Most of the pond building conversations that I have read talk about packing clay over any bedrock that they uncover. Pourous bedrock can allow water to seep out and cracks in the non-porous rock can do the same if the underlying soils allow that water to travel.
Your concern is very valid. My cousin had a small pond that had uncovered bedrock in it and, at best, it was an eyesore. A real heavy rain would fill it up, but a day later it was way down...so much so that fish could not survive the winter. This concept may be where alot of that 150K came from. Do you have clay availbale on the property?
