Would concrete and tarmac be impermeable rock types?
concrete is permeable to a certain extant but can vary in permeability if a waterproofing agent is added to the concrete mix before it is thrown and set, precast concrete is also less permeable than cast-insitu (cast on site) as the precast elements are cast in moulds and vibrated to remove air. but eventually water will always seep through. Tarmac, tar mixed with gravel, is theoretically im permeable, but only if it is mixed, laid down and compressed correctly.