Lets say a good earthen dam and core is built in a draw thats going downhill and has porus soils.
Each side and the bottom of the draw has solid non porus material that the core was dug into.
When the pond fills, the porus soils that are in the basin area of the pond would fill with water also, possibly very slowly.
I think this happens at some pond sites.
I would not think of this as a leak, since the water cannot escape through the sides, bottom, or past the dam.
