294 J. 8. Brown — Sea and Ground Water. 



that fluctuations of this kind in sandy material are less 

 than a few tenths of a foot. In fractured rocks, however, 

 fluctuations of several feet may occur. The best data 

 on tidal fluctuations in wells are given by Veatch, 18 who 

 says thart they are caused in three ways — "by transmis- 

 sion of pressure through open cavities or passageways 

 affording a free communication between the wells and 

 the ocean ; by a checking of the rate of discharge of the 

 normal ground-water flow through porous beds freely 

 connecting with the ocean ; and by a deformation of the 

 strata due to the alternating loading and unloading of 

 the tides.' ' The last method accounts for the rhythmic 

 rise and fall of flowing wells near the sea. 



Experiments indicate that fluctuations in salinity in the 

 water from wells can only very rarely be correlated 

 directly with the rise and fall of the tides. The tidal 

 period is too short for its immediate effect upon the salt 

 content of ground water to be propagated in a measurable 

 amount much beyond the limits of the low-tide and high- 

 tide shore line. However, when wells near the sea are 

 pumped so heavily that the water table is kept below sea 

 level for periods of several days or more and salt water 

 is thus enabled to flow toward the wells, the rate of this 

 flow increases at high tide due to the increased head of 

 salt water and a fluctuation in salinity having the perio- 

 dicity of the tides is established. 19 



Undoubtedly tides are very important in maintaining 

 the zone of brackish water that occurs along the coast. 

 They act partly through direct infiltration of salt water 

 at high tide (Ag. 4), and partly through oscillations in 

 the contact zone due to variations in hydrostatic pressure. 

 These oscillations promote intermixture of fresh and salt 

 water. 



18 Veatch, A. C. : Fluctuations of the water level in wells, with especial 

 reference to Long Island, New York, U. S. Geol. Survey Water-Supply 

 Paper 155, p. 69. 1906. 



19 Burr, W. H., Hering, Eandolph, and Freeman, J. E., Report of the com- 

 mission on additional water supply for the city of New York, pp. 406-423, 

 New York, Martin B. Brown Co., 1904. 



