Be careful that your contractor doesn't build you a leaker. I suggest that you do some reading in the Pond Building Section for the correct pond building methods for excavating and compacting clay soil. It is science and art. Fixing a leaker pond can cost as much or more that the initial construction.
Start by reading through threads in this link. Learn the proper techniques for soil excavation and soil compaction. Many dirt excavators do not know proper pond building and soil compaction techniques. You may have to guide your pond builder to get what you want. Or search for one that is very experienced. Read some of the leaky pond threads to get an education. It is apparent that lots of people pay to have a leaky pond.
http://forums.pondboss.com/ubbthreads.php?ubb=showflat&Number=92487#Post92487
