The tap
Where does Long Island drinking water come from?
Rain and snow soak into one sand aquifer under Nassau and Suffolk — the EPA named it a sole-source supply in 1978. There is no upstate reservoir behind it; Nassau is mostly sewered, and about three-quarters of Suffolk homes still use a septic tank or a cesspool.
Updated 2026-09-15 · Pipedeck
| Public water | Private well | |
|---|---|---|
| Where it starts | Same 1978 sole-source sand | Same sand |
| Between sand and glass | A district plant | Nothing unless you add it |
| Typical in | Almost all of Nassau; many Suffolk streets | County plan figure about 45,000 wells |
| Wastewater on the lot | Can still be a septic tank | Often a cesspool or tank too |
| Useful next page | Tap treatment if you want a filter | The well guide |
Nassau and Suffolk take drinking water from the same sand. The federal government labeled that sand a sole-source aquifer in 1978. The kitchen meaning is blunt: the glass is rain and snow that soaked into the island. It is not Catskill tunnel water. It is not a river intake. Census Vintage 2025 counts 1,398,939 people in Nassau and 1,546,090 in Suffolk. That is close to three million people on one supply.
A public water district still pulls from that sand. A private well is the same sand with no plant in between. The county line is not a different island. The useful split at the house is which pipe you have, then whether wastewater leaves the lot in a sewer or goes back into the ground.
Sole-source means no backup
Sole-source is a federal designation, not a bottled-water brand. It means the two counties have no second drinking-water supply sitting behind this one. No mainland pipeline. No backup reservoir. A filter cartridge treats the glass you already have. It does not import a different island.
People who grew up on New York City water assume tunnels. Nassau and Suffolk are not on that system. There is no Catskill or Delaware intake for this kitchen. If the house is in either county, the tap is this sand after a plant, or this sand from a well cap, and nothing else.
The year 1978 is the label. It is not a cleanup finish date. The island drank this sand after 1978. It still does. A new sewer on one block does not create a river. A new plant treats what arrives at that plant. The supply remains the sand under the house.
Rain and snow recharge the only supply
Sand stores water and moves it. Rain that hits a roof, a driveway, a lawn, or a cesspool field soaks down. That recharge is the drinking water. It is not a metaphor. It is the physical path from weather to glass.
Fill a kettle in Kings Park, Levittown, Patchogue, or Greenport and the water started as recharge into sand. The name printed on a district bill is the plant and the pipes. It is not a different island. If there is no bill, the well is still that sand.
What goes into the ground on a lot can show up later in a well field or a well cap down the street. A treatment plant can strip some of what arrives at a public well. A private well does not get that plant. That is the difference that matters at the kitchen sink, not a slogan about “island water.”
Nassau sewers; Suffolk still mostly does not
Drive Nassau and you are usually on a sewer. About 85 percent of Nassau homes are. Drive Suffolk and the pattern flips. Suffolk is only about 26 percent sewered. About three-quarters of Suffolk homes still use a septic system or a cesspool. County figures put on-site systems near 360,000.
What leaves a sewered Nassau bathroom usually leaves the lot in a pipe, toward a plant. What leaves an unsewered Suffolk bathroom often does not leave the island. It goes into a tank or a cesspool, then into the same sand that later shows up in the glass. That is ordinary plumbing. It is not a puzzle you need a consultant to read.
If the town sends a sewer bill, wastewater is leaving the lot in a pipe. If you pump a tank, or you have a cesspool lid in the yard, wastewater is still going into the sand. You do not need a map of every district to see that at the house. The 26 percent sewered figure is the county structure. It is not a promise that your block already converted.
Public water over a tank is still this sand
A house can sit on a septic tank and still drink public water. A house can drink from a well and wait years for a main. Those are two different setups on the same aquifer. Unsewered is not a synonym for a private well. A private well is not a synonym for a cesspool. Plenty of Suffolk lots are both. Plenty drink public water over a tank. South-central Suffolk mixed the two for years.
A water-district bill does not mean the water came from somewhere else. It means a plant sits between the sand and the tap. Nassau is mostly on public supply. Suffolk mixes public mains with private wells. Nassau has almost no private wells left — on the order of 500. Suffolk’s county water plan still cites about 45,000 private wells, on the order of 200,000 people. Those are plan numbers, not a door-to-door census. If you do not get a water-district bill, treat the house as a well.
When the county sampled private wells from 2016 to 2022, about 47 percent showed some PFAS. Two hundred and two of those wells were over the drinking-water standard. Some East End hamlets were denser. In Mattituck, one round found 20 of 38 private wells over the PFAS limit. Around Medford and Bellport, forever chemicals showed up in wells, and public mains reached some of those streets years later. A neighbor on the main is not your pipe. A main that arrived years later does not rewind the years the well was the tap.
