Bentonite is composed of clay minerals. These minerals can actually incorporate water into their mineral structure.
Some dry bentonites can swell to as much as 8x their original volume when combined with water.
This volume increase works well in leaky soils when you till the bentonite into the dry soil of the lake bottom. The bentonite sits in the small spaces between the soil particles and then swells up to fill the gaps when the pond refills with water.
This process DOES NOT work well if the bentonite is already saturated when trying to fill the gaps in the leaking areas. (Imagine water leaking through your kid's sand box. Putting a few layers of marbles on top of the sand will not stop the leak.)
The linear molecules in the polymer treatments have several advantages over bentonite when used in a water-bearing pond. First, they may expand up to 300x their original volume. Secondly, these are usually a "chain" type of molecule.
I believe the second reason is the most valuable in sealing a flowing leak. Imagine the sink/shower drains in your house once they are partially blocked. At that point, the drain is easily fully clogged by pliable linear items. The hairs, potato peelings, etc. stack up across the hole. As the obstruction cuts the flow rate, the linear items pack in even tighter until they completely clog the drain that had been passing water.
The chain molecules in the polymer treatments act like hairs packing up the tiny flow paths through the soils of the leaky pond.
Most of these treatment types also add a crosslinked polymer. This type of polymer may increase its size and length over a period of several days. Imagine having a hair blockage in your shower drain, and then doubling the volume of each hair. That blockage is not moving!
Hopefully that background will help in the selection of your treatment method. Good luck getting your pond squared away!
