Concrete is not the best option for natural pools, it's expensive, and not certified for potable water use. Concrete is also susceptible to cracks and damage, making it costly to maintain. Instead, geomembrane liners are incredibly flexible and durable, and are much easier to install. They are resistant to UV damage, and aren't at risk of leaching any toxic chemicals that may harm the plants in your regeneration zone.
Clay is another popular option for natural pools, however it is difficult to install, and can be seriously damaged if the clay is able to dry out and crack.
