Long Island Fresh and Saltwater Relations/Interactions

By New York Water Science Center May 29, 2017

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).

a graph of Groundwater flow patterns and the freshwater-saltwater transition zone in an idealized coastal aquifer — 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).