Hi DR3131,
With regards to your sandy soils condition, I encountered the same problem. I hired a guy named Harmon Mansell to dig my pond. I found him on the web and learned a lot. He sealed my sandy soil with sodium bentonite. After he dug the hole, he spent a day on the dozer and mashed in a couple of tons with his cleats. Worked like a champ. One thing I learned, most pond builders are dirt pushers. Make sure you hire a guy that knows what he's doing, i.e. coring the dam. My neighbor two doors down built a pond at the same time, (Feb '04), and lined his with 4" of clay. It ain't holdin' jack. Best of luck.

Matt Patterson

Links: Harmon's site:
http://www.site-mart.com/mansells/bentonite.htm

Excellent Bentonite resource site:
http://www.malibuwater.com/PondBentonite.html