Because saltwater has a greater density than freshwater, fresh groundwater in coastal aquifers will overlie any saltwater that is present in the aquifer at depth (Figure 14).
Independent studies by two scientists around 1900 (Ghyben, 1888; Herzberg, 1901) have shown that in coastal aquifers or small oceanic islands, for every foot of freshwater above sea level, there is about 40 feet of freshwater below sea level. This relationship of freshwater to saltwater head (the Ghyben-Herzberg relationship) makes coastal aquifers very susceptible to salt water intrusion due to pumping. For example, if pumping in a coastal aquifer lowers the water table by 1 foot, the thickness of the freshwater body within the aquifer will decrease by approximately 40 feet, thus allowing the denser saltwater at depth to “intrude” into the aquifer. The hydraulics of this relationship therefore dictates that careful management of groundwater pumping on small islands or a narrow connecting strip of land (like the North or South Forks of Long Island) or headlands (like Manhasset Neck or Great Neck) is required to avoid the intrusion of saltwater into the freshwater aquifer.
Because the coastal plain aquifers of Long Island slope to seaward, the front of the saltwater body forms a “wedge” that is typically kept offshore by the higher freshwater heads of central Long Island. However localized heavy pumpage can cause rapid saltwater intrusion. In southwestern Queens County, for example, past heavy pumpage has caused saltwater intrusion into the Jameco and Magothy aquifers (Perlmutter and Geraghty, 1963).
Saltwater moves into the unconfined aquifer from the Atlantic Ocean and into the shallow part of the top confined aquifer from the major bay (figure 15). The two freshwater-saltwater interfaces at the seaward boundary of each of the confined aquifers also move landward as saltwater is drawn inland from offshore areas (Barlow, 2003).
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Table of Contents
State of the Aquifer, Long Island, New York - Introduction
- Precipitation
- NWIS - the USGS Data Archive
- Surface Water - Streamflow
- Groundwater Levels
- Water Table and Surface Maps
- Water Use
- Groundwater Budget
- Inflow to the Groundwater System
- Outflow from the Groundwater System
Below are other science projects associated with this project.
Hydrologic Cycle
Long Island Precipitation and Recharge
Long Island Surface Water
Long Island Groundwater
Long Island Hydrogeologic Units
- Overview
Because saltwater has a greater density than freshwater, fresh groundwater in coastal aquifers will overlie any saltwater that is present in the aquifer at depth (Figure 14).
Independent studies by two scientists around 1900 (Ghyben, 1888; Herzberg, 1901) have shown that in coastal aquifers or small oceanic islands, for every foot of freshwater above sea level, there is about 40 feet of freshwater below sea level. This relationship of freshwater to saltwater head (the Ghyben-Herzberg relationship) makes coastal aquifers very susceptible to salt water intrusion due to pumping. For example, if pumping in a coastal aquifer lowers the water table by 1 foot, the thickness of the freshwater body within the aquifer will decrease by approximately 40 feet, thus allowing the denser saltwater at depth to “intrude” into the aquifer. The hydraulics of this relationship therefore dictates that careful management of groundwater pumping on small islands or a narrow connecting strip of land (like the North or South Forks of Long Island) or headlands (like Manhasset Neck or Great Neck) is required to avoid the intrusion of saltwater into the freshwater aquifer.
Because the coastal plain aquifers of Long Island slope to seaward, the front of the saltwater body forms a “wedge” that is typically kept offshore by the higher freshwater heads of central Long Island. However localized heavy pumpage can cause rapid saltwater intrusion. In southwestern Queens County, for example, past heavy pumpage has caused saltwater intrusion into the Jameco and Magothy aquifers (Perlmutter and Geraghty, 1963).
Sources/Usage: Public Domain.Figure 14. Groundwater flow patterns and the freshwater-saltwater transition zone in an idealized coastal aquifer. A circulation of saltwater from the sea to the transition zone and then back to the sea is induced by mixing of freshwater and saltwater in the transition zone (Barlow, 2003).(Public domain.) Saltwater moves into the unconfined aquifer from the Atlantic Ocean and into the shallow part of the top confined aquifer from the major bay (figure 15). The two freshwater-saltwater interfaces at the seaward boundary of each of the confined aquifers also move landward as saltwater is drawn inland from offshore areas (Barlow, 2003).
Sources/Usage: Public Domain.Schematic illustration of some of the modes of saltwater intrusion in an idealized multilayer, regional aquifer system caused by groundwater pumping at wells (Barlow, 2003).(Public domain.) _______________________________
Table of Contents
State of the Aquifer, Long Island, New York - Introduction
- Precipitation
- NWIS - the USGS Data Archive
- Surface Water - Streamflow
- Groundwater Levels
- Water Table and Surface Maps
- Water Use
- Groundwater Budget
- Inflow to the Groundwater System
- Outflow from the Groundwater System
- Science
Below are other science projects associated with this project.
Hydrologic Cycle
The water cycle has no starting point, but we'll begin in the oceans, since that is where most of Earth's water exists. The sun, which drives the water cycle, heats water in the oceans. Some of it evaporates as vapor into the air; a relatively smaller amount of moisture is added as ice and snow sublimate directly from the solid state into vapor. Rising air currents take the vapor up into the...Long Island Precipitation and Recharge
Precipitation is water released from clouds in the form of rain, freezing rain, sleet, snow, or hail. It is the primary connection in the water cycle that provides for the delivery of atmospheric water to the Earth. Most precipitation falls as rain. Water seeping down from the land surface and reaching the water table adds to the groundwater and is called groundwater recharge. Groundwater is...Long Island Surface Water
Streams either gain water from inflow of groundwater from the underlying aquifer or lose water by outflow to the underlying aquifer. Many streams do both, gaining in some reaches and losing in other reaches. Furthermore, the groundwater flow directions near any given stream can change seasonally as the altitude of the water table changes with respect to the stream-surface altitude or when rapid...Long Island Groundwater
Approximately 30% of the world’s water is stored as groundwater. Groundwater moves very slowly, on the order of feet per day, however it is still part of the hydrologic cycle. Most of the water in the ground comes from precipitation that infiltrates downward from the land surface.Long Island Hydrogeologic Units
Long Island’s aquifer system consists of a seaward-dipping wedge of mostly unconsolidated stratified sediments comprised of sand, gravel, silt and clay.