128 



in which K is the hydraulic conductivity and the remaining terms are given 

 in Fig. 8.6. Eq. 8.12 presents the idealized situation for a sharp 

 interface; however as noted before, there will always be a fairly gradual 

 transition from fresh to salt water. 



The approach yielding the results presented above for a circular 

 island can be applied also to represent a two-dimensional peninsula. 



8.3.4 Upconing 



One possible effect of groundwater pumping is that "upconing" of a 

 lower saltwater layer may occur in which the decreased pressure caused by 

 pumping draws the salt water into the intake. The maximum discharge, Qmax' 

 by a single well to avoid upconing is given by 



<max 



< TTd^KCAp/p) (8.13) 



in which d is the distance from the bottom of the well to the static 

 interface . 



With increasing sea level, assuming that rainfall recharge and other 

 parameters are unchanged, the freshwater region portrayed in Fig. 8.6 will, 

 be elevated by the same amount as the sea level rise; and with the same 

 absolute intake depth, the potential for upconing would increase. However, 

 by elevating the well intake point the same amount as sea level rise, the 

 upconing situation would be unchanged to a first approximation from that 

 with the lower sea level. Increased groundwater pumping in amounts greater 

 than the net renewal rate to the aquifer will elevate the interface. An 

 example is at Honolulu where the depths of production wells have decreased 

 gradually from 450 m to 85 m as the freshwater layer thickness has been 

 decreased by excessive pumping. 



8.3.5 Single Extraction Well Near a Coast 



An analytical solution has been developed by Strack (1976) to 

 represent the flow field in a confined aquifer in the vicinity of a 

 coastline as affected by the withdrawal from a single well, see Fig. 8.7. 

 For the case in which the unperturbed flow per unit length of coastline is 



