CONCOURSE — The vacant 4-acre lot on the Bronx bank of the Harlem River is normally a pretty desolate place — empty concrete caged in by barbed wire and overshadowed by the elevated Major Deegan Expressway.
Then suddenly, this month, the site sprouted a mini-prairie and wetland.
The pop-up greenery was installed by a Bronx environmental group to capture and clean the thousands of gallons of dirty rainwater that stream off the highway and into the river during storms.
It’s also meant to inspire locals to reimagine the space, a former rail yard just north of the 149th Street Bridge dubbed Pier 5, which is now used as a storage space for the Parks Department and a staging ground for a traveling circus.
“This shows what’s possible,” said Chauncy Young, a board member of the volunteer-run Bronx Council for Environmental Quality (BCEQ), which spearheaded the $200,000 pop-up project.
The installation includes two swimming pool-sized plastic basins, which are divided into categories. One is a wetland and the other is prairie.
The wetland basin contains a lightweight organic material called GaiaSoil atop a layer of Styrofoam, which allows a bed of native plants to float above a foot of water. The prairie basin is also filled with plants, but contains less water.
A massive drainage pipe that collects runoff from a 10th of an acre of the expressway has been diverted to deliver its water into the basins, which can hold about 135,000 gallons of rainwater.
The layer of 5,000 plants covering the basins prevents the collected rainwater from becoming a breeding ground for disease-carrying mosquitoes.
The plants also naturally treat the runoff — likely polluted with trash, fuel and debris from the highway — before allowing the filtered water to evaporate back into the atmosphere.
Now, all that dirty rainwater will be kept out of the river and cleaned, without relying on expensive drainage systems or treatment facilities, organizers said.
“Nothing like this has ever been put in place before,” said Paul Mankiewicz, the biologist who created GaiaSoil and helped design the pop-up wetland.
U.S. Geological Survey scientists, with help from local Hostos Community College students, will monitor how much water is captured, what is in it and how well the system cleans it.
That data could then be used to design a permanent wetland installation at the site and, perhaps, to make a case for installing similar systems under the other 700 miles of elevated roadway in the city.
“The thing that we’ve been saying for a long time,” said Dart Westphal, a BCEQ board member, “is that many places that are good places to have new parks could also serve other purposes, particularly capturing storm water.”
The pop-up wetland is just one piece of a grand plan to revitalize the Bronx waterfront along the nine-mile Harlem River.
On Wednesday evening, the group held the first of several public sessions to solicit locals’ ideas for a Pier 5 park.
Some suggested fishing areas, basketball courts, an amphitheater or kayak and boat docks, though organizers warned that a waterfront rail line severs the pier from the river.
Nayeh Yisrael, a Hostos student, said she just wants a grassy place to lounge between classes.
Noel Hamlet, a Bedford Park resident, proposed a planetarium or solar power generators at the future park.
“This is good,” he said about the planning session. “It’s got to start somewhere — with ideas.”