Long Island, New York, is the largest island in the continental United States. It is located at the southeastern tip of New York State and projects out across the Atlantic continental shelf. The Native American name for Long Island is Paumonauk, meaning "fish-shaped island." The south side of Long Island faces the Atlantic Ocean and the north side of the island forms the southern coastline of the Long Island Sound. The eastern end of Long Island tapers off into two peninsulas known as the north and south forks. Long Island is approximately 120 miles long and 15 miles wide, and has a total area of about 1,400 square miles. Four counties make up Long Island: Kings, Queens, Nassau and Suffolk.
Long Island Geology
Long Island is made up of a series of sand and gravel aquifers. All of Long Island's water supply comes from underground water held in aquifers. An aquifer is a geologic formation, which can hold, transmit and yield water in usable quantities. Clay layers between certain aquifer layers act to retard some water movement beneath the island. Stacked one on top of the other like layers in a cake, three major and one minor aquifer make up the Long Island aquifer system.
In sequence from shallowest to deepest, the LI aquifers are: the Upper Glacial, the Magothy and the Lloyd Aquifers. A minor aquifer, the Jameco, lies beneath the Upper Glacial, along the western south shore of Long Island. All of the aquifers slope downward toward the southern portion of the island. This causes the deepest parts of the aquifers and the groundwater system to reside along the southern shore of Long Island, and extends out into the subsurface material beneath the ocean floor.
The aquifer formations beneath Long Island were deposited at different times in the area's geologic history. The deepest aquifer layer, the Lloyd, rests on a bed of consolidated bedrock, primarily igneous and metamorphic. The bedrock was laid down in the Precambrian Era, making it older than 600 million years. The sand deposits of the Lloyd Aquifer were laid down in the Cretaceous Period, about 80-100 million years ago. The sand and gravel of the Magothy aquifer was deposited in the upper Cretaceous Period, about 50-80 million years ago. The youngest aquifer formation, the Upper Glacial, was laid down during the last ice age, 10-15 million years ago. It contains course sands, pebbles, rocks and occasionally boulders, carried to Long Island and left behind by the glaciers. The water table, or the top of the groundwater system, is found in the Upper Glacial aquifer.
A thick deposit of clay (100-200 feet thick) called the Raritan Clay exists between the Magothy and Lloyd aquifers. The Lloyd sands and the clay bed above it are together known as the Raritan Formation. The Lloyd aquifer is found along the north shore of Long Island at depths averaging 200-300 feet. Along the southern shoreline the Lloyd aquifer is approximately 1,500 feet below the land surface. Along the south shore a second clay bed separates the Upper Glacial aquifer from the Jameco aquifer. It is called the Gardiners Clay. Clay units have very low permeability and act as aquitards, minimizing water exchange between the layers. (view Figure 1).
Long Island Groundwater
All Long Island aquifers receive their fresh water from precipitation. Long Island receives, on average, about 44 inches of precipitation a year. Of this, about half of the precipitation, or approximately 22 inches of rain, percolates into the ground and is recharged into the groundwater system. The remaining precipitation is either evaporated, taken up by plants, or runs off into creeks, bays and estuaries. In areas where the water table and the ground surface meet, streams, ponds and wetlands are formed. In an undisturbed natural setting, (e.g., before human activities) all of Long Island's groundwater would ultimately reach the coast where the groundwater would mix with and the ocean (this process is called underflow - view Figure 2). Due to human activity, this process has been significantly changed so that not all water in the groundwater system is returned to the ocean.
Today, groundwater is withdrawn from the system constantly. Over 138 billion gallons of water is taken each year from beneath Nassau and Suffolk Counties. In coastal areas, as water is drawn up for use, less groundwater is available to be discharged into the estuaries. The resulting loss of water and pressure allows saltwater from the ocean to flow into the aquifer, causing the groundwater to become saline, resulting in a condition called "saltwater intrusion".
New water from precipitation is constantly recharging, or replenishing, the aquifers. Unfortunately, as water recharges the system, it can easily carry contaminants with it into the groundwater. Since it is the shallowest and closest to most sources of contamination, the Upper Glacial aquifer is the most heavily contaminated of the three. The next most seriously contaminated aquifer is the Magothy, which is the layer below the Upper Glacial. The Magothy aquifer supplies over 90% of the water used in Nassau County and about 50% of all water used in Suffolk County.
Although water can flow from one aquifer to another, groundwater on Long Island generally travels in a easterly direction. This horizontal movement offers some protection for deeper aquifers, however pollutants can still contamination aquifers below the Upper Glacial.
Updated by tbono 3/30/10