SEA WATER FROM GROUND SOURCES 



177 



reached, the depth of fresh water below sea 

 level at any point on the island will be propor- 

 tional to the fresh-water head above sea level at 

 that point and the ratio between the depth and 

 head of the fresh water will depend upon the 

 relation between the specific gravities of the 

 fresh and salt waters. 



The following explanation, by Brown (1925, 

 p. IG), of the relation between salt water and 

 fresh water nnder a small sand island is appli- 

 cable both to figure [2A] and figure [2B] : 



Let -ff= total thickness of fresh water. 



/t=depth of fresh water below sea level. 

 ^=height of fresh water above mean sea 

 level. 

 Then H=h + t 



But the column of fresh water H must be 

 balanced by a column of salt water h in order 

 to maintain equilibrium. Wherefore, if g is the 

 specific gravity of sea water and the specific 

 gravity of fresh ground water is assumed to be 



1, 



H=h + t=hf/ 



t 



whence /(: 



r-1 



In any case ^—1 will be the difference in spe- 

 cific gravity between fresh water and the salt 

 water. 



If it is assumed that the specific gravity of 

 sea water is 1.025, which is about an average 

 figure, then /(=40^ In other words, for every 

 foot that the fresh Avater stands above sea level, 

 it extends 40 feet below sea level. This ratio 

 is so extreme that it is not practicable to show 

 it in the various parts of figure [2]. For con- 

 venience, therefore, the first three parts of this 

 figure have been drawn with a ratio of 1 to 10 

 between the head and depth of the fresh water. 

 This would be the true condition if the specific 

 gravity of the sea water were 1.100 instead of 

 about 1.025. The fourth part of this figure is 

 drawn with a ratio of 1 to 5 between the head 

 and depth of the fresh water and represents an 

 imaginary specific gravity of sea water of 1.200. 

 The general relation between fresh and salt 

 water shown by these diagrams is not affected 

 in the least by this assumption of a specific grav- 

 ity of sea water greater than the range that 

 occurs in nature. The specific gravity of sea 

 water varies from place to place, so that the 

 figure of 1.025 used in the example above is only 

 an approximate average. 



In nature a body of land composed entirely of 

 permeable material to any great depth is rare. 

 The occurrence of beds or layers of impermeable 

 material does not change the basic principles 



just discussed, but it does modify their applica- 

 tion. If the island shown in figure [2B] were 

 underlain by clay or bedrock that reached a 

 level above the bottom of the fresh-water body, 

 conditions such as those shown in figure [2C] 

 would occur. Along the coast the position of 

 the contact would be determined by the head of 

 the fresh water, just as in an island composed 

 entirely of sand, but under the center of the 

 island fresh water would extend all the way 

 down to the impermeable layer and would not 

 be in direct contact with salt water. 



The modification of conditions by impermeable 

 formations is even more marked on the coasts 

 of larger bodies of land, where water-bearing 

 sands may lie under and between as well as above 

 layers of impermeable material and may slope 

 upward to remote intake areas well above sea 

 level. Along such a coast the conditions in a 

 permeable sand underlain by impermeable ma- 

 terial would be similar to those in the sand 

 island underlain by impermeable material, except 

 that the fresh water would be in contact with 

 salt water only on the side exposed to the ocean. 



Figure [2D] shows two conditions which 

 occur in water-bearing sands confined between 

 layers of impermeable material. This diagram 

 differs essentially from the others in that it 

 shows the conditions that occur when the fresh- 

 water in the sand is under artesian head rather 

 than under water-table conditions. In the upper 

 sand in this diagram, the salt water and fresh 

 water are in balance, just as in the preceding 

 examples. Salt water fills the lower part of this 

 sand and fresh water fills the upper part of it. 

 The position of the contact is determined by the 

 head of the fresh water, which in turn is deter- 

 mined by the elevation of the intake area. The 

 similarity between the conditions in this sand 

 and those in the U-tube in figure [2A] is easily 

 apparent. 



In an artesian sand, the water is prevented 

 from rising to the surface by the overlying im- 

 permeable bed. It is under a head that would 

 cause it to rise in a well to a level above the 

 bottom of the confining bed. The imaginary 

 surface that would pass through the surface of 

 the water in a well drilled to the sand at any 

 point throughout its extent is called the "piezo- 

 metric surface." The piezometric surface is 

 therefore a pressure-indicating surface, and its 

 elevation at any point indicates the head on 

 the water in the sand at that point. At the 

 intake area of the sand it merges into the water 

 table which, though not imaginary, might be 

 considered a part of the piezometric surface. 

 In a section such as figure [2D] the line repre- 



