OK, something got whacked on the way over, your Ponds lowered surface definitions were hosed. Either way I saw the issue in your post, and I've help a user over at the Discussion Groups on this same challenge. Sometimes the Paste operation fails for various reasons. I accept this, and work with it in this manner:
When I need a 'composite surface' the best option is to use the underlying data as the base material for the composite surface definition. In other words I build a NEW surface and incorporate all the items that define the surfaces I might otherwise be tempted to try to paste together.
I suggest that you rename your Mounds Surface, and export it via XML, then change its name back.
Now Import that Mounds Surface impostor, and add to it's definitions the contour data from your two lowered ponds. I toggled off all of the surface smoothing, edge swapping to insure they were not at play, and the end result is as shown.
The other issue is often the pasted data overlaps the destination surface definition in bad ways and creates these same voids.
