Go to the website and read all that is available for sealing ponds.
http://www.texassodiumbentonite.com/ The recomended amount of bentonite for a porus sand is 8# to the sq foot. That means 10 tons. It is only $80 ton at the mine in Alpine, Texas but the trucking will cost you as much as the bentonite. Then you need to mix it with a few inches of the topsoil and pack the heck out of it. You must decide for yourself if it is worth the money and effort. My sand layer was staying wet in the worst drought conditions possible so I was sure it could transport water.
http://srac.tamu.edu/tmppdfs/9718650-105fs.pdf This is from another SRAC document:
Coarse soils containing large amounts of
sand and/or gravel are unsuitable. If the soil can be formed into a tight ball that maintains its shape or is moldable, it is suitable for pond construction. A rule ofthumb is that soil must contain at least 20 percent clay.
If it will mold or pack into a ball you might get by just packing with a padfoot or sheepsfoot roller.
