whereas oils will swirl on a thinner intact film. Most occurrences are oxide films."
So when you see an oily scum on the surface of a puddle, stir it with a stick. If it breaks into plates, the water isn't polluted with oil, and is ok except perhaps for those iron-eating bacteria!