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Analytical and Numerical Models 



Analytical and numerical models to describe the location of the 

 saltwater- freshwater interface have been developed for many different 

 applications. Most of the analytical models are based on the 

 Ghyben-Herzberg relation, which predicts that the location of the interface 

 below sea level is approximately 40 times the value of the freshwater head 

 above sea level, and on the Dupuit assumption, which assumes horizontal 

 flow in the aquifer. The location of the interface can be predicted for 

 both confined and unconfined aquifers (Bear, 1979). Two-dimensional 

 solutions based on complex variables and conformal transformations have 

 been developed (e.g., Henry, 1964; Van Der Veer, 1977). The lateral and 



vertical movements of the interface in response to pumping can be analyzed 

 using solutions developed by Strack (1976) and Schmorak and Mercado (1969), 

 respectively. The effects of pumping from the saltwater side of the 

 interface to increase the allowable pumping rate from the freshwater side 

 of the interface have been investigated by Vandenberg (1975). Numerical 

 solutions have been developed that allow dealing with heterogeneities in 

 aquifers and with two-dimensional and transient conditions (e.g., Shamir 

 and Dagan, 1971; Segol et al . , 1975; Segol and Finder, 1976; Mercer 

 et al., 1980). Three-dimensional effects have also been considered 



(Huyakorn et al., 1987). 



Protective Measures 



Measures to protect coastal well fields from saltwater intrusion 

 generally can be classified as physical or hydraulic barriers (Kashef , 

 1986). Physical barriers include sheet piles, cofferdams, and grout 

 curtains to retard the landward movement of seawater. Hydraulic barriers 

 include recharge wells and spreading basins, which maintain a pressure 

 ridge along the coastline perpendicular to the direction of saltwater 

 intrusion, and discharge or extraction wells, which create a continuous 

 pumping trough with a line of wells adjacent to the sea (Todd, 1980). 

 Case histories in Long Island, California, and Florida (Todd, 1980) 

 illustrate the impacts of saltwater intrusion and describe a number of 



