4.9.2 Saltwater Intrusion 



Saltwater intrusion, as used here, is the advance of salt water into 

 groundwater aquifers (in another context it refers to displacement of 

 fresh water or less saline water by more saline water). 



Surface aquifers vary in level with seasonal changes in rainfall, 

 sometimes rising above the land surface and flooding it, but usually 

 remaining below at a depth of several inches to many feet. There may also 

 be a number of deeper, geologically confined aquifers, which are separated 

 from each other and the surface aquifer by more or less horizontal layers 

 of impermeable rock or clay (Figure 29). Typically, the confined aquifers 



Figure 29. A typical profile of aquifers of Long Island, New York 

 (Source: Reference 86). 



Lake 



Ocean 



Vertical scale exaggerated 



Unsaturated soil 

 Surface water 

 Confining layer 

 Salt water 

 -♦ General direction of groundwater movement 



Upper glacial aquifer 

 Gardiners clay 

 Jameco aquifer 



Magothy 

 aquifer 



Raritan clay 

 Lloyd aquifer 



are sloped seaward with fresh water flowing 

 cept the seafloor. The interface between s 

 under natural conditions is a zone of trans 

 pressures on the aquifer normally prevent s 

 into the fresh water, but overpumping may r 

 intrusion from the saltwater layer. Along 

 there has been seawater intrusion in aquife 

 7 others are known to be threatened, and 15 

 intrusion sites [85]. Most other coastal s 

 intrusion problems along their seacoasts. 



4.9.3 Subsidence 



out of them where they inter- 

 alt water and fresh water 

 ition (Figure 30). The head 

 alt water from intruding far 

 educe this pressure and allow 

 California's populated coast 

 rs in at least 12 localities, 



others are regarded as potential 

 tates have actual or potential 



Subsidence of the land surface from over withdrawal results because 

 the land loses the subsurface support provided by groundwater (Figure 31) 



131 



