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The ultimate source for new freshwater entering the aquifer systems is precipitation. The process of water replacement is called recharge, where water slowly percolates through the unsaturated soils, reaches the water table and enters the vast groundwater reservoirs of sand, gravel and clay. Water slowly migrates deeper into the aquifer formations over time. Groundwater moves through the aquifer systems under the influence of pressure and gravity. The water in the Upper Glacial and upper portions of the Magothy is unconfined and influences the level of the water table. Water in the deeper Magothy and the Lloyd Aquifer as well as the Jameco Aquifer is confined; meaning a confining bed of clay overlies it. This water is pressurized, and such aquifers are known as artesian aquifers.

The water table is not flat, as the term implies. Rather, the water table surface is contoured, rising and falling in a mirror image of the land surface itself, only not as sharply. The water table dips down close to sea level along the south and rises up beneath the high ridgeline running west to east, just north of the Long Island Expressway. The water table may be 70 feet or more above sea level along the moraines. Further north, along the north shore, the water table dips down again as it approaches the Long Island Sound.

The highest point in the water table is called the "groundwater divide". It follows the long, low hill-like formations known as the terminal moraines that run the length of Long Island. Two moraines are found on Long Island, remnants of the two advances of the glaciers during the last ice age. One moraine (the Ronkonkoma Moraine) is southern-most and ends at Montauk Point on the South Fork. The northern moraine (the Harbor Hill Moraine) ends at the tip of the North Fork. Precipitation falling in this area of Long Island has the ability to infiltrate very deeply and can recharge the Magothy and the Lloyd quicker than in other areas. These areas are called "deep-flow" recharge zones.

The groundwater divide acts as an invisible boundary within the aquifer system. Water entering the groundwater on the north side of the groundwater divide eventually migrates northward toward Long Island Sound. However, water that enters the system on the south side of the divide moves toward the south, eventually discharging into the south shore bays or the Atlantic Ocean.

Rain falling along the island's perimeter, outside of the deep-flow recharge zone, replenishes, mainly, the Upper Glacial aquifer. Most precipitation, however, falls within the large deep recharge zone, where it percolates deeply into the Magothy and Lloyd aquifers. Unfortunately, this means that contaminants on the land within the deep-flow recharge area can also infiltrate the Magothy and Lloyd easily. This, consequently, reduces the percentage of clean groundwater recharging the system.

Water moves from areas of high pressure to areas of low pressure. The high elevation of groundwater in the middle of the island creates high pressures that drive the water downward into the deeper aquifers as well as laterally, toward the coast lines, where the pressure is reduced. In pre-development times (around 1900), the high pressure in the center of the deep recharge zone was enough to push the freshwater in the Magothy and the Lloyd a considerable distance out under the surrounding coastal waters (underflow).

Currently, public water supply wells and massive sewer systems cover much of Long Island. The 3.4 million people on Long Island use a tremendous amount of water. This places great stress on the water supply in many parts of the island and reduces the pressure needed to cause underflow of freshwater into the coastal waters surrounding Long Island. When underflow is reversed and salt-water intrusion occurs, local water supplies are put in jeopardy or lost. Parts of southwestern Nassau County, the Great Neck peninsula, the Port Washington peninsula, the necks in Huntington and the North and South Forks are all vulnerable to or experiencing serious salt water intrusion problems.

Population Growth and Contamination Problems

Population growth on Long Island leveled off or increased slightly in 1999, reversing a trend of significant population loss during the late 1980s and early 1990s. Water demand, however, has continued to increase in both Nassau and Suffolk Counties. In Nassau County, for example, the average daily water use was 124 gal./day per person in 1979. According to the draft Nassau County Master Water Plan (1998), by the year 2020, water use is expected to increase to 135 gal./day per person, which is over 16 million gallons more per day than was withdrawn in 1999. In Nassau County, the recharge rate is between 250-300 million gallons per day, of which only about two thirds recharges the area for public water supply. This trend will continue to make salt-water intrusion a growing problem for coastal water supply systems.

One of the other consequences of increased water demand on the aquifers is when contaminants enter the system; they have a tendency to move through the aquifer systems more quickly. Public water supply wells draw their water, predominantly, from the Magothy, rather than the Upper Glacial, because the Magothy is less contaminated. Even still, a significant number of public supply wells are contaminated with Volatile Organic Compounds (VOCs). Although the deeper Magothy is less prone to contamination than the Upper Glacial aquifer, pollution is really just a function of time. Eventually, synthetic organic contamination will penetrate more of the Magothy as well.

Both the quantitative and qualitative problems facing Long Island groundwater in the coming years can be minimized or avoided if actions are taken soon. But, only major changes in the way land is developed, in how contamination is released to the environment and in how much water is used, will be enough to stop the continuing decline in water quality and quantity. Protection of the deep-flow recharge areas must be dramatically increased in order for these areas to recharge clean water into the primary aquifer systems (the Upper Glacial and Magothy). Aggressive reductions on consumption through water saving devices, clean up of priority areas and county recharge programs to help maintain recharging of the groundwater are all important approaches to maintaining a sustainable water supply on Long Island.

Updated by tbono 3/30/10