Build Your Own Natural Swimming Hole
Douglas Buege and Vicky Uhland
Natural pools are home to frogs, water striders and dragonflies, making them a treasure trove of riches for children to explore.
Whether you like to practice your dolphin dives or lounge away the day on a raft, swimming is one of summer’s perfect pleasures. With a minimum of materials you can build an idyllic water oasis right in your own backyard and thwart summertime’s sultry dog days.
Although fairly common in Europe, natural swimming pools are in their infancy in the United States. Ask most American swimming-pool contractors to build a backyard pool and chances are they’ll roll out a long list of goods, including rebar, sprayed concrete, fiberglass and a complex filtration system. But in recent years, a few builders and a growing number of homeowners have learned how to build pools that look more natural and blend nicely into the landscape.
Natural swimming pools take a bit of forethought in their construction but can be well worth it in the end. Gravel, stone and clay are used in place of concrete or fiberglass, and aquatic plants and a simple aeration system is all that’s needed to keep the water clean.
In a natural pool, plants enrich the water with oxygen, support beneficial bacteria and other micro-organisms that consume debris, and give habitat to frogs, dragonflies and other water life. The result is a beautiful, ecologically diverse system that is relatively inexpensive to construct. A natural pool can be constructed for as little as $2,000 if you do it yourself, while conventional pools can cost tens of thousands of dollars. But the savings don’t end there. A natural swimming hole is fairly low-tech, and once established requires only a modicum of management. You won’t have to drain the pool each autumn and, except for topping it off now and then, you’ll fill the pool only once.
The easiest and least costly way to build a swimming pool is simply to hollow a hole in the ground. You can make your pool as deep as you want, but the key is to make sure the sides slope, which will prevent cave-ins and offer easy entrance and exit. The ratio should be a 1-foot vertical drop for every 3 horizontal feet.
“It’s not a bathtub effect, but more like a soup bowl,” says Tom Zingaro, partner with Denver-based Blue Lotus Designs (www.BlueLotusDesigns.net), a pool- and pond-architecture company.
One of the main reasons traditional swimming pools are constructed with a steel framework is to ensure the walls stay vertical and perpendicular to the bottom surface of the pool. Construct a pool with sloping sides and you’ll eliminate the need for steel reinforcement.
Plants in your pool
Reserving at least 50 percent of your pool’s surface area for shallow plants, either at one end or in a ring around the sides, eliminates the need for conventional filtering systems. You’ll need to separate the swimming area of your pool and the filtration area, or plant zone, with a rim that comes to within an inch of the water’s surface. This rim keeps plants in their place but allows water from the swimming area to move to the plant zone for filtering. As water passes through the fibrous root structure of the plants, micro-organisms concentrated on the plants’ roots act as a biological filter, removing contaminants and excess nutrients from the water. Decomposer organisms, also found in the plants’ root zones, consume the bacteria, effectively eliminating underwater waste buildup.
The plant zone’s bottom contour should gradually reach a maximum depth of 18 inches near the swimming zone. The outermost 6 inches of the plant zone will provide a home for taller aquatic plants and ought to be no deeper than 2 to 3 inches. Submerged and floating vegetation occupy the deeper area.
In addition to cleaning the water and beautifying your pool, the shallow plant zone’s water is quick to warm in spring and provides habitat for frogs and many invertebrates. These animals appreciate the shallow and quiet water for breeding and repay the favor by eating mosquito larvae.
Bubble, skim, filter
For best results, your pool’s water needs to circulate through the plant zone so their roots can cleanse it – this will require a small pump and a bit of plumbing. Water can be channeled from your pump into the plant zone with PVC tubing. Zingaro recommends using flexible PVC in cold climates – check with your local plumbing supplier for the right material. In any climate, bury the tubing in the soil about 18 inches deep.
You should also aerate the water so the water organisms’ oxygen needs are met and to avoid problems with anaerobic bacterial growth and stagnation. Underwater aeration, which uses less energy than constructed waterfalls and circulates water more effectively, involves diffusing air at the pool’s bottom.
You can build your own aerator, using an air compressor (1/4-horsepower for a pool smaller than an acre) and high-strength tubing connected to a diffuser located in the deepest part of the pool, where swimmers are not likely to damage it. A brass manifold connected to the compressor helps to regulate the air flow. Don Schooner at Inspired By Nature (www.IBNature.com), an Ohio-based pond and lake restoration company, suggests aerating the pool four to eight hours a day: in the morning, when oxygen demand is greatest, and again in the evening.
