SALT SOAKER

Source: landscapearchitecturemagazine.org

16 mars 2021 –

IN CHICAGO, A SYNTHETIC GROWING MEDIUM WILL PROVIDE A HEALTHY BUFFER BETWEEN CONTAMINATED SOILS AND RIPARIAN PLANT LIFE.

For nearly 100 years, the Morton Salt facility on the North Side of Chicago, with its massive rain slicker and umbrella sign, has been an iconic presence along the industrial corridor that traced the North Branch of the Chicago River. The warehousing and packaging facility closed in 2015, and within a few years, the company announced an ambitious adaptive reuse plan for the site, turning it into a mixed-use campus featuring a concert venue and office spaces. (The sign will stay.) It will also be home to Morton’s R&D facility, relocating from Chicago’s suburbs, where the company will research water softener salt, pool salt, deicing salt, and salt solutions for other industrial applications.

The project’s innovation will extend to the outdoors: The landscape of the campus will include a synthetic growing medium developed by Omni Ecosystems. According to the company, it’s the first site in Chicago that’s been approved for the use of special stormwater soils designed to mitigate runoff and stormwater from combined sewer overflows. Working with the Chicago Plan Commission and the Department of Buildings, Omni Ecosystems will use 60,000 cubic feet of Omni Infinity Media, largely composed of an ultra-light, kiln-dried mineral similar to volcanic rock. This medium will allow a rich wetland and riparian ecosystem to thrive on top of a degraded and polluted site that’s been capped with concrete and asphalt.

The Omni Infinity Media is mostly air—it has 78 percent void space, compared to standard topsoil, which has 25 percent. “It’s quite literally and physically a sponge,” says Michael Skowlund, ASLA, the director of landscape architecture at Omni Ecosystems.

While standard soil can absorb about six inches of water per hour, Omni Ecosystems claims that its Infinity Media can drain a fire hose: 51 inches per hour. “You can continuously dump water on it, and it will never saturate and it won’t overflow,” Skowlund says. This rate of absorption is meant to withstand a 100-year flood, allowing water to get into sewer systems rapidly, or even better, to return to the river without picking up pollutants. This is critical in cities such as Chicago with systemic combined sewer overflow problems, where strong downpours cause sewage to back up into the Chicago River.

The Morton Salt site is being developed by R2 and Blue Star Properties, with Lamar Johnson Collaborative as the architects and Omni Ecosystems as landscape architects, and Skowlund says the growing medium is likely to be installed by next spring. The four-plus-acre site is divided into two separate “watersheds,” Skowlund says, with the western half draining into the sewer system and the eastern half draining into the river. The growing medium is inoculated with microbes and fungi that round out a diverse soil biology palette. “We affectionately call them bugs,” he says. The “bugs” will nourish a native collection of sedges, switchgrass, little bluestem, and more.

Because of its extremely light weight, Omni Infinity Media has been used on green roofs for years, “but to me, that’s not the genius behind it,” Skowlund says. “The stormwater capture component is the genius behind it.”

Sites with dirty and contaminated soil (such as the Morton Salt tract, which contained a tannery before the salt company moved in) are where the Infinity Media is particularly cost-effective. That’s because with it, you can cap off a site and place a porous, well-drained medium on top that aids plant growth without costly soil remediation, or the need for bulky and expensive StormTrap systems that create underground concrete stormwater bunkers yet still require new topsoil to be placed on top. Jenny Trautman, the development manager for R2, says that instead of spending money belowground on a StormTrap, R2 preferred to “beautify the site with things you can see.” Essentially, the Infinity Media allows designers to specify a material that doubles as a growing medium and a StormTrap—one that absorbs and contains water with granular, air-filled minerals instead of a hollow bunker.

The Omni Infinity Media sits on top of a concrete and asphalt cap. Image courtesy Omni Ecosystems.

Skowlund compared cost estimates for Infinity Media and a traditional gray infrastructure pipe-it-out approach for the Morton Salt site, he says, and the growing medium proved to be viable; it was 30 percent less expensive than traditional infrastructure. Based on the impermeable area of the site and its proximity to the river, Skowlund’s project was required to detain 2,250 cubic feet of water (or 16,800 gallons) during a major storm event, but it’s projected to be able to deal with more than twice as much stormwater. “In concept it’s great, but it has to save money,” he says.

Compared to traditional gray infrastructure methods and soil remediation, “This is just a more sustainable, clean way of adding greenery to sites,” Trautman says, “because there’s a lot of waste going to a landfill in the previous scenario.”

Once it’s renovated, the Morton Salt site will host office space, a concert venue, and a riverwalk. Image courtesy Omni Ecosystems.

As Omni Ecosystems gets more experience using this growing medium for stormwater management, Skowlund hopes they’ll be able to refine exactly how much is really needed at a site, and not a pebble more, to make it even more cost-effective. Already, they’ve learned that less growing medium is required for plants (especially trees, which can subsist on as little as 12 inches of Infinity Media), because the material’s structure allows for expedited nutrient transfer and root growth. And if things go well at the Morton Salt site, they should have plenty of opportunities. Morton Salt is “a unique site,” he says, “but it’s the tip of the iceberg, because pretty much every site we work on in Chicago is a dirty site.”