If the house is already on public water, skip a well kit unless you actually have a well. The kitchen job, if you want one, is a filter that matches the chemical. The sand does not change because the bill says public.
A cesspool and a well share the lot
If both are on the lot, you drink from the sand and return water to it. Over the decades the deeper sand has carried more nitrate. A household does not need a chart for that. It is one reason a Suffolk septic system on this aquifer is not set-and-forget plumbing.
Nitrate in the Magothy — the deep drinking formation public-supply wells often use — rose more than 80 percent from 1987 to 2013. That rise is the on-site wastewater signal in the drinking sand, not a one-year spike. Nitrogen from a cesspool stays on the island. It recharges. Nassau’s sewers explain why that county is not the whole story. Suffolk’s 360,000 on-site systems are the load that still sits on the sand.
Nitrate is not PFAS. It is not 1,4-dioxane. Forever chemicals and solvent leftovers share the sand. They do not share a single next step at the sink. A nitrate rise is not a filter box. A filter box is not a cesspool. Keep those jobs apart so the kitchen buys the right tool, or none, instead of a slogan.
One hamlet sewer vote is not a county conversion
Sewering arrives hamlet by hamlet, not as one island switch. Oakdale’s sewer vote passed about 88 percent in December 2025 when grants paid for construction. Great River voted no in 2019. Those are two votes, years apart, with different money on the table. Grants moved Oakdale. Great River did not get that same deal in 2019.
The aquifer does not wait on the referendum calendar. A hamlet with money on the table is not a county conversion. Until later connections actually land, Suffolk is still about 74 percent unsewered. The 360,000 on-site systems remain the structure under most Suffolk lots. A yes vote in one place does not sewer Patchogue, and a no vote in another does not rewrite Nassau’s 85 percent.
If you are trying to read your own street, ignore the island-wide headline. Look at whether a sewer bill exists, whether a tank gets pumped, and whether a well cap sits in the yard. Those three facts tell you how this lot uses the sand.
Listed sites sit on the sand. They are not a second tap
People hang a round number on this sand: 500 Superfunds. That is the wrong label. The count used here (Deziel, 2025) is 34 federal listed sites plus 449 state listed sites — 483 in all. That is not 500 Superfunds and not 500 sites on the federal Superfund list. Those listings sit on the same aquifer. They are not a second drinking-water supply.
Calverton, at the old Navy/Grumman plant, has local well and plume numbers: 1,4-dioxane at 14.9 ppb against a 1 ppb standard, and one PFAS compound (PFNA) at 7,580 ppt against a 10 ppt line. The plume runs toward the Peconic. Remediation study work has been pushed toward 2039. Movement on the order of a foot a day is the site story. It is not a reason to diagnose every hamlet, and it is not a pitcher.
Bethpage is a different Navy–Grumman leftover: a TCE plume on the order of 4 by 2 miles, and mostly public supply. A 2013 health study around that plant is local incidence, not a census of every kitchen on the island. If the Bethpage house is on a district main, a well kit is the wrong first tool. If a well cap is actually in the yard, that lot is the exception, not the town.
Named sites matter at those addresses. They do not give Nassau and Suffolk a river. This page stays on the sand.
The next step is the pipe you have
If the house is on a well, start on the well guide. Suffolk County well testing is 631-852-5810 — that is the county office, not this site, and that $100 panel is a different list of tests than a mail-in kit built for PFAS and 1,4-dioxane. This page is not that office. Legal mail for Pipedeck is 65 Cedar Street, Kings Park, NY 11754. There is no public phone here.
If the house is on public water and you want treatment at the sink, the PFAS page is the carton marks. The 1,4-dioxane page is reverse osmosis, because a pitcher fails that chemical. This URL is the sand under both jobs. It is not a shopping cart, and it does not sell a filter from this screen.
At the house you either test a well or match a filter to the tap. You do not get a second supply by waiting.
If you have a wellA PFAS filter for the tap1,4-dioxane reverse osmosis
This page has no buy link. Guides that sell a test or a filter disclose commissions on those URLs. How that works
Common questions
Where does Long Island drinking water come from?
Rain and snow that soaked into one sand aquifer under Nassau and Suffolk. The EPA named it a sole-source supply in 1978. There is no backup drinking water.
What does a sole-source aquifer mean for Nassau and Suffolk?
It means the two counties have no second drinking-water supply. No upstate reservoir. No river intake. A kitchen tap and a private well both pull from that sand.
Did the 1978 sole-source listing clean up Long Island drinking water?
No. 1978 is the federal label, not a cleanup finish date. The island drank this sand after 1978. It still does. A new cartridge or a new sewer on one block does not import a different supply.
