For years people have walked by NYC’s urban lakes and ponds without knowing what the green stuff in the water really is. But dog owners in Prospect Park may know all too well: It’s toxic algae bloom.

“My dog walker who takes like 10 dogs out at a time, had the dogs over at the lake. They all got sick. She didn't know about it,” said Molly Jensen who takes her dog Roxy to the park daily.

What causes the algae to bloom in the first place? It turns out it’s from too much phosphate in the water, which has amassed from plant material in the lake bed and constant flow from the city’s water system.

“New York City tap water has phosphorus in it to keep lead from leaching out of the pipes, which is a really good thing for drinking water,” explained Justine Heilner, Senior Landscape Architect for Prospect Park Alliance. “But that is also the food for these microorganisms. So it kind of causes them to just grow out of control. It's like fertilizer for tiny plants, including blue-green algae, duckweed and the other things that can kind of take over a system.”

Molly Jensen and Ellen Lee walk their dogs Roxy and Mini-Pearl in Prospect Park daily.

The toxic, blue-green algae is hard to control without cordoning off sections of the lake during hot summer months when the algae is in full bloom. “If they get close to the lake, they're going to drink water,” explained Ellen Lee who also walks her dog Mini-Pearl in the park every morning.

Compounding the problem is the shallowness of park’s lake and ponds. In the late 19th century, Frederick Law Olmsted and Calvert Vaux designed them as “aesthetic pools,” described Christian Zimmerman, VP of Capital & Landscape Management for Prospect Park Alliance.

As a result, the water heats up allowing for more growth of the algae bloom, particularly as temperatures become hotter. “We need to solve this problem because it's not going away. It's getting worse and worse,” continued Zimmerman.

Managing blue-green algae blooming in the watercourse is typical urban waterway problem. While filters could be used, they are seen as too high-maintenance, requiring constant monitoring, and too expensive to replace.

Now, the park has teamed up with Brooklyn College to pilot a new nature-based solution called ecoWEIR which they hope will address the issue. “We were excited and they were excited to have a place where we could test it out, because it could be something that could be done at a large scale,” said Heilner.

The technology filters incoming water underground and uses plants grown on top to absorb excess nutrients, similar to other green infrastructure technology. However, unlike current green infrastructure technology which is highly variable in how much water actually gets filtered, ecoWEIR contains and holds the water until it can be released via a smart sensor.

Current methods for controlling harmful algae bloom often involve putting more things into the water: straw bales to absorb excess nutrients or even Clorox to kill bacteria for example. “People always concentrate on the water body itself,” said the inventor of ecoWEIR, Professor Jennifer Cherrier at Brooklyn College’s Department of Earth and Environmental Sciences. “But if you don’t stop the pollutants from going into the water in the first place, it’s just going to be never ending.”

Comparing current green infrastructure to ecoWEIR. Figure A is a typical method where results are variable. Figure B is the ecoWEIR system that contains and controls filtration.

The pilot in Prospect Park is using a pond liner for the ecoWEIR system, which can also be contained with natural materials like clay or benzonite. After installing a specialized pipe and valve system with the smart sensor, gravel and sandy soil are placed at the bottom, creating a reservoir to collect water, followed by a top layer of well-drained sandy soil. Native grasses are then planted in the top sandy soil layer, with their roots tapping into the bottom layer of stored water, minimizing the need for watering or irrigation.

The water-saturated soil in the bottom layer naturally undergoes an anaerobic process that removes the excess nutrients that can be absorbed by the plants. On a timed cycle—24 hours in the case of Prospect Park—the filtered water is released back into the watercourse leading to a lake or pond.

“I got the idea from doing a lot of research in wetlands and just trying to create the same function of the system, but without standing water,” said Cherrier. This system allows for multipurpose use of land, an important element for cities like NYC where space is limited. In other applications, ecoWEIR could even be used to reduce urban heat island effects in public spaces or buildings.

The pilot project in Prospect Park is being funded through a two-year grant by New York State Parks. The Prospect Park Alliance is testing two sites with different plants, one area with meadowy native grasses, and another using turf grass. Both sites will be planted next spring, but ground has already been broken in preparing the ecoWEIR system’s 4’ deep underground 20’x40’ containers.

Site Plan of the ecoWEIR pilot project in Prospect Park.

Zimmerman is looking forward to seeing how the pilot progresses. “If I could take 20 acres of the Long Meadow and put in a big filtration system underground, which you wouldn't see, it's just going to be the substrate and this really happy grass” fed by phosphates drawn up from the ground, he said. “And then it spills back into the watercourse through a certain amount of time. That would be phenomenal for us.”

Results of the water quality will be monitored by researchers at Brooklyn College and NOAA. If the pilot shows a reduction in harmful algae bloom by next year, the ecoWEIR system in Prospect Park could become a model for other parks in the city and beyond.

“It’s going to take a lot of tools in the toolbox,” said Cherrier of addressing the algae bloom. “But this is definitely one tool that I'm hoping that people use because it's a lot cheaper and it's nature based…Nature knows how to clean itself. We just have to give it the proper conditions so that it can do it.”

Hear reporter Clarisa Diaz talk about the Prospect Park project on WNYC's Morning Edition:

Clarisa Diaz is a designer and reporter for Gothamist / WNYC. You can follow her on Twitter @Clarii_D.