Here's a site that mentions 'soda ash' as a pond sealant. Univ. Tenn. Ponds
[URL=[URL=http://www.cqconcepts.com/chem_sodiumcarbonate.php]]Sodium Carbonate
According to Univ. of Tenn., if you have a high calcium level, it's useful. Hmmm, all I can find is 1 reference that high PH soils have calcium. I have access to some marginal low clay content loam just above my future pond site. Think I'll send off a soil sample for testing this April while in Alabam. ;\)

Self-educated rednecks, the real intelligentsia.