98 DOMESTIC WATER SUPPLIES FOR THE FARM 



Location of Driven Wells. Since the wells of the driven 

 type are tightly cased from the pump at the surface to the water- 

 bearing layer below, it is manifest that there can ordinarily be no 

 entrance of polluting matter either at the top or by seepage 

 through the sides. 



Since contaminating matter can enter only at the bottom of 

 the well it must previously filter through a thickness of material 

 equivalent to the full length of the pipe; and, as wells of this type 

 are commonly sunk only in sands and gravels, both of which are 

 naturally good filters, it follows that unless polluting matter is 

 carried into the ground in large quantities there is relatively 

 little danger of contamination in wells of this sort. It has been 

 found that in spite of exceedingly unsanitary surroundings of 

 wells of this type in many sandy districts, the waters were reason- 

 ably safe. 



In fact, except in villages where the whole upper part of the 

 ground-water body is polluted, there is comparatively little danger 

 of contamination if the casing is carried to a point below the 

 polluted zone. Experience has shown that this zone is usually of 

 no great thickness and a well drawing its supply from ten feet 

 below the ground-water level will often prove safe where one just 

 reaching the water-table would be dangerously polluted. 



A driven well with its strainer 15 feet below water level (when 

 the latter is at its lowest point) will be practically free from 

 danger; and, if it can be carried to this depth, its exact location 

 will generally be immaterial, and it may be placed where most 

 convenient. 



Sinking the Driven Well. - - The pipe, which is commonly 

 from i to 3 inches in diameter and provided with a driving point 

 at the end, is usually driven into the ground by a wooden or iron 

 maul, operated either by hand or by power. In the larger and 

 deeper wells a heavy weight, operated by a revolving drum and 

 hoisting sheave, or by various horse-power appliances, is used. 

 Additional lengths of pipe are screwed on to the preceding sec- 

 tions from time to time until the required depth is attained. 



