Build Your Own Natural Swimming Hole

Sealed with a clay

Once you’ve dug the hole for the swimming pool and the plant zone, you have a couple of options 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 prices vary, and, depending on your soil type, you should expect to pay between 20 cents and nearly a dollar per square foot. Liners can cost 25 cents to $1 per square foot, depending on their composition and weight.

Bentonite bonds with soil particles to create an impervious barrier that will prevent the pool’s water from seeping into the ground. Some soils may contain enough clay that simply compacting the pond bottom will enable it to hold water. 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 soils.

Bentonite also can be troublesome when the surrounding soil is very dry. In arid climates, Zingaro recommends applying bentonite beneath a woven or textured plastic liner to keep it 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 synthetic liner, select one made of ethylene propylene diene monomer rather than PVC. EPDM is a synthetic that’s twice as expensive as PVC, but its ultraviolet stability and flexibility make it worth the cost. If your soil is particularly rocky or is filled with sharp roots, select a 45- or 60-millimeter liner. You can use a 30-millimeter liner if your soil is 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 soil 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 synthetic liner is in place, cover the pool’s bottom with 4 to 5 inches of gravel or pea rock. The gravel provides a habitat for beneficial micro-organisms, which help decompose leaves and other organic matter that sinks to the bottom. Make sure to use washed gravel to avoid cloudy water caused by suspended particles. Expanded shell aggregates are also likely to be clean enough to use in your natural pool.

Finishing touches

In addition to lining the pool with gravel, many people opt to build cobblestone steps for pool access. 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.