What nobody told you is that you must build a good dam by digging down past all of the black soil to clay. You must then scrape all the black soil away in your pond and obtain clay to build a dam with. So your dam must be made of clay, sitting on clay. Lots of pond builders scrape down to clay and build a core wall of clay, then cover and surround it with dirt so that plants will more easily grow on the dam.
Just adding more leaky black dirt on top of your existing leaky black dirt will do you no good. Gotta be well packed clay, all the way to the existing clay subterrainian levels.
