276 Ground Water, Wells and Farm Drainage. 



342. Geological Conditions Which Give the Best Wells 



The largest and best supplies of well water are usually found 

 in the extensive sandstone formations and wherever these 

 are within easy reach the well should be sunk into them 

 deep enough to have 20 or more feet of percolating sand- 

 stone surface. Next to the sandstone formations as sources 

 of water supply stand the fissured limestones which either 

 overlie sandstones or are so related to the surface soil that 

 water from them can percolate down into the fissures and 

 through them reach the well when sunk so as to connect 

 wkh a system of these fissures. 



Again beds of sand between beds of clay often give large 

 supplies of pure cold water. 



In many localities artesian or flowing wells can be se- 

 cured and some of the conditions under which these origi- 

 nate are represented in Fig. 107. 



343. Conditions which Influence the Capacity of a Well 

 The rate at which water can enter a well depends upon five 

 prime factors: (1) The size of the grains of the water- 

 bearing beds and the pore space. (2) The depth of the 

 well in the water-bearing bed. (3) The amount the water 

 is lowered in the well when pumping. (4) The diameter 

 of the well. (5) Whether the well is in or near a system 

 of fissures. 



344. Influence of Size of Grains and Pore Space on the 

 Capacity of the Well. From the fact that the flow of water 

 through sands is nearly proportional to the squares of the 

 diameters of the soil grains, and is greater the larger the 

 pore space, it is clear that these are very important factors 

 in determining the capacity of wells. It has been computed 

 that when all other factors are the same the capacities of 

 two wells, in sands having the diameter of grains of .15 

 mm. and .25 mm. and pore spaces of 30 per cent, and 32 

 per cent., are to each other as5.234tol8.01or one is over 

 three times the other. It is therefore clear that when the 

 sand grains and pore space are small the other well factors 

 must be made enough larger to compensate. 



