Since we bought a house in 2009 (then 5 years old), I have noticed a slow accumulation of (what i'm assuming is) oil sitting in the grass and on the stepping stones on the outside of our house, where there is a small pipe that constantly drips water. On the other side of this wall is our garage and water heater. The area that is oily never dries and now encompasses a 5x3 ft area. The stepping stones in that area always have a thick black rainbow sheen to them. We are stumped. Any ideas about what is causing this?