Nor’easter Delivers Big Boost to LI Aquifer

The powerful nor’easter that soaked Long Island last weekend is expected to add nearly 40 billion gallons of water to Suffolk County’s aquifer, providing a major boost to the region’s underground drinking-water supply.

Some communities received more than five inches of rain, including 5.53 inches in Eastport and 5.25 inches in East Setauket.

Frank Mancini, superintendent of the Riverhead Water District, said groundwater levels have been running low following an extended period of dry conditions.

“There’s been a regional drought for at least a year and water levels are low, so this was much needed,” Mancini said.

Long Island’s aquifer depends on precipitation for recharge, and Mancini said roughly half of the rain that falls typically reaches the groundwater system. The rest is lost through runoff, evaporation and vegetation.

The 40-billion-gallon estimate is based on five inches of rain falling across Suffolk’s roughly 912 square miles of land. One inch of rain over a square mile equals about 17.4 million gallons, putting five inches at roughly 79 billion gallons. With about half ultimately reaching the aquifer, the recharge would be close to 40 billion gallons, Mancini calculated.

“This time of year, water use is at a minimum,” Mancini noted. “The summer’s over, and the ground’s not frozen, so it’s a good time to receive this type of recharge.”

Mancini said Suffolk’s aquifer is generally capable of providing substantial quantities of water, though conditions become more challenging toward the East End, where the land narrows and there is less area available to capture precipitation.

“Every bit of precipitation helps,” he said.

Recharge is not instantaneous. Rainfall first filters through soil and sand into the shallow groundwater system before gradually moving into the water supply.

Mancini said the Riverhead Water District uses about 20 million gallons on a peak summer day. At that rate, the nearly 40 billion gallons expected to recharge Suffolk’s aquifer would equal about 2,000 days — or roughly five-and-a-half years — of peak-day water use for the comparatively small district.