WATEE 7 



during that life they are remarkably active. A polluted 

 river may continue to supply a town with drinking water 

 which produces no harm, until pathogenic microbes find 

 their way in. 



Wells are described as deep, shallow, and artesian. The 

 source of water in these we have previously considered. 

 A deep well is generally stated to be anything over 50 

 feet, and a shallow well as being under 50 feet deep. 

 This definition is rather arbitrary. Sutton* tells us it is 

 evident that no very definite distinction can be drawn 

 between deep and shallow wells. There are many con- 

 siderations which qualify the definition of a deep well. It 

 may be considered essentially as one the water in which 

 has filtered through a considerable thickness of porous 

 material, and whether the shaft of such a well is deep or 

 shallow will depend on circumstances. If the shaft passes 

 through clay or other impervious stratum, and the surface 

 water is thus rigidly excluded, the well should be classed 

 as deep, even if the shaft is only a few feet in depth, 

 because the water in it must have passed for a consider- 

 able distance below the clay. On the other hand, however 

 deep the shaft of the well is, it must be considered as 

 shallow if water can enter it near the surface, or large 

 cracks or fissures give passage to surface water through 

 the rock in which the well is sunk. 



In shallow wells the depth of the soil through which the 

 surface water is filtered before entering is rarely sufficient 

 to get rid of its impurities by oxidation, though it is often 

 su£6.cient to decolourize it, and by its absorption of carbonic 

 acid give it a bright, sparkling appearance, which may be 

 most deceptive. 



The chief source of shallow well pollution is the cesspool. 

 These latter are often purposely made pervious, so that the 

 liquid portions find their way into the soil and only the 

 solids remain. In this condition the cesspool will not 

 require to be cleaned out perhaps for years. The percolating 

 liquid joins the ground water, which as previously stated is 

 * ' Volumetric Analysis.' 



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