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Of Soil and Water: outdoor swimming in a naturally filtered urban pool

July 16, 2015

Of Soil and Water: a new freshwater swimming pool in central London. Image: Ooze Architects

The thought of taking a dip in an outdoor swimming pool on a construction site in the middle of London isn’t necessarily everyone’s idea of a good time.  However, a new initiative called “Of Soil and Water: King’s Cross Pond Club” has recently opened just such a pool on the site of one of London’s most extensive redevelopment schemes.  And the most innovative part of this scheme?  The new pool is filtered entirely by natural processes, using an array of planted vegetation both above and below the waterline to keep the pool clean enough to safely swim.

Part-public amenity, part-land art and part-open air natural experiment, Of Soil and Water is a small, self-enclosed ecosystem in a new 40 metre pool, which is designed to be self-purifying, despite the multiple stresses and pollutants emitted from the urban environment.  The pool is the result of a collaboration between the Ooze architects (Eva Pfannes and Sylvain Hartenberg) and artist/architect Marjetica Potrč, as part of a series of art events commissioned by King’s Cross Central Limited Partnership.  Recently opened, the pool provides a small beacon of urban freshwater nature, nestled amongst cranes and concrete footprints of high-rise buildings in construction close to Kings Cross station, and will remain open to the public until 2017.

Describing the project’s concept, artist Marjetica Potrč said of the work “We have to rethink how we live with the city and with nature. Here, we are collaborating with nature, and the artwork encourages the viewer to participate in that experience. Water is a source of life but it is also a metaphor for regeneration. We want to understand people’s influence upon nature but also our balance with nature.”

Speaking recently to the Guardian, the Ooze architects suggest that visitors can swim in “a living laboratory where they are aware of their relationship with nature, and about consequences of their interactions with nature”.  This emphasis on a ‘living landscape’ (however small in scale), is designed to allow the natural features of the pool to change over time.  This relatively open process-based ecology that underpins the design was chosen the architects to “show a micro-landscape in the becoming; the succession of the different stages of natures related to different soils and waters. The experience of visitors will change continuously within the 18 months.” 

King’s Cross Pond Club and Cubitt Park, King’s Cross. Image: The Londonist

Whilst this idea is laudable in many ways (and chimes with many of the non-linear and process-based approaches currently dominating ecology and restoration), there will doubtless be a tension between allowing for the trajectories the pool ecosystem can take over time, whilst ensuring the ecosystem services it provides, namely the naturally purified water.  It’ll also be interesting to watch how the biodiversity of the pool changes over time.

Located close to a network of canals, and within a few miles of a number of lakes, ponds and rivers, the pool will likely be colonised by mobile invertebrates like damselflies and water boatmen before too long.  One question might be: if outside plants and animals begin to colonise the pool – lets say even birds and small mammals start to nest (and feed, and leave faeces etc) there – how far does this ‘living landscape’ allow for their presence, whilst still maintaining water quality?

In all probability, given the pool’s short lifespan, this is unlikely to be a major issue, but it does flag up the idea that whilst we might undertake environmental management that emphasises natural processes and uncertainty, there is still the need for managers to choose which processes to prioritise, and to what ends.  In this way, we see further parallels with the pool and wider questions that environmental restorationists are asking in their work.


Architect’s sketch for the pool’s design. Image: Ooze Architects

Plant filtration systems for freshwaters are not new (indeed, you could argue that they’re the original water treatment works…), but are being increasingly adopted in environmental management which tackles multiple stressors.  Put simply, many aquatic and marshland plants can take up excess nutrients, chemicals and toxins from the water in which they grow, removing these dissolved pollutants from being available in (and harming) the wider ecosystem.

For example, in 2009 Alan Berger, a landscape architect at MIT in the USA, proposed an initiative to use vegetation planting to help improve water quality in 2600km of polluted canals and waterways that thread through the Pontine Marshes, south of Rome in Italy.  Using a large grant from the European Union’s LIFE+ project, Berger and colleagues designed a landscape-scale ‘Wetland Machine’, filtering all the water in the marshes through a 2.3 km² area of wetland built-in winding channels and planted with vegetation that is particularly efficient at taking up and storing pollutants and toxins such as marsh grass.

Berger’s winding design ensures that the water flows through the wetland at a sufficiently low-speed for the pollutants and toxins to be taken up by the plants.  Berger’s work in Italy is still ongoing, but represents one of the largest and most ambitious examples of natural water filtration management in the world (see more information here).


The different filter zones which keep the Kings Cross pool clean. Image: Kings Cross Pond Club

To loop back to the (comparatively small) Of Soil and Water pool at Kings Cross: how have the designers used natural processes to filter the pool’s water, so that it is safe to swim in?  The pool is split into three zones: a swimming zone; a regeneration zone; and a filter zone.  In the regeneration zone, largely free-floating plants including water lilies and mare’s tail absorb nutrients from the water, and pondweeds oxygenate the pool.  Algal growth is limited by allowing microorganisms and zooplankton to flourish, which in turn graze on the algae.

In the filter zone, a layer of gravel collects a growing biofilm of microorganisms, fed by nutrients brought into the pool by the bathers and the urban environment and oxygen in the water.  The biofilm mineralises any organic matter in the pool, and helps reduce pathogenic germs, whilst the limestone gravel releases calcium into the water which binds to dissolved phosphates.  Here too, plants which filter nutrients and toxins from the water are grown, including flag irises, water mint, marsh marigold and various rush species.


Natural filtration techniques. Image: Kings Cross Pond Club

On close inspection, whilst Kings Cross pool does not rely entirely on natural processes to filter the bathing water.  Instead, a series of pumps and water skimmers circulate the water and help remove floating impurities, and a phosphate filter keeps phosphorous concentrations low in the pool, preventing algal growth.

Despite this, when viewed as a whole, the Of Soil and Water pool is clearly ambitious, environmentally minded, and perhaps above all, fun.  Engaging people with urban nature and ecosystems that are otherwise unnoticed or taken for granted is an important step in helping foster responsibility and care for the environment.  And here, at a small pool of water amongst the high-rises, bulldozers and cranes, is an example of natural processes being able to thrive, both for the enjoyment and appreciation of people, and – hopefully – for the health and biodiversity of the wider urban environment.

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