We have almost 7 acres near Mt. Shasta,CA (a volcano). We want to put in a 1/4 (?) acre pond and need to know how deep clay would have to be to create a nice seal. The site is an old gravel pit and the substate is lava flow alluvium with very fast drainage into the soil. The pit was 20-30 feet deep and when filled with boulders and gravel it was not compacted. Some have recommended a commercial pond liner, but labor costs seem high. Is it worth looking into trucking in clay? How much will I need? Will it work over our site? Thanks.