Master Stroke: Borden Park Natural Swimming Pool, Edmonton, Alberta

The first view you get of Edmonton’s latest outdoor pool is a long black gabion wall. But it’s no ordinary wall: it has a particularly even texture and refined construction that hints at something special on the other side. And indeed, there is. It’s the country’s first natural swimming pool—a system where plants, rather than chemicals, are used to treat the water.

It’s a sweltering hot August afternoon when we visit. Coming by transit from the north, the approach to the pool has no sidewalk, no signage, and no formal pathway, so it’s a bit disorienting to find our way to the entrance. But we hear the clear and familiar sounds of swimmers behind the wall, and eventually join a line of visitors. Admission is a thirty-minute wait—demand is high for this novel place—but the building is oriented to provide ample shade for waiting alongside the east-facing wall.

Borden Park Natural Swimming Pool, Edmonton, gh3*
A level ground plane unites the shower deck, pools, and beach areas into a single controlled composition.

A swimming hole has long existed in Borden Park. In the late nineteenth century, residents enjoyed dipping into a pond in their dungarees; after the park was formally established in 1906, one of the city’s first three outdoor swimming pools was opened in the same spot.

In the 1950s, that simple structure was upgraded to a flat-roofed pavilion, with red brick walls and broad overhangs. Parts of the 1950s building are incorporated in the southwest corner of the current pool, designed by gh3*, a Toronto-based architecture and landscape architecture firm. The mid-century spirit also lives on in the new pavilion’s straightforward form: a simple rectangular bar, constructed from black limestone gabion. Occasional glazing slices reveal the full mass and thickness of the seasonal building’s envelope—which, unlike a typical layered building skin, is made solely of gabion. At the entry, a series of steel plates pivot open to reveal a through-space, leading directly past the admissions desk to the pool’s courtyard. Setting foot in this vestibule is akin to passing through a fortification into a spa-like club. The architecture is rigorous, regimented and elegant.

Borden Park Natural Swimming Pool, Edmonton, gh3*
Steel plates pivot open at the entrance to the reception and change pavilion, emphasizing the depth of its enclosing gabion wall.

Height-wise, the new building and surrounding fence align with the original 1950s structure. This creates a continuous datum that lends the pool a clear feeling of “insideness,” while being open to the sky and overhanging trees above. The idea of expansive flatness is further emphasized at the ground plane. The sand, the flush edge of the pool water, the pool deck, and the wood shower deck are on a continuous level, creating an evenness that echoes prairie fields.

The pavilion is organized in a rational manner: inside its gabion walls, two circulation corridors run the length of the building. Distinct interior volumes include the admissions desk, changerooms and washrooms. Finished surfaces of marine-grade plywood were rubbed with black and white paints to expose the wood grain in high contrast, yielding a highly textured effect.

Borden Park Natural Swimming Pool, Edmonton, gh3*
The natural filtration system includes pools stocked with debris-trapping pebbles and microorganism-consuming plants.

From the exterior gabion walls to the interior concrete floors and sinks, the most important material in the project is stone. “Everything is built on the idea of stone in its incremental forms,” says architect Pat Hanson, FRAIC. “When you think about clean water, you think about mountain water—water rolling over rocks and this relationship of cleanliness and hard mineralized surfaces.”

To preserve the integrity of the water, showers are mandatory. After soaking under sleek, cane-shaped on-deck showerheads, we enter the wading pool. The water has a soft quality to it, similar to a lake, and is perfectly cool on this blazing hot day. This natural, clean water is as much an achievement as the building itself, and as my infant son dips into it, I am glad for the designers’ efforts.

Borden Park Natural Swimming Pool, Edmonton, gh3*
he pavilion includes three bays of universal change rooms. Since the pool is only open in the summer, the structure is not heated or conditioned, and the gabion walls are exposed on the interior.

Swimming pools in Canada have typically been rendered safe through the use of harsh chemicals like chlorine. This sterilizing comes at the cost of red eyes and bleach-scented skin. The Borden Park Pool water is different. Before city-supplied water enters the closed loop system, it is dechlorinated and phosphates are removed. From the pool, water is continuously filtered through a three-chamber sedimentation system that removes large particles. It can then go one of two ways. In one direction, it gets sprayed onto specially selected aquatic plants at the top of a large Neptune sand filter. The plants remove microorganisms by absorbing them as nutrients. The water slowly filters through layers of granite rocks, which catch smaller dirt particles and microscopic impurities. The other pathway leads to the on-deck filtration system consisting of a sand-and-stone submersive pond and planted hydrobotanic pond. After travelling to a holding basin, the water can then move to heating, a UV purifier, and another phosphorous absorber. Rather than being sterile, this water is living and clean.

Borden Park Natural Swimming Pool, Edmonton, gh3*
The changeroom walls are made of marine-grade plywood that has been rubbed with black and white paints to bring out the material’s natural grain.

One of the project’s biggest achievements was getting Alberta Health Services and other authorities onside. Canada’s swimming pool regulations are among the world’s most stringent—much stricter than the regulations in Europe, where natural pools have existed for decades.

To operate this Canadian-first-of-its-kind pool, Borden Natural Swimming Pool was classified as “recreational waters,” like a lake. “In the end, we applied for the building permit as a ‘constructed beach with variances’. And the variances were the pools,” Hanson explains. “We had the potential to back up the system with chlorine if it didn’t work. And it has worked out, but it was a bit of a calculated risk.”

Borden Park Natural Swimming Pool, Edmonton, gh3*
In one stage of the filtration, sprinklers spray the pool water onto planted beds. The plants take up microorganisms in the water as nutrients, helping to purify it for reuse in the pool. Gabion separating walls connect the landscaped filtration elements to the architectural language of the pavilion.

As in any lake, there are small amounts of algae in the water, which contribute to the feeling of softness. With more swimmers and the residual phosphor from their bodies, algae growth accelerates. Water samples are tested three times a week in collaboration with the University of Alberta; if the amount of algae is high, the water turns dark green and cloudy. While this is not a health hazard, lifeguards can’t see far into dark water and public safety becomes an issue. In these instances—as on the day of our visit—the pool’s deep end must be closed to the public.

Borden Park Natural Swimming Pool, Edmonton, gh3*
Simillar to a lake, the pool water contains small amounts of algae, which give it a feeling of softness and a blue-green tinge.

From my vantage point on a floating pool noodle, this is a reasonable trade off for a chemical-free pool. The cloudiness of the water is a perfectly natural growing pain for an innovative idea in its debut season. I hold my baby while he splashes water in our faces, and we are happy.

Edmonton-based architect Cynthia Dovell, MRAIC is a principal at AVID Architecture. She sits on the Edmonton Arts Council’s public art committee and teaches design studios for the RAIC Centre for Architecture at Athabasca University.

CLIENT City of Edmonton, Robb Heit (Project Manager) | ARCHITECT TEAM Pat Hanson (FRAIC), Raymond Chow (MRAIC), John Mckenna, DaeHee Kim, Joel Di Giacomo, Bernard Jin (MRAIC), Nicholas Callies | STRUCTURAL/MECHANICAL/ELECTRICAL/CIVIL Morrison Hershfield | LANDSCAPE gh3* | INTERIORS gh3* | CONTRACTOR EllisDon | NATURAL POOL CONSULTANT Polyplan | AREA 770 m2 | BUDGET $14.4 M | COMPLETION July 2018