Determining whether you need a pond liner depends on the soil type in your area. In many places, liners may not be necessary. The goal, whether using a liner or not, is to prevent seepage that will cause the pond to drain. Soils with enough clay may naturally prevent seepage, which removes the need for a pond liner.
However, other times, the soil may be of a well-draining type. This includes rocky soils, soil with a high sand content, or soils with certain types of clay in which the clay particles form small gravel-like clusters rather than a sealing surface that holds water. In these conditions, you will need to use either soil amendments to seal the pond, or a pond liner. Soil amendments can often be cost prohibitive and labor intensive to put in place, so a pond liner may be your best bet to make sure the pond seals well.
