When we began planning the water features at Montserrat Reserve, conventional chlorinated pools were never on the table. Our natural pool design grew from a simple conviction: if we were building an eco retreat near Chacahua, every element had to work with the landscape rather than against it. What we ended up creating is a chemical-free swimming pool that uses biological filtration, integrates directly into the existing stone formations of our coastal Oaxacan jungle, and feels less like a built amenity and more like a place the land always intended to hold water.
This article walks through our entire process — from the ecological reasoning behind the decision to the specific filtration mechanics, construction challenges, material choices, and seasonal maintenance realities of running a natural swimming pool in a tropical environment.
What Is a Natural Pool?
A natural pool — sometimes called a natural swimming pond or bio-pool — is a swimming area that uses biological processes instead of chemical treatments to keep water clean and safe. The concept originated in Austria in the 1980s and has since spread across Europe, where thousands of public and private natural pools operate year-round.
The core principle is straightforward. You divide the pool into two zones: a swimming zone where people swim, and a regeneration zone where aquatic plants and beneficial microorganisms filter the water. Water circulates between these two zones continuously. The plants absorb nutrients that would otherwise feed algae, while biofilms on gravel substrates break down organic matter. The result is water that is clear, soft on the skin, and free of chlorine, bromine, or any other synthetic disinfectant.
Unlike a conventional pool, a natural pool is a living ecosystem. That distinction shaped every decision we made during design and construction.
Why We Chose a Natural Pool Over a Conventional One
The decision aligned with our broader sustainability commitments, but it went deeper than philosophy. Here in the coastal lowlands of Oaxaca, a conventional pool creates specific problems that a natural pool solves.
Chemical runoff. Chlorinated water that splashes out, backwashes, or drains eventually reaches the soil and, in our case, the nearby lagoon system. Chacahua’s lagoons support mangrove ecosystems, migratory bird populations, and local fishing communities. Introducing chlorinated discharge — even in small amounts — was something we refused to do.
Energy consumption. Conventional pools in tropical climates require aggressive chemical management and mechanical filtration that runs 8 to 12 hours per day. A natural pool’s circulation pump operates at a fraction of that energy load because biological filtration does most of the work passively.
Guest experience. We wanted swimming at Montserrat Reserve to feel like an experience rooted in the place itself — not a generic resort amenity dropped onto the landscape. A natural pool, surrounded by native plants and integrated into the rock, delivers something that chlorinated concrete simply cannot.
Longevity and resilience. Concrete pools in tropical coastal environments face constant battles with salt air corrosion, UV degradation of liners, and chemical imbalance from high temperatures. A natural pool built with stone and lined with EPDM rubber is more durable in these conditions, and the biological system actually strengthens over time as the ecosystem matures.
The Design Process: Reading the Land First
We did not start the natural pool design with a shape drawn on paper. We started by spending weeks walking the property, studying water flow during rain events, mapping the existing stone formations, and observing where water naturally wanted to collect.
Working with Existing Stone Formations
Our site sits on a gently sloping parcel where volcanic and sedimentary stone outcrops emerge from the jungle floor. Some of these formations create natural basins. Others form ridges that channel rainwater during the wet season. Rather than blasting or removing stone to create a pool cavity, we identified a formation where two large rock shelves created a natural depression roughly 14 meters long and 6 meters wide.
We expanded this depression modestly — removing loose material and reshaping the edges — but the fundamental bowl shape was already there. The stone walls on the north and east sides became integrated features of the finished pool. Guests swim alongside rock faces that are millions of years old, covered in moss and ferns above the waterline.
Mapping Water Flow
Understanding how rainwater moves across the site during Oaxaca’s wet season (June through October) was critical. We needed to ensure that stormwater runoff would not overwhelm the pool with sediment, and that any overflow from the pool would follow natural drainage paths rather than eroding new ones.
We installed temporary channels and observation stakes during one full rainy season before finalizing the pool’s position. That patience paid off. We identified a natural overflow route on the southwest corner that we later formalized with a stone spillway. During heavy rains, excess water exits the pool through this spillway and follows its historical path downslope — eventually reaching a seasonal arroyo that feeds into the lagoon system at safe, uncontaminated volumes.
Defining the Plant Zones
The regeneration zone occupies approximately 40 percent of the total water surface area. This ratio is on the higher end of the standard range (30 to 50 percent), and we chose it deliberately. In tropical climates, higher water temperatures accelerate nutrient cycling, which means algae can bloom faster if filtration is insufficient. A larger regeneration zone gives us a comfortable margin.
