Forgot to say.
We have sealed many ponds wiyh sand wash available at your local sand pit. Have it prossesed sample down to a 230 sieve . Apply it to pond bottom 8-12" thick and knit with existing soil all the way up sides to top deck if in doubt just do a 40 ft circle at bottom and watch results. granted there is no hydro pressure. It should cost you approx. 250$ but might show promise
Scott Trava
Catskill Pond
