178 



JOHN CLARK AND RONALD EISLER 



senting the piezometric surface is the hydraulic 

 gradient of the water in the sand along the 

 section. As there is no flow in the upper sand 

 in this figure, the line representing the piezo- 

 metric surface is level and extends from the 

 intake area toward the ocean as far as the 

 fresh water extends in the sand. 



In the lower sand in figure [2D] the head of 

 the fresh water is suflScient to cause a flow of 

 fresh water into the ocean below sea level, form- 

 ing a suboceanic fresh-water spring. The fresh 

 water fills the water-bearing formation down 

 to the bottom edge of the overlying impermeable 

 layer and far enough below this level to permit 

 the water to flow out into the ocean. Here 

 again, the salt water fills the lowest part of the 

 formation, but as the pressure in the main body 

 of fresh water is greater than that in the salt 

 water at the outlet, the salt water fills only that 

 part of the formation in which the head of the 

 fresh water has been reduced below the pressure 

 of the salt water by the resistance of the sand 

 to its movement. The line representing the 

 piezometric surface for this sand slopes gently 

 downward from the intake area to the point 

 where the thickness of the sand carrying fresh- 

 water is reduced by the intrusion of salt water. 

 From that point to the point of discharge, the 

 slope increases. . . . 



Any general lowering of the head of the fresh 

 water in a sand exposed for a part of its extent 

 to the waters of the ocean will permit the salt 

 water to advance farther inland and occupy more 

 of the sand. The lowering may be caused by 

 natural conditions, such as a dry year or a 

 series of dry years, but lowering due to such 

 causes is not likely to have any serious conse- 

 quences, unless it occurs in conjunction with 

 artificial withdrawal of water from the sand. 

 This is usually accomplished through wells, 

 either by pumping or by the natural flow from 

 artesian sands. Pumping water from a water- 

 bearing sand lowers the head of the water in 

 it materially in the immediate vicinity of the 

 point of pumping and, to a decreasing extent, for 

 a considerable distance away. If this lowering 

 of head or "cone of depression" occurs above or 

 extends beyond the zone of contact, it will disturb 

 the balance between fresh and salt water and 

 permit the salt water to move up through the 

 formation toward the well. The radius and 

 depth of the cone of influence increase as the 

 i-ate of pumping from the well is increased . . . 



The specific gravity of sea water varies slightly 

 from place to place and sometimes at different 

 depths at the same place, but it is never much 

 greater than that of fresh water. For the pur- 



pose of this report, the specific gravity of fresh 

 water may be considered to be 1.000. In the 

 summer of 1!)13. Bigelow " found that the specific 

 gravity of the water off the Atlantic coast of 

 the northern United States at different places 

 and at different depths ranged from 1.019 to 

 1.028. 



Owing to the very small difference between the 

 specific gravity of fresh water and that of salt 

 water, a slight change in the head of the fresh 

 water produces a very considerable change in 

 the position of the zone of contact. If a water- 

 bearing sand is exposed to sea water having a 

 specific gravity of 1.025, the level of the fresh 

 water in it must be maintained at 2.5 feet above 

 mean sea level if the zone of contact is to be 

 held at a depth of 100 feet below sea level. A 

 fresh-water head of 5 feet above mean sea level 

 would be sufficient to hold back the sea water to 

 a depth of 200 feet below sea level. Similarly, 

 if the fresh water head in such a sand were 

 lowered only 2.5 feet, it would permit the salt 

 water to rise 100 feet. If the fresh water head 

 in the sand were lowered to sea level, the salt 

 water would rise to sea level. In a gently sloping 

 confined sand, such as the upper sand in figure 

 [3C], a vertical rise of 100 feet might represent 

 a movement of the salt water several miles 

 inland . . . 



It has already been pointed out that salt water 

 fills the sand around and beneath the lens of 

 fresh water under a small island composed en- 

 tirely of sand. The effect of pumping a well in 

 such a sand island is shown in figure [3A]. As 

 the depth of the zone of contact at any point 

 on this island is detemnined by the head of the 

 fresh water at that point, any lowering of the 

 water table on the island would cause a rise 

 of the salt water beneath the point where the 

 lowering of head occurred. The section, in figure 

 [3A] is shown passing through a shallow well 

 that is being pumped at a rate suflScient to draw 

 the level of the fresh water down to a point 

 somewhat above sea level. The cone of depres- 

 sion caused by pumping this well causes the 

 salt water to rise from beneath the island in a 

 shape similar to the cone of depression but in- 

 verted and proportionally much greater in its 

 vertical dimension. This rise in the zone of 

 contact might well be called a salt-water cone 

 of elevation. The distance which the salt water 

 rises is proportional to the amount of lowering 



« H. B. Bigelow. Exploration of the coastal water 

 between Nova Scotia and Chesapeake Bay, July and Au- 

 gust 1913, by the U.S. Fisheries schooner Grampus. 

 Harvard College, Museum Comparative Zoology Bulle- 

 tin, vol. 69, No. 4, figs. 55, 61. 1915. 



