The root of the problem is that the land is reclaimed mine land, or least that's the story. Given that, the company that was in charge of restoring it only needed to place 12" of topsoil. All of that is now in the pond, or worse, gone off the site. You are talking about draining a pond, mucking it out with a serious set of dozers, probably D8s. Possibly a large excavator at some point in there. Spreading out material for a few months to dry, then either scraping it back up, loading it in trucks, or using scrapers to get it back on the track. It might be cheaper to just haul in new dirt.