OUR GROUNDWATER SUPPLY: How it works
By John L. Turner
This is the first in a two-part series on Long Island’s water supply.
When thinking of Long Island’s groundwater supply – its drinking water aquifers – it helps to visualize a food you might eat while drinking water – say, a three-way turkey sandwich tiers – a slice of cheese on top, a juicy, thick disc of tomato in the middle, a slice of turkey on the bottom, all resting on a piece of hard, crusty bread.
Well, substitute the upper glacial aquifer for cheese, the thicker Magothy aquifer for tomatoes, the Lloyd aquifer for turkey, and a “bedrock basement” for bread and you have the system of Long Island Tiered Groundwater. It is this collection of underground aquifers – these sections of the sandwich – that are the only source of water for all the uses for which the people of Long Island use water. Hydrologists estimate that there are approximately 90 trillion gallons of water contained in Long Island’s groundwater supply.
Our sandwich model described above is not entirely accurate as there is another layer called the Raritan clay formation separating the Magothy and Lloyd aquifers. This layer of clay, about 200 feet thick, retards the movement of water (for a number of reasons water moves painfully slowly through clay) and is called an aquitard. So in our sandwich model, let’s make the thin but impactful clay formation a layer of mustard or mayonnaise. Except for this clay containment layer, Long Island is essentially a million-acre pile of sand whose geology is generally distinguished by subtle changes in the composition, texture, and porosity of its materials. geological – various mixtures of silt, clay, sand, gravel. and pebbles that affect transmissivity or water movement rates.
The bedrock basement (the bread in our sandwich) that underlies all of Long Island is metamorphic rock estimated to be around 400 million years old. It dips northwest to southeast dipping at about 50 feet per mile. Thus, while the thickness of freshwater aquifers in northwest Queens is only a few hundred feet, it is around 2,000 feet in western Southampton.
On the North and South Forks and the barrier islands of the South Rim, fresh water does not extend to bedrock as it does in Nassau County and much of the west and center of Suffolk county. It is shallower on the barrier islands, with the freshwater lens extending only several tens of feet.
On the North Fork it goes a bit deeper before the water turns salty and is deepest on the wide South Fork where the freshwater lens extends down to about 550-600 feet. The depth of the aquifer is influenced by the number of feet above sea level of the water table. There is a hydrological formula, called the Ghyben-Herzberg Principle, which states that for every foot of water above sea level, there are 40 feet of fresh water below.
Water from underground aquifers is not stored in large underground pools or caverns, as is the case in other places in the country with markedly different geology. On the contrary, the water is located between the tiny interstitial spaces existing between the innumerable particles of sand. that collectively make up Long Island. Given this, it is not surprising that groundwater flows (under the influence of gravity) slowly downwards and laterally (depending on where the water is in the aquifer) moving on the order of a few feet per day at most, but generally in the ballpark of about one foot per day.
It takes tens to hundreds or even thousands of years for water to move through the system, depending on where it first landed on the surface of the island. Water pumped from the seashore of the lowest aquifer – the Lloyd Aquifer – may have fallen as rain many years before the start of the ancient Greek Empire.
In the late 1970s, several government studies helped us better understand some of the basics of how the groundwater system works. One of the important lessons from this research is that it is the middle two-thirds half of the island that is most important for recharge – this segment is known as the “deep flow recharge zone” because a raindrop that lands here will travel vertically downward recharging the vast underground water supply.
The middle of this area is known as the “groundwater divide”; a drop of water that lands south of the divide will move down and then laterally in a southerly direction discharging into one of the bays on the south shore or into the salty groundwater beneath the Atlantic Ocean while a northerly drop will eventually move into Long Island Sound or the sandy sediments below.
Hydrologists have determined that for every square mile of land (640 acres), an average of about two million gallons of rainwater lands on the surface, with about one million gallons recharging the groundwater supply daily. What happens to the other million gallons? It evaporates, runs off into streams and other wetlands, or is taken up by trees and other plants which need it to support vital processes such as transpiration (a large oak tree needs about 110 gallons of water a day to survive).
In contrast, raindrops that land in locations closer to the coast, such as Setauket, north of Smithtown, south of Brookhaven, Babylon, or other locations along the north and south shores, do not part of vast underground water reservoir; instead, after percolating through the ground, the water moves horizontally, discharging either into a stream that flows to the salt water or into the salty groundwater that surrounds Long Island. These landscape segments are called “shallow flow recharge areas”.
The highest elevations along the Ronkonkoma Moraine (Long Island’s central spine created by glacial action about 40,000 years ago) are also the highest points of the water table, although the contours rise in the water table are an attenuated expression of the earth’s surface. Thus, in the West Hills area of Huntington where Jayne’s Hill is located, the highest point on Long Island at just over 400 feet, the water table elevation is about 80 feet above sea level. the sea.
Below the water table is the saturated zone and above the unsaturated zone where air, instead of water, exists in the tiny spaces between the sand particles (in the case of Jayne’s Hill, the unsaturated zone saturated extends approximately 320 feet). It is the water (more precisely its weight) in the upper regions of the saturated zones that pushes on the water below, causing the water in the lower parts to move first laterally or laterally and then back up into salty groundwater under the ocean. Due to the weight of the water, the freshwater-saltwater interface is actually off both coasts, which means you can drill from a rig a mile offshore from Jones Beach and tap into fresh water if you had to drill several hundred meters deep.
A wetland forms at the intersection of the land surface and the water table. It could be Lake Ronkonkoma, the Nissequogue or Peconic River, or one of more than a hundred streams that drain the aquifer flowing into bays and harbors around Long Island. So when you look at the surface of Lake Ronkonkoma, you are looking at the water table – the top of Long Island’s groundwater system. Since the elevation of the water table can change due to varying amounts of rain and snow and pumping by water providers, these wetlands can be affected; in wet years they may enlarge and shed more water while in droughts the wetlands may dry out largely, which happened on Long Island in the 1960s.
It’s clear, given the isolated nature of our water supply – our bubble of fresh water surrounded by hostile salt water – that we are the captains of our own destiny. Our underground water supply is the only source of water to meet all of our collective needs and desires. There are no magical freshwater underground connections to Connecticut, New York State, or New Jersey. We are not related, and it is unlikely that we will ever be able to tap into New York City’s water supply, provided by the Delaware River and several upstate reservoirs. As the Federal Environmental Protection Agency has stated, Long Island is a “single-source aquifer.” To paraphrase the late Senator Daniel Patrick Moyhihan: “The people of Long Island all drink from the same well. Indeed, we do.
The next article will detail the quality and quantity issues facing our groundwater supply.
A resident of Setauket, John Turner is conservation chair of the Four Harbors Audubon Society, author of “Exploring the Other Island: A Seasonal Nature Guide to Long Island” and president of Alula Birding & Natural History Tours.