Natural pools: how does the water stay clean without chlorine?

How does a natural pool stay clean without chlorine? Because an ecosystem does the work. The plants consume the nutrients algae would need to bloom; life polices the water; you don’t sterilise, you balance. I can say this without a diagram or a brochure: in 2005 I dug a pond of about eight square metres at the bottom of my garden, thirty centimetres to a metre deep. Twenty years on, the water is clear in every season. No chlorine, no UV. Twenty years of watching are worth, to my mind, any number of diagrams.

The workforce, in this pond, has names: Iris pseudacorus, Caltha palustris, Pontederia lanceolata, Cyperus involucratus, Equisetum scirpoides, Potamogeton lucens, waterlilies; some arrived on their own, as stowaways. Their roots plunge into the water and draw out its nutrients; starved of that resource, algae never settle in. Below the surface, rocks shelter the micro-fauna that does the rest. And let’s be honest: chlorine-free does not mean technique-free. A closed-circuit pump lifts the water to a raised basin, from which it falls back into the pond as a thin sheet of water, oxygenating it and keeping it moving. No sterilisation, but a circulation. Goldfish live there too; in a pond designed for amphibians, their presence would be up for discussion, and that is for another article.

Maintenance amounts to half a day a year: pulling out vegetation, to leave the plants room to grow, because their growth is what cleans the water. You harvest the filter. Then let me be plain: this is an ornamental pond, not a swimming pond. Nobody swims in it. But that is exactly what it proves: the biological balance has held, with no additives, for two decades. And where a chlorinated pool subtracts life from a garden (sterile water where small wildlife comes to drink at its own risk), this pond adds some: dragonflies, self-sown plants, and in spring, amphibian spawn along the bank.

A natural swimming pool transposes the principle to bathing scale: a swimming zone, and beside it a planted regeneration zone doing the work my whole pond does on its own. Let me say plainly where I speak from: I have never built a natural swimming pool. What I bring here is the eye of a designer and landscaper who has watched a principle hold for twenty years, not a site report. Because the change of scale is real: swimmers bring nutrients an ornamental pond never sees. The more you expect constant water quality, the more seriously the balance must be designed: the ratio of swimming area to planted area, the profile of the banks, and the question to ask before the first line is drawn: who will maintain it, how often, on what budget. As with the soil, you do it owning everything it entails.

Living water, finally, should be described in advance. It is amber, not turquoise; it has its spring algae while the plants get going again; it invites guests you did not ask for. Announced, all of this becomes the charm of the place; discovered, a defect. Whoever wants shower-gel water, smooth and blue, should know it before digging: living water gives something else.

The framework remains: French law has not settled the status of natural swimming ponds, and the formalities depend on the size of the project and on the local authority; ask at the town hall before you dig, and out of plain good sense, design for young children’s safety even where nothing requires it. Then plant, and let life take up its post. For twenty summers I have watched mine at work: the falling water sings, the waterlilies open, the water stays clear. Is it reasonable to keep pouring chlorine to get less than that?