125 



One result of interest is that if the discharge, Q, diminishes due to a 

 drought, groundwater interception by pumping, or other cause, the outflow 

 region, Xq, is reduced so that the aquifer tends to conserve the freshwater 

 region. Also, from Eq. 8.8, it is possible to calculate the increase in 

 elevation of the salt- freshwater interface if Q is decreased. This is 

 relevant to a pumping field with intakes at a particular depth. 



As shown in Fig. 8.5, for an unconfined aquifer, the effect of a sea 

 level rise would be to displace the fresh- saltwater interface landward a 

 distance dependent on the slope, s, of the topography. Thus for a sea 

 level increase, S, the interface would be elevated by a distance S and 

 displaced landward a distance. Ax, where approximately 



Ax = S (8.11) 



s 



For a confined aquifer, which flows to the ocean under pressure, 



saltwater intrusion into the aquifer is not expected to present the same 



magnitude of problem as for the unconfined aquifer. As shown in Fig. 8.1b, 



the only requirement is that the piezometric head at the point of aquifer 



outcropping must be equal to the depth of water at this point. With an 



increase in sea level, the rate of outflow can decrease, thereby increasing 



the piezometric head at the point such that little if any intrusion will 



occur . 



8.3.3 Oceanic Islands 



Many oceanic islands are composed of relatively permeable limestone 

 with a freshwater layer overlying a saltwater layer. The freshwater layer 

 must be maintained by rainfall or the layer thickness would diminish to 

 zero with increasing time. Referring to Fig. 8.6, the freshwater layer 

 must outcrop at or below the mean sea level. 



Considering a circular island of radius, R, with a rainfall recharge 



rate, W, and employing the Dupuit and Ghyben-Herzberg relations, an 



approximate freshwater boundary can be determined. The distance, z, to the 



interface is given by 



2 2 

 z2 W(R - r ) (8.12) 



2K[l+^][^] 

 P P 



