SEA WATER FROM GROUND SOURCES 



175 



turn line and as an emergency overflow drain 

 from the aquarinm. 



This plan for installing: the well was 

 achieved snccessfnlly, and except for a 

 troublesome problem which was not an- 

 ticipated — the presence of ferrous iron in 

 the water source — would have adequately 

 supplied our needs. This problem will be 

 discussed in detail later. 



THEORY OF SALT-WATER OCCURRENCE 



In attemptmg to locate salt water, it 

 was necessary to drill much deeper than 

 had been anticipated. A series of test 

 wells showed that, even at the very shore, 

 with water level at 11 feet below ground 

 surface, it was necessary to bore deeper 

 than 40 feet to locate water of acceptable 

 salinity. A similar situation is docu- 

 mented by Barksdale, Sundstrom, and 

 Brunstein (1936) for ground-water sup- 

 plies of the Atlantic City, N.J., region. 

 Since this paper is difficult to obtain, rele- 

 vant portions are given here. 



The problem of obtaining fresh water from 

 sands that are exposed for a part of their extent 

 to the waters of the ocean has been studied in 

 many parts of the world. The earliest scien- 

 tific work on this problem was done in Europe, 

 where the basic principles were first pointed out 

 in 1887 by Badon Ghyben,' a Dutch captain of 

 engineers, and in 1900 by Herzberg,^ who appears 

 to have had no knowledge of the earlier work. 

 The basic principles that govern the relation of 

 salt water to fresh water in a water-bearing 

 sand have now been fairly well established. 

 They are discussed by Brown in papers published 

 in 1922 ^ and 1925.* 



At the contact between the fresh and salt wa- 

 ters the zone of diffusion is surprisingly narrow. 



1 Badon Ghyben, W. Nota in verband met de voor- 

 genomen put boring nabij Amsterdam. K. Inst. Ing. 

 Tijdschr, 1888-89, p. 21. The Hague, 1889. 



" Herzberg, Baurat. Die Wasserversorgung einiger 

 Nordseebader. Jour. Gasbeleuchtung und Wasserver- 

 sorgung, Jahrg. 44. Munich, 1901. 



3 Brown, J. S. Relation of sea water to ground wa- 

 ter along coasts. American Jour. Sci. 5th ser., vol. 4, 

 pp. 274-294. 1922. 



* Brown, J. S. A study of coastal ground water with 

 special reference to Connecticut. U.S. Geological Sur- 

 vey, Water-supply Paper 537. 1925. 



In Holland, Pennink ' found a range of salinity 

 from 100 to l."i,000 parts per million of chloride 

 in distances varying from 60 to 100 feet. In 

 the present investigation ranges from 800 to 

 8,000 parts per million and from 1,900 to 7,300 

 parts per million were Observed in 4 feet of 

 depth. 



Salt water is heavier than fresh water and 

 tends to fill the lower parts of a formation. The 

 fresh water in the sand floats on the salt water 

 much as ice floats on water, with most of its 

 volum-C submerged. The position of the contact 

 is determined by the head of the fresh water 

 above mean sea level and by the relative specific 

 gravities of the two waters. This is the prin- 

 ciple developed by Badon Ghyben and Herzberg. 



This theory is illustrated in figure [2, A and 

 B]. Figure [2A] shows a simple U-tube with 

 both ends open to the air. The two legs of the 

 tube are filled with two liquids of different spe- 

 cific gravities. The liquids in the tube will come 

 to rest in such a way that the pressure at the 

 bottom of one leg is exactly equal to and bal- 

 anced by that at the bottom of the other leg. 

 The surface of the lighter liquid will, therefore, 

 necessarily stand higher than that of the heavier 

 liquid. Furthermore, as the heavier liquid fills 

 the lower part of the tube in both legs up to 

 the level of the contact between the liquids, the 

 pi-essure at this level is equal in both legs, and 

 the heights of the two columns of liquid above 

 the level of the contact are inversely proportional 

 to the specific gravities of the liquids. 



In a small island or narrow peninsula com- 

 posed entirely of permeable sand and surrounded 

 by sea water, this same balance of pressure oc- 

 curs between sea water and the lighter fresh 

 water. Figure [2B] represents a cross section 

 of such an island and shows that the salt water 

 not only fills the sand around the island but also 

 extends entirely under it below the lens-shaped 

 body of fresh water. In such an island, the re- 

 sistance of the sand to the flow of water causes 

 the fresh water from rainfall to build up a head 

 above sea level sufficient to cause it to flow out 

 into the ocean at the shores of the island. It 

 also prevents the mixing of the salt and fresh 

 waters in the sand below sea level by wave action. 

 As the sand is permeable in all directions, the 

 fresh-water head will cause a downward flow of 

 fresh water until it fills the sand to a depth 

 at which its head is balanced by the head of the 

 salt water. When equilibrium has thus been 



5 Penninlv, J. M. K. De "prise d'eau" der Amster- 

 damsche duin Waterleiding. K. Inst. Ing. Tijdschr., 

 1903-4, pp. 183-238. The Hague, 1904. 



