SEA WATER FROM GROUND SOURCES 



179 



of the water table, and the ratio between the 

 lowering of the water table and the rise of the 

 salt water is determined by the relation between 

 the specific gravities of the fresh water and the 

 salt water. The position of the zone of contact 

 beneath the island is not affected outside of the 

 area of the cone of depression produced by pump- 

 ing the well. 



A lowering of the water table on the island 

 by the pumping of a well will, of course, pro- 

 duce the same effect whether the well is shallow 

 or deep. As long as the water table is not low- 

 ered to sea level, there will be a space filled with 

 fresh water between the bottom of the cone of 

 depression and the peak of the salt water cone 

 of elevation. It is evident, therefore, that a 

 shallow well on such an island might yield fresh 

 water, whereas a deeper well at the same point 

 and pumped at the same rate would penetrate 

 the salt-water cone of elevation and yield salt 

 water . . . 



Figure [3B] shows the result of pumping 

 water from a well on a sand island underlain 

 by impervious material. A well very near the 

 coast of such an island would, of course, en- 

 counter conditions similar to those in an island 

 composed entirely of sand. Obviously, the safest 

 location for a [fresh-water] well on such an 

 island would be near its center. In this loca- 

 tion a well might be pumped at a rate that vpould 

 draw the water table down even below sea 

 level without drawing in salt water, provided 

 the cone of influence did not extend to any point 

 at which it would be lowering the fresh-water 

 head above the zone of contact. Under such 

 conditions a "fresh water barrier" is said to 

 be maintained between the well and the salt 

 water. If the location of the well or the rate 

 of pumping from it is such that the cone of 

 influence extends beyond the zone of contact, 

 the fresh-water barrier will be broken dowm, and 

 salt water will be drawn into the well. The 

 diagram shows the conditions that will occur 

 when a well on the island is pumped at a rate 

 which would cause its cone of influence to extend 

 beyond the nearest part of the zone of contact. 

 The salt water would move in under the island 

 and form a cone of elevation beneath the well, 

 similar to the one that would be formed on the 

 sand island shown in figure [3A] except that 

 it would be cut off at the bottom by the im- 

 pervious layer. Here again a shallow well might 

 yield fresh water, whereas a deeper well at the 

 same place pumped at the same rate would yield 

 salt water . . . 



In an artesian sand the pumping of a well may 



not draw the head down to such a depth that 

 the sand around the well is drained, as under 

 water-table conditions. Usually pumping a well 

 that taps an artesian supply merely lowers the 

 head at and around the well and creates a cone 

 of depression in the piezometric surface without 

 unwatering any of the sand. Figure [3C] illus- 

 sti-ates conditions under which salt water might 

 be drawn into such a sand. In the upper sand 

 in the figure a well in the part of the formation 

 that contains salt water would, of course, yield 

 salt water from the beginning. Salt water would 

 be drawn into a well farther inland if the cone 

 of influence of the well extended beyond the 

 edge of the zone of contact. If the well were 

 near the zone of contact, as shown in the dia- 

 gram, this might occur with only a very slight 

 lowering of the fresh-water head in the for- 

 mation as a whole. 



DEVELOPING THE WELL 



From a shallow test well, we found the 

 ground- water level at an average depth of 

 11 feet, and about 0.9 feet above tide level 

 of the bay. Accordingly, with bay salin- 

 ities ranging from 20 to 28 parts per thou- 

 sand, depth of the fresh-water lens was 

 estimated to be from 35 to 45 feet below 

 the ground-water level or from 46 to 56 

 feet below the surface. 



We planned to drill the well to a depth 

 between 50 and 75 feet depending upon 

 the quality of water located and the drill- 

 ing problems encountered. Because of the 

 fragility of the PVC pipes, the well pipe 

 was water-jetted into place (within the 

 6-incli iron housing). This process util- 

 izes a bentonite clay and water mixture. 

 Consequently, in sampling the ground wa- 

 ter for salinity as drilling proceeded, it 

 was necessary to completely flush the sand 

 of the clay-water mixture by extensive 

 pumping of the well before drawing a 

 sample. 



The water, therefore, was tested for salt 

 content at only a few intervals during the 

 jetting operation. Jetting was terminated 

 at 70 feet because of excessive loss of the 

 bentonite mixture through a layer of 

 coarse gravel (more than three-eighths 



