Brookhaven Supervisor Edward P. Romaine has asked Suffolk County Executive Steve Bellone to support a joint effort to install septic systems at four public parks near Long Island’s largest freshwater lake.

In the Jan. 19 letter, Romaine said Lake Ronkonkoma’s water quality is plagued by nitrogen flowing from septic systems in the watershed and pathogens from restrooms in the parks.

“A coordinated project to install I/A [innovative/alternative] systems at four parks along the lake’s shoreline would be an important step towards restoring the lake to its former place as a premier destination for residents and tourists,” Romaine wrote in the letter. “It would also show that we can work cooperatively to overcome some of the region’s water quality issues.”

In a Tuesday interview, Romaine said the project would consist of replacing cesspools and installing new septic systems at the bathrooms in the parks.

Islip, Smithtown, Brookhaven and the county own land in the parks. Romaine estimated each of the four systems in the parks would cost at least $50,000.

Having all four municipalities join together would be a “regional action” with the goal of reducing nitrogen pollution, Romaine said.

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“The lake is stressed out and polluted. As a boy, a long time ago, it was crystal clear. I remember swimming there,” Romaine said.

Islip and Smithtown officials did not respond to requests for comment.

Lake Ronkonkoma is Long Island’s largest freshwater lake. It has a rich history and was once known for its swimming, fishing and boating. It is a kettle-hole lake carved by receding glaciers at the end of the ice age and was once the centerpiece of a thriving early 20th-century resort.

Now the lake is polluted with rainwater runoff and trash. Parts of the lake are 60 feet deep.

In his letter, Romaine said he and Bellone “share a vision . . . to restore clean water to our lakes, ponds, streams and bays for the benefit of our residents.”

Romaine’s letter dovetails with the county’s own efforts to improve water quality at the lake. Suffolk officials have filed grant proposals to pay for the installation of new septic systems at the same parks.

Suffolk County Legis. Leslie Kennedy (R-Nesconset) said Tuesday that she reached out to Brookhaven regarding how to prevent storm runoff from entering the lake if the county is indeed awarded the grant money.

In 2015, the state Department of Environmental Conservation confirmed the presence of blue-green algae in the lake, and high rates of nitrogen also have been blamed for stagnant water.