We designed the regeneration zone as a shallow shelf (300 to 600 millimeters deep) that wraps around the south and west edges of the swimming area. This placement was strategic: the south and west exposures receive the most direct sunlight, which the aquatic plants need for photosynthesis and nutrient uptake. The deeper swimming zone (up to 2.2 meters) stays partially shaded by the surrounding tree canopy, keeping it cooler and less hospitable to algae.
The Filtration System
Our filtration system combines passive biological processes with minimal mechanical assistance.
The Regeneration Zone
The regeneration zone is built in layers. At the bottom, we placed a 150-millimeter bed of washed volcanic gravel (tezontle), which is locally abundant in Oaxaca. Tezontle is ideal for biological filtration because its porous surface provides enormous surface area for beneficial bacteria colonies — the same nitrifying bacteria that power aquarium filters, but at landscape scale. These bacteria convert ammonia and nitrites (from organic debris, body oils, and sunscreen residues) into nitrates, which the plants then absorb as fertilizer.
Above the gravel substrate, we planted a mix of aquatic species selected for their filtration performance and their compatibility with our coastal tropical climate:
- Thalia dealbata (powdery alligator flag) — tall emergent plant with strong nutrient uptake, native to the Americas
- Pontederia cordata (pickerelweed) — hardy, fast-growing, excellent phosphorus absorber
- Cyperus papyrus (dwarf papyrus) — dense root mass ideal for microbial colonization
- Nymphaea species (tropical water lilies) — floating leaves shade the water surface, reducing algae growth
- Ceratophyllum demersum (hornwort) — submerged oxygenator that competes directly with algae for dissolved nutrients
We avoided non-native invasive species entirely. Every plant in the regeneration zone was sourced from nurseries in Oaxaca or propagated from specimens found in regional waterways.
Mechanical Circulation
A single low-wattage pump (running on solar power during daylight hours) circulates water from the bottom of the swimming zone to the far end of the regeneration zone. The water then moves slowly through the planted gravel bed — a journey that takes roughly four to six hours — before returning to the swimming area through a series of submerged inlets. This slow transit time is essential. It gives the bacteria and plants sufficient contact time to process nutrients.
The pump moves approximately 8,000 liters per hour, which means the entire pool volume (roughly 120,000 liters) turns over about 1.6 times per day. For a tropical natural pool, this turnover rate keeps the water consistently clear without excessive energy use.
We also installed a surface skimmer at the downwind edge of the pool to collect fallen leaves and organic debris before they sink and decompose. In a jungle setting, leaf litter is the single largest source of nutrient loading, so removing it mechanically at the surface is far more efficient than relying on biological processes to break it down at the bottom.
Construction Challenges
Building a natural pool in coastal Oaxacan jungle presented challenges that you would not encounter in a temperate European or North American setting.
Substrate and Excavation
The mix of stone, volcanic soil, and dense root networks made excavation slow and largely manual. We used hand tools and a small excavator for the deeper sections, but much of the shaping work — especially where we wanted to preserve stone features — was done by hand by a crew of local masons who understood the material intimately.
Waterproofing
We lined the pool with 1.5-millimeter EPDM rubber, a synthetic membrane that is fish-safe, UV-resistant, and flexible enough to conform to the irregular stone surfaces. EPDM was chosen over PVC because it performs better in sustained high temperatures and does not release plasticizers over time. The liner was custom-welded on-site to minimize seams, and we anchored it with stone coping along the edges so that no rubber is visible from any angle.
Humidity and Curing
Tropical humidity affects every material on the build. Mortar and concrete cure differently when relative humidity sits above 80 percent for months at a time. We scheduled all masonry work for the dry season (November through May) and used a modified mortar mix with lower water content to account for ambient moisture.
Access and Logistics
Our property is reached by a dirt road that becomes challenging during the rainy season. All heavy materials — gravel, stone, EPDM liner, the pump and plumbing — had to be delivered during dry months. We stockpiled materials on-site well before construction began, which required careful planning and covered storage to protect components from humidity and insects.
