Pond #3 - under construction

The first pond was not designed as well as it could have been so is a bit unsightly and inefficient (although the wildlife don't seem to mind that much). However the many sharp hooved and clawed animals visiting have torn the strong lining and so the pond no longer holds the water it did originally. I am constructing another pond in the same field and again converting an area damaged by construction of a Quad Bike feature by the previous owners. This time I am using Bentonite to create an impermeable membrane although the expert doing the bulk of the work is strongly suggesting that a stone layer sealed by a form of concrete is a must. The photo shows the pond in construction. Pond #2 is a small rigid lined pond intercepting our spring that flows when it rains. I think it is mainly favoured by flies rather than any higher form of aquatic animal.