The Field

As the notion of “sustainable sites” becomes increasingly important, the Natural Water Feature can provide the rejuvenating and life-affirming elements of water, while maximizing a site’s sustainability factor. Borrowing design techniques from natural and constructed wetlands and the increasingly popular “natural swimming pool,” the Natural Water Feature offers a green alternative to the traditional, chlorinated, and resource consuming display.

So what exactly is a Natural Water Feature? While the definition is still being debated, there are three main aspects upon which everyone agrees:

First, the display will use a non-chemical water treatment system. The Natural Water Feature relies on a living ecosystem in lieu of adding chlorine and acid. Chemical pollution becomes a non-issue—nature does the work.

Second, the Natural Water Feature should use minimal mechanical gear and electricity. This means eliminating or downsizing the mechanical filter and its pump, the effect pump, and other equipment—which translates into using less energy.

Third, the display should use recaptured water where possible, and in general, minimize the use of potable water. Some localities demand this for any water feature.

To understand how non-chemical based water treatment works, one must first understand certain biological principals. Image 1 shows the biological cycle of “producers,” “consumers,” and “decomposers.” The producers are both plants and algae, taking in sunlight, consuming nutrients, and producing O2 and biomass, which ultimately become detritus.

While plants and algae compete for available nutrients, the Natural Water Feature works to maintain plant health and longevity. Properly selected aquatic plants such as Juncus can outcompete algae and the algae that manage to grow on limited nutrients are then largely consumed by zooplankton such as daphnia. An added benefit to the Natural Water Feature is to provide a habitat for local insects and other pond critters – an important aspect for the Sustainable Sites Initiative.

At cycle’s end, beneficial bacteria digest consumer byproducts (detritus) and reduce them to nitrates, essentially making plant fertilizer. When this cycle has reached a proper balance, which can take a growing season or two, algae is minimized and the water is clear most of the time. Algae will always be part of the ecosystem (but not the dominant species), so if you are looking for a perfectly sterilized water feature, chlorine will be needed.

With that said, to create a successful Natural Water Feature, one must design a system which sustains aquatic plants, zooplankton, and beneficial bacteria. The key design element is a regenerative zone, composed of sand, gravel, and water plants. The sand and gravel substrate will support plant roots, which is where the majority of beneficial bacteria thrive. Soil or fertilizer should not be part of the substrate, as nutrients are not to be imported from outside.

Gustafson Guthrie Nichol used this substrate concept for their Natural Water Feature at the Gates Foundation Iris Campus project in Seattle, Washington. The 12” deep substrate is comprised of primarily 1.5” gravel and planted with Juncus. Since water quality needs to stay seasonally consistent, a mechanical bag filter and ultraviolet (UV) sterilizer were added. The bag filter, in contrast to traditional sand filtration, does not waste precious rain harvested water during a cleaning cycle. The UV system was employed since it adds no chemical residual and therefore has no impact on pond biology. As indicated in images 2 and 3, the aquatic plants are shown thriving, producing high water quality, with only isolated algae blooms – this is expected after installation of a garden that has not yet matured. Note that the design approach used in this project will not always be successful in all locations or micro-contexts, as the development and design of Natural Water Features will depend on the region and surrounding environment.

When we renovated the CMS Collaborative mock-up pool to be non-chemical, we needed to consider the location and context. The pool was set in the Santa Cruz, California sun and adjacent to a large birch tree with continuously falling leaves, so to aid water purification, we added perforated piping below the substrate (refer to image 4). In this scenario, the top-sand provides mechanical filtration, and the substrate below (where the beneficial bacteria thrive) can actively remove nutrients that otherwise make algae food. The result is a Natural Water Feature using harvested water with excellent water quality that only requires a 1/6th HP pump in comparison to the standard filtration which would require 6x the power – significantly lowering operating costs.

This technique of water flowing through the substrate has been used on several projects; for instance, Atelier Dreiseitl’s Tanner Springs Natural Water Feature in Portland, Oregon. Water flows through a gravel matrix and around planted roots before re-introduction into the larger pond. Again, the substrate’s beneficial bacteria and roots efficiently consume nutrients (now denied to algae), making the beautiful plant based natural system and clear water (see header image).

Water that goes down, can also come up through the substrate – and for the same good result. One example is Hoerr Schaudt’s Uptown Normal’s Traffic Circle. The design creates a living plaza where naturally-cleansed storm water, public space, and transportation co-exist. The Circle’s chemical-free filtration bogs polish water collected from surrounding streetscapes and return it to the site as a public water feature. Later, this water is recycled into an irrigation system for the adjacent streetscapes. The inner elements are comprised of an upper pool supplying a non-planted, stepped sluice. The surrounding ring contains plants and substrate through which water percolates up and meanders to a subterranean reservoir, which also supplies the non-planted areas. An abandoned storm sewer provides the rain catchment cistern and this fountain requires little, if any, municipal water.

Oehme van Sweden’s New York Botanical Garden Natural Water Feature also pushes water up through a planted substrate, but at a much larger scale. This feature has three tiers with the top tier being the substrate and wetland. The water pushed up through the planted substrate then flows down into the more formal intermediate and main basins containing no plants. Water filtration through the upper wetland maintains a certain level of water quality. Similar to the Gates Foundation, this Natural Water Feature also uses a bag filter and a UV sterilizer.

It is important to address maintenance early in the design process to better educate the owner on what is involved. By comparison, standard chemically treated fountains simply sterilize the water and have little variations in required maintenance tasks. Natural Water Features have consistent maintenance tasks such as clearing dead vegetation and sometimes vacuuming debris from the substrate. Like planted landscapes, there are different maintenance routines based on seasonal or other variations, leading to varying tasks to be complete.

Periodic water quality reports can help anticipate future maintenance requirements. The report should include pH, TDS, nitrogen and phosphorus levels. Ongoing readings will let you know whether a new system has stabilized and if nutrients are entering the water from elsewhere (e.g. fertilizer runoff entering the feature or decaying vegetation from falling leaves).

As noted above, designers should be willing to tolerate some algae, especially at the start of the growth season. One can use additional treatment methods such as adding extra beneficial bacteria and algaecides to promote reduced algae levels, but it is still important to emphasize that low levels of algae are signs of a healthy eco-system. Interestingly, overly lush plants can suggest an overabundance of available nutrients, which can promote an algae bloom.

By Chris Cook, PE, Affiliate ASLA, an Associate Principal with CMS Collaborative, Inc.