How Deep Is the Water Under Your House in Nassau County?

Only where the water table sits close to your floor, and in much of Nassau County it does not. USGS wells read in spring 2016 found it 0.37 feet down at one harbor-edge well and more than 160 feet down on the hills, so the same damp floor can be groundwater in one place and a problem inside the house in another.

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What the federal depth map says about wet lower levels

Knowing roughly how far down the water sits tells you which fixes are worth pricing and which are aimed at nothing. The U.S. Geological Survey drew exactly that picture for Long Island in April and May 2016: a map of depth to the water table, with the shallow ground shaded red. In the map's own words, the red areas are where potential substructure flooding is more likely. The reverse is the useful part for most Nassau homeowners. Where the map shows the water tens of feet below the surface, there is no groundwater close enough to push up through a floor, and the damp has to be coming from somewhere else.

How far down it measured across the county

The map rests on real wells: USGS built it from 275 observation wells plus stream and lake gauges and the coastline, and it plots its own field readings, taken to the hundredth of a foot, on the sheet. Across the center of the county those spring 2016 readings ran roughly 19 to 48 feet down, including about 25 feet in Garden City and Levittown and 47 feet in Hicksville. On the ridge in the northwest they went past 140 feet in Lake Success, and a Syosset well read almost 165 feet. The shallow readings on the North Shore came from wells at the harbor and Sound edges, all standing 18 feet above sea level or lower.

Why two wells a mile apart can disagree by 165 feet

On Manhasset Neck, a Port Washington well read the water 0.37 feet below the ground in May 2016, while a Flower Hill well about 1.3 miles away read it 165.27 feet down that April. Both draw on the same upper sand and gravel layer. The difference is the land, not the water: USGS found that on the Neck the water table tends to parallel the land surface, so it rises under the hills but the hills rise faster. That is also why a headline about Nassau having the island's highest water table, which USGS does report for the northwest of the county when it is measured against sea level, says nothing about how far it sits below your floor. Height and depth are different numbers.

What the map cannot tell you about your own house

USGS is plain about the limits. The depth map is accurate at a scale of 1:125,000 with a vertical error of plus or minus 5 feet, because the land elevation data and the well data behind it were mapped at different scales. Along stretches of the south shore with no well data, including the marshes and the barrier island, the map simply assumed the water stood at sea level. And it is one season in one year. So we will not tell you the depth under a particular street or lot, and nobody should from this map alone. What it does is set the odds: shallow near low shoreline ground, deep under the hills and much of the middle of the county.

Is the water table rising under Nassau?

You may have read that rising groundwater is flooding basements nearby. That finding is real, but USGS made it for Kings and Queens Counties, after public supply pumping there was halted, and its report makes no such finding for Nassau. In Nassau the agency measured the opposite effect in the sewering era: in one 32 square mile sewered area, groundwater fell 11.8 feet against an unsewered area to the east between 1953 and 1972, and a later USGS model put the lowering from sewers at up to 18 feet in central Nassau. We are not aware of a USGS finding of a county-wide rise since, and we do not claim a trend either way.

What to do with the number

Use it to decide what to rule out first, not what to buy. If the house sits on low ground near the water and the damp shows at the lowest floor, groundwater belongs on the list and a sealer painted on from inside may be fighting a table a few feet down. If the house sits up the slope or in the middle of the county, a sump pump is aimed at water that is not there, and the search belongs in the house: the slab, the pipes, the gutters and grading, and condensation. A licensed assessor is the person whose job includes finding that source, and the practical direction test is in the next guide.