Materials
Every material choice was evaluated for environmental impact, durability in a tropical coastal environment, and local availability.
| Material | Use | Source |
|---|---|---|
| Volcanic stone (basalt) | Pool walls, coping, decorative boulders | Quarried within 80 km of site |
| Tezontle (volcanic gravel) | Regeneration zone substrate | Local supplier, Oaxaca |
| EPDM rubber (1.5 mm) | Waterproof liner | Imported, fish-safe grade |
| Reclaimed hardwood | Decking, entry steps | Salvaged from regional demolition |
| 316 stainless steel | Pump fittings, skimmer housing | Specified for salt-air resistance |
| HDPE pipe | Circulation plumbing | UV-stabilized, food-grade |
We avoided concrete wherever possible. Where structural support was needed beneath the liner, we used compacted gravel and dry-stacked stone rather than poured foundations. This approach is more labor-intensive but eliminates the carbon footprint of cement production and allows the pool structure to flex slightly with ground movement — an advantage in a seismically active region.
Landscape Integration
The goal was for the pool to look as though it had always been part of the site. Several design decisions support this.
The pool edge is entirely irregular. There are no straight lines, no uniform coping, no tile trim. The stone edging varies in height, texture, and species of moss or fern growing on it. In some places, the pool edge disappears under overhanging vegetation. In others, a flat stone shelf extends into the water, providing a natural seating ledge.
We preserved every mature tree within five meters of the pool. The canopy coverage these trees provide is not just aesthetic — it reduces water temperature by two to three degrees Celsius during peak sun hours, directly suppressing algae growth. The trees also support birds, insects, and epiphytes that make the pool area feel alive in a way that a manicured pool deck never could.
Pathways leading to the pool from the villas follow existing animal trails that we widened slightly and surfaced with packed earth and stone. There are no concrete walkways, no artificial lighting poles. Night lighting around the pool uses low-voltage LEDs embedded in the stone coping, angled downward to avoid light pollution and disturbance to nocturnal wildlife.
Seasonal Considerations
Operating a natural pool on the Oaxacan coast means adapting to two distinct seasons.
Wet season (June through October): Rainfall is heavy and frequent. The pool receives significant direct rainfall, which dilutes nutrient concentrations and actually helps water clarity. However, runoff from surrounding slopes can carry sediment into the pool. Our stone spillway handles overflow, and we increase skimmer operation during this period. Plant growth in the regeneration zone explodes during the wet season — we prune aquatic plants every two to three weeks to prevent them from overtaking the swimming zone.
Dry season (November through May): Evaporation increases, and we top up the pool with water from our rainwater harvesting cisterns. Nutrient concentrations can rise as water volume drops, so we monitor phosphate and nitrate levels weekly. This is also when we perform annual maintenance: inspecting the liner, cleaning the pump intake, dividing overgrown plants, and removing accumulated sediment from the gravel substrate using a pond vacuum.
Water temperature ranges from 24 degrees Celsius in January to 31 degrees Celsius in August. At the higher end of that range, we keep a close watch on dissolved oxygen levels. The submerged oxygenating plants (hornwort and similar species) help, and the pump’s return flow is designed to aerate the water as it re-enters the swimming zone through a shallow cascade over stone.
Maintenance Realities
We are honest about this: a natural pool requires more ongoing attention than a conventional pool, but it is a different kind of attention. There is no chemical testing, no pH adjustment, no shock treatments. Instead, maintenance is ecological.
Weekly tasks include skimming the surface, checking pump operation, and visually inspecting plant health. Monthly tasks include trimming plants, testing nutrient levels with simple aquarium test kits, and clearing the skimmer basket. Annual tasks include partial draining for liner inspection, gravel bed vacuuming, and plant division.
The labor is more physical and more seasonal than conventional pool maintenance, but it requires no specialized chemicals, no hazardous material storage, and no disposal of contaminated backwash water. Every byproduct — trimmed plants, vacuumed sediment — goes directly into our composting system and eventually back into the organic garden.
What We Learned
Three years into this project, the natural pool has exceeded our expectations in water clarity, ecosystem stability, and guest response. The water is clearer than any chlorinated pool we have seen in the region. Dragonflies hunt over the surface. Tree frogs occasionally visit the regeneration zone. Guests describe it as the most memorable part of their stay.
If we were to build it again, we would make the regeneration zone even larger — perhaps 50 percent of total surface area — to give ourselves an even wider margin during the hottest weeks of the year. We would also install a secondary settling chamber before the regeneration zone to pre-filter large debris and reduce maintenance on the gravel bed.
But the fundamental approach — reading the land, building with local stone, letting biology do the work of chemistry — proved sound. It is slower, more demanding, and less forgiving of shortcuts than a conventional build. It is also more beautiful, more resilient, and more aligned with the kind of place we are trying to create.
For anyone considering a natural pool design in a tropical setting, our advice is simple: spend more time observing your site than drawing plans. The land will tell you where the water wants to be. Your job is to listen.