I'll help with one question at a time. Betonite seal? Do you mean betonite, packed clay, or betonite mixed with soil seal??
1. Betonite seal will be your thinnest most sensitive and easiest penetrated seal.
2. Packed clay, depending on thickness should be your thickest, most durable seal. (6' to 8' thick is best)
3. Betonite-soil mix will be in between the above two. This will depend on the depth thickness of the mixing of bentonite into the soil or layers.
Betonite seals are sensitive to cracks/leakage due to water loss from evaporation.
If the original seal is "tight" the integrity is best maintained if the pond is kept full and the water level is not allowed to drop much.
I've seen 40' tall spruce tree roots spread out up to 25 feet from the tips of the branches.
