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Freshwater-Saltwater Interactions along the Atlantic Coast

A Regional Assessment of the Ground-Water Resources Program

Freshwater-Saltwater Home | Saltwater Intrusion | Ground-Water Flow and Discharge to Coastal Environments |
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Saltwater Intrusion
Under natural conditions, the seaward movement of freshwater prevents saltwater from encroaching coastal aquifers, and the interface between freshwater and saltwater is maintained near the coast or far below land surface. This interface is actually a diffuse zone in which freshwater and saltwater mix, and is referred to as the zone of dispersion (or transition zone) (figures 2 and 3). Ground-water pumping can reduce freshwater flow toward coastal discharge areas and cause saltwater to be drawn toward the freshwater zones of the aquifer. Saltwater intrusion decreases freshwater storage in the aquifers, and, in extreme cases, can result in the abandonment of supply wells. Saltwater intrusion occurs by many mechanisms, including lateral encroachment from coastal waters and vertical upconing near discharging wells (figure 4 and figure 5).
Ground-water flow patterns in an idealized coastal aquifer
Ground-water flow patterns and the zone of dispersion in an idealized, homogeneous coastal aquifer.

Freshwater and saltwater mix in the zone of dispersion by the processes of diffusion and mechanical dispersion. A circulation of saltwater from the sea to the zone of dispersion and then back to the sea is induced by mixing within this zone. (Figure modified from Cooper, H. H., 1964, A hypothesis concerning the dynamic balance of fresh water and salt water in a coastal aquifer: U.S. Geological Survey Water-Supply Paper 1613-C, p. 1-12).

Figure 2 -- Ground-water flow patterns in an idealized aquifer
Figure 3 -- Zone of dispersion in the Biscayne aquifer
Figure 4 -- Vertical movement (upconing) at a discharging well
Figure 5 -- Vertical movement of saline water in the Floridan aquifer system

For more Information, Contact
Paul Barlow, Coordinator
Atlantic Coastal Zone Assessment
U.S. Geological Survey
10 Bearfoot Road
Northborough, MA 01532
(508) 490-5070

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