Water-monitoring finds pollution in Vannie Lake water

The water of Van Cortlandt Park’s lake has recently shown a spike in phosphorus, a common component of organic waste in sewage, and while the source of every pollutant may be difficult to pinpoint, many are flowing into the lake from upstream in Yonkers, according to a recent study. 

Overall phosphorus levels in the lake registered at negligible and nearly undetectable levels for months, but soared to about 1.3 milligrams per liter recently, according to yearlong water-monitoring study results presented by environmentalists from Friends of Van Cortlandt Park at Community Board 8’s environmental committee meeting on Nov. 16. The environmental group carried out the study together with Manhattan College, which hosted the Nov. 16 meeting. 

Pollutant levels in the lake “not crazy high for an urban stream,” but high nonetheless, John Butler, an ecological project manager at Friends of Van Cortlandt Park, said. 

The lake and its tributary, Tibbetts Brook, have also shown occasional increases in nitrate, according to the study. Nitrate levels tend to go down in summer months, when plants absorb the nutrient, and rise again in the fall and winter, Mr. Butler said. 

Contaminants in the lake come from runoff from the park’s parade ground, or are discharged from a pipe that discharges water at the lake’s western edge. And a huge share seems to come from Yonkers – carried by Tibbetts Brook downstream into Van Cortlandt Lake. 

“We have a large inflow that’s coming in from upstream,” Mr. Butler said. 

Yonkers authorities have been responding quickly to some reports of contaminants during the past year, he said. But while Van Cortlandt Lake is one of the cleanest in the city, Yonkers’ waterways are more heavily tainted – and to leak contaminants and invasive species downstream into the northwest Bronx park. 

In summer, a pipe in Yonkers leaked sewage into Tibbetts Brook, causing the levels of fecal coliform and enterococcus bacteria to rise in Van Cortlandt Lake, Mr. Butler told the meeting. 

His group reported the pollutants’ increase to the New York State Department of Environmental Conservation in mid-August, and the Yonkers Engineering department “got right on it,” hiring contractors to fix a leaky sewer pipe, he said. 

But Van Cortlandt Lake remains infested by water chestnuts, an invasive species, which apparently spread into Van Cortlandt Park’s lake from Yonkers’ Tibbetts Brook Park lakes. 

Getting rid of invasive plants may be a thorny problem: Once the plants are removed, “something else might move in that would be even worse,” Mr. Butler said. 

The group has been collecting water samples weekly from different sections of the lake and of Tibbetts Brook and taking samples for analysis to a Manhattan College Lab, in the yearlong study that started last December. 

At the environmental committee meeting on Nov. 16, the panel voted unanimously to adopt a letter of support for the water-monitoring project, with committee members expressing hope the it may help researchers receive grants to continue their work beyond next month.