130 



q and with the well located a distance Xq from the coast, the critical 

 withdrawal, Q^,, is defined by the following two equations 



f4- 7(1 + 7) = 2 ^ - [1 - (^)2] in [ \ ^1^.1° ]] (8.14) 



1 - (^)^ ] (8.15) 



lo^o ^o 



In the above equations , D is the vertical distance from mean sea level to 

 the aquifer bottom boundary. 



8.3.6 Saltwater Barriers 



In areas where solution channels exist that convey large quantities of 

 fresh water to sea, the possibility exists of employing underwater dams. 

 This approach has been tested successfully in the limestone caverns of 

 Port-Miou River near Marsielles, France. 



8.4 CASE STUDIES 



There are two general approaches for reducing saltwater intrusion into 

 coastal aquifers: 1) modifying ptomping practice, and 2) construction of 

 flow barriers. The most simple and direct approach is, where appropriate, 

 to reduce groundwater withdrawal during periods of drought when intrusion 

 would tend to occur. It is clear that with increased sea level, the 

 frequency and/or duration that intrusion would tend to occur will increase. 

 Unfortunately, drought occurrences are precisely the times that demand on 

 groundwater resources tends to be increased. An alternate approach is to 

 adopt a pumping plan to minimize intrusion through hydraulic or structural 

 barriers . 



Five case studies will be presented of approaches to cope with 

 salinity intrusion into aquifers. 



8.4.1 Long Island. NY 



Todd (1980) has described the effects of excessive pumping near the 

 more populated western end of Long Island including a reduction in the 

 water table to 10 m below sea level. The response has been abandonment of 



