New federal data has confirmed what Long Islanders whose basements flooded this spring probably suspected: The water table rose this year, with local groundwater levels on average 2.5 feet higher than they were last April.

Months after March's soaking storms, scientists with the U.S Geological Survey said groundwater remains at near-record highs on Long Island and in New York City. "We have just begun this month to see water levels decline," said Steven Terracciano, supervisory hydrologist in charge of the agency's Coram office.

Terracciano said this year's spike is due in part to the big snowstorms and March rains that came before the growing season. "Plants aren't using any of the water and so it moves more efficiently down through the water table," he said.

The annual survey measured groundwater levels in April at more than 600 wells across Long Island and New York City. The most dramatic spikes were observed at slightly elevated areas such as Calverton, where clay-like soils prevent water from draining quickly. The biggest jump was measured at one well south of Sag Harbor, where groundwater was 9.2 feet higher than last year - something unlikely to trouble residents because the water table was still 40 feet below the surface.

It was a different story in Riverhead, where water flooded homes on Horton Avenue and made a lake in the middle of Osborne Avenue. One lane has finally reopened after months of pumping, said Town Supervisor Sean Walter.

USGS researchers said that even small rises in the water table caused trouble in low-lying areas on the South Shore. That's because basements, septic tanks and other underground structures lie closer to the water table in places like South Valley Stream or Center Moriches, where the depth to groundwater measured just 5 feet in April.

"We got two feet of water in the basement and it wouldn't go down for at least a month," said Phyllis Tamburello, 58, of Mastic Beach. "I had $10,000 worth of damage in my basement." Her insurance did not cover the damage because the flood was caused by groundwater, Tamburello said.

Groundwater also continues to soak some lawns, basements and roads in Smithtown and Shelter Island.

Scientists expect the water table to go down over the next few months. But they caution that this kind of flooding could become more common.

Many scientists expect that global increases in temperature will result in rising sea levels and more frequent and intense storms - conditions that the agency said would increase the potential for groundwater flooding. "If the predictions bear out, these anomalous conditions that we're experiencing now may not be anomalous in the future," Terracciano said.

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