SOURCES AND SAFETY OF UNDERGROUND SUPPLIES 



should penetrate to a level below that of the ground- water sur- 

 face in the driest seasons. (See Fig. 16.) 



FIG. 16. Diagram showing relation between depth and permanence of wells. A, 

 Well sunk to ordinary water level, but failing at times of drought; B, well sunk to 

 level of water in dry years and never failing. 



Waters of Clays. Clay usually contains large quantities of 

 water, but its pore spaces are so fine or small that water soaks into 

 it or out of it so slowly that it is impervious in the sense that little 

 or none of that which it contains can be utilized as a source of 

 supply. Considerable amounts are frequently reported in clays, 

 but they usually come from more or less sandy layers. In some 

 places sand that approaches clay in fineness and that is some- 

 times mistaken for clay yields considerable amounts of water. 



FIG. 17. Diagram showing action of clays or shales in confining water in sand or 



sandstone. 



Clay is of the greatest importance, however, not as a water bearer, 

 but as a confining layer which prevents the water from escaping 

 (see Fig. 17), or as a layer collecting the water from overlying 

 porous beds and bringing it to the surface. 



When, because of the absence of other sources, it is necessary 

 to obtain supplies from clay, a well sunk should be of as large 

 diameter as possible and should be continued far enough beneath 

 the point at which water is obtained to insure ample storage 

 capacity. (See Fig. 18.) Dug wells are usually most satisfactory 

 where the clay is near the surface, but such wells should be care- 

 fully covered and guarded from all sources of pollution 



Waters of Tills. Till is a heterogeneous mixture of clay, 



