I am no structural engineer, but that sure looks like sheets of concrete were simply laid down over rocks. Where is the support for the weight of all that water?
You may find an answer or somebody who knows the answer or somebody who can get the answer on this link.
http://www.freerepublic.com/focus/news/3524221/posts
Go to the end of the link and work backwards a bit to find the real technical gurus on the thread.
The concrete slab over the bedrock is fine, although they may consider drilling the bedrock and doweling that into the concrete slabs to eliminate future movement.