The water in Natural Swimming Pools needs to be contained in some way so that it does not need to be constantly topped up.
In some locations, there is sufficient clay in the soil so that pools do not need to be waterproofed and can be constructed by simply digging into the ground. The water retention of the soil can be improved by puddling – a technique to make clay less permeable. The challenge with soil bottomed pools is that they do leak slightly through capillary action, the water never clears completely, and there is a risk of nutrient pollution from the surrounding soil. Plants however, do very well in soil bottomed pools, and if not disturbed too much, clay bottomed NSPs can be very pretty.
The majority of pools are however sealed using various building materials. Commonly waterproofed concrete, fiberglass or liners. The choice of which building material to use depends on the design of the pool, the ground conditions, budget and other relevant technicalities. There is more information on liners elsewhere on this site.
PVC and other non-flexible liners such as LDPE are cost effective per m2, but require technical hot welding equipment and have no inherent flexibility. They therefore do not wear well, especially if exposed to UV, which tends to make them brittle and susceptible to cracking and tearing. They are also difficult and expensive to repair because they require heat welding. PVC has toxic by-products, and leaches toxins into the water. Only very select PVC liners are rated as compatible for use with sensitive aquatic species and are expensive as a result.
The most important aspect of a Natural Pool is the choice of waterproofing because no matter how beautiful the design, and how efficient the biology, if the pool leaks it is not a pool, but a hole in the ground.
Most biological systems require pumping 24 hours a day to supply the microbial community with nutrients and oxygen, and remove wastes and CO2 – just like the heart pumps blood to the cells of the body. Because there is no algae in the water that needs to be filtered out by a high pressure pump and sand filter, well designed NSPs use much smaller and more efficient pumps. Because these pumps are smaller, it becomes feasible to run these pumps off wind and solar.
In some very simple systems, pumps can be omitted. In this case, impurities in the water are moved via natural processes such as osmosis, wind or temperature differentials, but this requires the establishment of a complete ecosystem and only works for pools larger than 100m2.
More Science Behind Natural Swimming Pools
The basic principle of NSPs is the management of nutrients in the water. Water, because of its molecular structure, water attracts and traps organic matter. It then dissolves the organic matter and accumulates the breakdown products in the form of nutrients. This is why, when chemical levels are not maintained at optimal levels, chemical pools quickly become green. Green water is caused by single celled planktonic microalgae, which use the nutrients as a food source to grow on.
The water in natural swimming pools is reticulated through biological filters made up of porous material. Water moves through this material transporting dissolved nutrients and gasses. The porous material offers a high surface area for microbes to colonise. Plants are planted into the porous substrate, and they use the byproducts of the biofilm digestion for their own growth. Once the plants are established, they attract animal life. The development of this ecosystem is phylogenetic, i.e., it mimics the evolution of life on Earth: individual microbes; microbial communities, plant life, animal life.
The difference between these systems and natural wetlands is the absence of soil. Soil plays an integral part in nutrient recycling in natural wetlands, which cannot be accommodated within the small space of a NSP filter ecosystem, and would cause the water to become murky.