Some folks use skimmers hooked up to an additional small pump, to suck off floating undesirables. While these devices are not essential, you might want to consider purchasing one if leaves or seeds from nearby trees and shrubs are likely to litter your pool. The skimmer removes detritus that would otherwise sink and contribute to algae growth.
Place your aerator, circulation pump and skimmer in a plastic container, such as a bucket or large plant container, and put a steel-mesh filter mat over the top, to keep debris out of your equipment. Expect to pay $1,000 to $1,200 for a quality underwater aeration system.
Installing pumps and compressors can be a tricky business because you’re running electrical devices near or in water. Hiring a skilled electrician will ensure the safety of the system.
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.
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.
To finish the edges of your pool, run a plate compactor around the perimeter. This will help with soil erosion, but it’s not enough to guarantee that extra soil won’t get into your pool. One option is to edge the perimeter with rocks, flagstone or wood planking. Better still, plant right next to the edge and let the plants stabilize the perimeter, says Martin Mosko, principal architect with Marpa Design Studio (www.Marpa.com), a landscaping company based in Boulder, Colorado. Plants work not only to anchor the soil, but create a natural setting for an old-fashioned, swimming-hole effect. Mosko says if you use plants instead of stone, choose plants that thrive in wet soil or make sure the water level is at least a foot below the pool’s edge so the perimeter plants don’t become waterlogged.
Prepare for planting
Once your pool is constructed, you’ll need to prepare the plant zone with 3 to 6 inches of soil. Choose your soil with care as soil can carry various contaminants. Avoid harvesting soil from areas where animals have been kept, or that contains a large proportion of organic matter that has not yet decomposed. If you have concerns, get the soil tested – contact your state’s health department. Once soil, gravel and hardware are in place, fill the pool carefully to avoid disturbing the soil and let the pool rest for a week before installing plants. During this time, you can test your hardware to make sure it works.
The secret ingredient
Sedges (Carex) and rushes (Scirpus), both aquatic plants, make great emergent vegetation for your pool’s perim-eter. You can also consider lesser cattails (Typha angustifolia) and aquatic irises, though be sure to ask which varieties won’t overcrowd other plants. Pickerelweed (Pontederia cordata), arrowhead (Sagittaria) and water primroses (Ludwigia) are all contenders for the shallowest areas of your pool. Be sure to include submergent plants such as common waterweed (Elodea) and hornwort (Ceratophyllum) for their high oxygen output.
In water 6 to 18 inches deep, plant a mix of floating, submergent and emergent plants. Water lilies (Nymphaea) adapt to any depth, so use them liberally. Floaters, such as pondweeds (Potamogeton) and common duckweed (Lemna minor), drift freely and quickly cover the surface of the plant zone.
Once you’ve purchased your plants, you can plant them in the filled pool. Stick to a plan, grouping plants according to height and type. Place your plants into the soil, anchoring them with plenty of gravel.
Get rid of the green
Both microscopic and macroscopic algae can cause your pond to turn green, but they can usually be controlled by limiting the pond’s nutrient levels, keeping oxygen levels high and nurturing a healthy plant zone.
Algae compete with other aquatic plants for nutrients and light, but spring algae blooms often decline as soon as water lilies and other plants emerge to shade them.
If you have more persistent problems with algae, consider adding plants, which will help consume excess nutrients. A second option is to keep phosphorous levels low by preventing fertilizer-rich runoff and urine from entering the pool. Increasing the pool’s aeration schedule will also help.
For more information on managing algae, visit the Ohio State University Extension Web site for the article at OhioLine.osu.edu/a-fact/0012.html.
Removing plant litter in spring and fall will help maintain the long life of your natural pool. Keep your water level constant and be prepared to add water as needed. Inexpensive test kits, available in garden centers, will allow you to monitor your pool’s nutrient levels, alerting you to problems.
In addition to maintaining the pool’s biological health, check the mechanical systems annually. Wipe diffusers with vinegar to remove deposits, check air hoses for cracks and obstructions, and examine all connections to the pumps. Given these precautions, your pool should provide you cool pleasure for years to come.
Environmental writer and educator Douglas Buege is an avid organic gardener and beekeeper. Freelance writer and editor Vicky Uhland has written about holistic lifestyles for a variety of publications. This article first appeared in the August/September 2002 issue of Mother Earth News.