117 



Methods of predicting saltwater intrusion are based on Darcy's law 

 (e.g., Todd, 1980) and include analytical (Henry, 1959; Henry, 1964; van 

 der Meer, 1978; Kozeny, 1953; Strack, 1976; Hunt, 1983) and numerical 

 (McDonald and Harbaugh, 1984; Lennon et al., 1987; Atkinson et al., 1986; 

 Shrivastava, 1978) models. The analytical models are useful for obtaining 

 insight into the interrelationship of various parameters, whereas the 

 numerical models are valuable for predicting detailed response for a 

 particular situation including the benefits of various management 

 strategies. The capabilities of numerical modeling appear to be limited 

 only by knowledge of the transmissive and porosity characteristics of the 

 medium, and the availability of the forcing function, and boundary 

 conditions such as inflow at the boundaries. Most of the efforts conducted 

 to date have not been directed toward evaluating the effects of sea level 

 changes; however, the capability appears to exist. The status of saltwater 

 intrusion in aquifers in all 48 coastal and inland states is presented by 

 Atkinson et al . (1986) as a figure for each state. 



In discussing the effects of sea level rise on saltwater intrusion, 

 the distinction between unconfined and confined aquifers is important, see 

 Fig. 8.1. Unconfined aquifers that discharge at or near the shoreline are 

 much more vulnerable to saltwater intrusion than are confined aquifers. 

 The idealized landward displacement of the salt -freshwater interface can be 

 predicted based on the sea level rise, the form of the interface at the 

 level of interest and the slope of the land. For confined aquifers, the 

 effect of sea level rise is to cause a feedback which tends to offset the 

 rise. The increased head due to the rise at the point of discharge causes 

 a decrease in the piezometric gradient, a reduction in the discharge rate 

 and a resultant transient that, for the same recharge rate, causes an 

 increase in the inland head. The net effect is to reestablish the same 

 discharge rate and same relative (to sea level) piezometric head as before 

 sea level rise. 