Is Long Island drinking water piped from upstate?
Not for Nassau and Suffolk. Those counties are not on the Catskill or Delaware tunnel system. Sole-source means there is no mainland pipeline and no backup reservoir waiting off-island.
Can Nassau and Suffolk get drinking water from the mainland?
No. Sole-source means there is no second drinking-water supply. No upstate reservoir. No river intake. No backup waiting off-island. A filter at the sink treats the glass. It does not import a different island.
How many people drink from this one sand aquifer?
Census Vintage 2025 counts 1,398,939 people in Nassau and 1,546,090 in Suffolk. That is close to three million people on one drinking-water supply. A public district still pulls from that sand. A private well is the same sand with no plant in between.
Does a water-district bill mean my drinking water came from somewhere else?
No. A district bill means a plant sits between the sand and the tap. It does not mean the water came from upstate or from a river. Nassau is mostly on public supply. Suffolk mixes public mains with private wells. Both still pull from this aquifer.
Is most of Suffolk still on septic?
Yes. About three-quarters of Suffolk homes still use a septic system or a cesspool. Suffolk is only about 26 percent sewered.
How many Suffolk homes are on a septic system or cesspool?
County figures put on-site systems near 360,000. That is the septic load on this aquifer, not a well-test product and not a Nassau sewer map.
Is Nassau County on sewers?
Mostly. About 85 percent of Nassau homes are sewered. Nassau still drinks from the same sand. A sewer sends wastewater to a plant instead of back into the ground on the lot.
If my Suffolk house is on septic, am I automatically on a private well?
No. Unsewered is not automatically a private well. A private well is not automatically a cesspool. Plenty of Suffolk lots drink public water over a septic tank. Plenty are both a well and a cesspool. Look at the bill and the yard, not a county average.
Can you have public water and a septic tank in Suffolk?
Yes. South-central Suffolk mixed public mains over a tank for years. A main on the street is the drinking pipe. A tank in the yard is still wastewater into the sand. Those are two setups on one aquifer.
If Oakdale voted yes on sewers, is Suffolk now sewered?
No. Sewering arrives hamlet by hamlet. Oakdale’s sewer vote passed about 88 percent in December 2025 when grants paid for construction. Until later connections actually land, Suffolk is still about 74 percent unsewered.
Did Great River vote for sewers in 2019?
No. Great River voted no in 2019. Grants moved Oakdale in 2025. Great River did not get that same deal in 2019. Two votes do not convert the county.
If I have a cesspool and a well, am I drinking the same sand?
Yes. You are drinking from the sand and returning water to it. Nitrate in the deeper Magothy sand rose more than 80 percent from 1987 to 2013. That is the on-site wastewater signal, not a separate island supply.
Why did nitrate in the Magothy sand rise?
Nassau sends most wastewater to a plant. Most Suffolk homes still send it into the ground. County figures put on-site systems near 360,000. Nitrate in the Magothy — the deep drinking formation public-supply wells often use — rose more than 80 percent from 1987 to 2013. Nitrogen that leaves a cesspool stays on the island.
Is nitrate the same problem as PFAS in this aquifer?
No. Nitrate is the wastewater story. Forever chemicals and solvent leftovers are later, different jobs. They share the sand. They do not share a single next step at the sink. A nitrate rise is not a PFAS filter, and a PFAS filter is not a cesspool.
Does rain recharge Long Island drinking water?
Yes. Rain and snow that hit a roof, a driveway, a lawn, or a cesspool field soak down into the sand. That recharge is the drinking-water supply. There is not a second reservoir behind it.
Do Kings Park, Levittown, and Patchogue drink the same aquifer?
Yes. Fill a kettle in Kings Park, Levittown, Patchogue, or Greenport and the water started as recharge into the same sand. The name on a district bill is the plant and the pipes, not a different island.
Does Long Island have 500 Superfund sites on its drinking water?
That round number is the wrong label. A 2025 count used here (Deziel) is 34 federal listed sites plus 449 state listed sites — 483 in all. That is not 500 Superfunds and not 500 sites on the federal Superfund list. Those listings sit on the same sand. They do not give the kitchen a second tap.
EPA named the sole-source sand aquifer in 1978. Census Vintage 2025 county populations. Suffolk County water-plan well counts; Nassau and Suffolk sewer and septic patterns, including about 360,000 on-site systems; Magothy nitrate 1987–2013; private-well sampling 2016–2022. Oakdale sewer vote December 2025; Great River vote 2019. Deziel 2025 listed-site count (34 federal + 449 state = 483), not 500 Superfunds. Calverton and Bethpage site figures are local, not a second supply.