The Charles River—the 80-mile river that hugs the north side of Boston before draining into the Atlantic Ocean—has a problem: it’s not always swimmable. Despite efforts for decades to clean the river, the Environmental Protection Agency still faces barriers to make it clean enough to swim year-round.

That’s not to say the EPA hasn’t made great progress; historically, the river was regularly contaminated with sewage.

“When there were big rain events, it would combine into the sewer system and blackwater would flow out [into the river],” says Kate Kennen, associate teaching professor of landscape architecture at Northeastern. “It was really an environmental mess.”

Now, “on certain days of the year, it’s swimmable,” she says.

But chemicals like phosphorus, which comes from fertilizers, animal feces and other sources, still get drained into the river via rainwater, Kennen says.

That’s why, for the past five years, Kennen and her team at Offshoots and Horsley Witten Group have spearheaded a new landscape project at Northeastern’s Henderson Boathouse, which abuts the Charles on Soldiers Field Road in Boston, that will help prevent contaminants from reaching the river in the first place. 

The team does this by strategically placing plants so that they serve a dual purpose: they look nice, but they also clean pollution from the stormwater before it enters the river.

That is what Kennen and her team did at a small vacant property in Cambridge, where she suspects that contaminated groundwater runs from an old factory. When Kennen was charged with landscaping for a new openspace, she placed a line of hybrid poplar trees so they ran perpendicular to the groundwater’s path. These deep-rooted trees were integrated into the landscape, but they also processed any petroleum in the water that ran through their roots.

Kennen applied similar concepts to the landscaping around Henderson Boathouse when it was time to install a new parking lot. The renovations, which ran for five years and finished last month, took into account aesthetics while also putting the river’s health at the forefront. 

Similar to the Cambridge project, the Henderson landscape uses herbaceous perennials to clean storm water of pollutants before it reaches the Charles. After a big storm, Kennen says, the runoff is loaded with nutrients like phosphorus, as well as small particles that have chemicals and nutrients bound to them. When this water gets into the river, it loads it with these pollutants. They encourage algae blooms and cyanobacteria that make the water unswimmable.

Permeable pavement now addresses that problem at Henderson. The parking lot has pavement with grass that creeps through the crevices, so water can be absorbed instead of running off to the river and carrying harmful chemicals with it. The cars are parked on permeable pavements as well, so any chemicals the cars bring to the environment get cleaned out. 

But the main feature of the new landscape is the bioretention area, a basin that captures and treats excess water before directing it into the river. Designed to treat the volume of water produced by a large storm event, the bioswale features an inlet that captures stormwater that runs off the parking lot. A series of dams punctuates the basin so each section can hold water and force it to seep into the soil.

From there, the soil and the plants work together to cleanse the water of different levels of pollutants before it runs into the Charles. The top layer of soil is biosoil, meant to allow water to seep through quickly while stripping it of pollutants like phosphorus. The water seeps down into the second layer—gravel—and into perforated pipes that overflow into the river when water isn’t directly infiltrated into the ground. 

The dissolved phosphorus, meanwhile, gets treated by the soils and  plants, which are all native species and are strategically spaced so that the plants in the middle are adapted to thrive in the wettest areas. 

This gives the bioswale a more rustic look. “It looks more wild,” Kennen says. But that belies its very specific function. “It’s a highly engineered system.”

Kennen recently brought her students there to demonstrate how the system works, and she hopes that it can continue to be an educational tool for students and visitors. 

“What’s so nice about this is this idea of a nature-based solution where you’re getting habitat and wildlife benefits at the same time as pollution removal,” she says. “You’re getting aesthetics, the plants are getting water, we’re getting the water cleansed, and it’s a model educational  landscape.”

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