How to Build a Natural Swimming Pool

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SEALING THE POOL

Once you've dug the hole for the swimming pool and the plant zone, you have a couple of options, depending on your soil conditions, to make sure the pool holds water: You can apply a layer of bentonite clay to seal the soil or lay a synthetic liner. Bentonite is usually the cheaper option, averaging 35 cents per square foot. Liners can cost 25 cents to $1 per square foot, depending on their composition and weight.

Bentonite works as a glue, bonding with the soil particles and preventing pool water from seeping into the ground. Some soils may contain enough clay that simply compacting the pond bottom will enable it to hold water. Talk to local pond builders to find out for sure. But beware: Bentonite doesn't bond well with sandy soil. Particularly sandy soil can require up to 12 pounds of bentonite per square foot, as opposed to 6 pounds in clay-rich soil. Bentonite also can be troublesome when the surrounding soil is very dry. In arid climates, Zingaro recommends bentonite be applied beneath a plastic liner that is woven or textured on the bottom. This liner keeps the bentonite from shifting. In more humid climates, bentonite can be applied directly to the soil. Before treating your pool with bentonite or any other clay powder, thoroughly compact the soil. You can do this with a lawn roller or a plate compactor. Then, while wearing a mask, spread a 2- to 3-inch layer of bentonite powder along the pool sides and bottom. Pack it down with a tractor or plate compactor. Then apply another foot of quality topsoil and compact again.

If you choose a liner, select one made of ethylene propylene diene monomer rather than PVC. EPDM is a synthetic rubber twice as expensive as PVC, but it's worth the extra cost. It has protection from ultraviolet rays, and unlike PVC remains flexible in cold weather. If your soil has a lot of rocks or roots, select a 45- or 60-millimeter liner. You can use a 30-millimeter liner if your soil is very sandy and smooth, and if you and your guests aren't likely to tear holes in a liner while frolicking in the pool. Before laying your liner, compact the sod and cover it with a layer of sand or an absorbent material such as old carpeting or newspaper. Newspaper is a good option: When wet, it bonds to the liner, providing extra protection if the liner develops a small hole.

After the bentonite clay or a liner is installed, cover the bottom of the pool with 4 to 5 inches of gravel. The gravel provides a habitat for beneficial bacteria, which help biodegrade leaves or other natural materials that sink to the bottom of your pool. Make sure you use clean gravel. Fill a 5-gallon bucket with a spigot with some of the gravel you intend to use. Open the spigot and run water through the gravel. If the water comes out dirty, you need to clean the gravel (a taxing, water-wasteful process) or find another source. Expanded shell aggregates and other manufactured gravels are likely to be clean enough to use in your natural pool. In addition to lining the pool with gravel, many people opt to build cobblestone steps for access into and out of the pool. A cantilevered dock built out over the water also provides an easy way to get in and out of the pool, and helps protect the pool's sides.

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