BACTERIOLOGY OF WATER AND SEWAGE 439 



Wells sunk to a sufficient depth and properly protected 

 from surface wash practically always will yield pure water, 

 provided there are no cracks or crevices in rock through 

 which surface drainage may reach the water in the well. 

 The latter do not frequently occur except in limestone 

 regions. Wells in sand, particularly if clay has been pene- 

 trated in digging, usually will contain water of high sani- 

 tary quality. Care must be exercised, however, to see that 

 privy vaults and cesspools are some distance removed. It 

 is true that passing polluted water through sand or even 

 gravel will rapidly remove the bacteria present by filtra- 

 tion ; nevertheless constant passage of heavily contaminated 

 or polluted water may in some cases cause microorganisms to 

 cross considerable distance and eventually lead to pollution 

 of wells. 



METHODS OF MAKING IMPURE WATER POTABLE 



It is evident that the most satisfactory method of making 

 and keeping water suitable for drinking purposes is entirely 

 to prevent initial contamination. As has already been 

 noted this sometimes is done for streams by protecting their 

 watersheds, being sure that no sewage of any kind enters 

 the stream. The same is true of lakes and other natural 

 sources of supply. In some cases, however, it is necessary 

 for cities and communities to make use of a supply of water, 

 such as a stream or river, which receives sewage at some 

 point farther up on its course. The water, therefore, is 

 more or less contaminated and must be purified before it 

 can be used. Purification is generally brought about in one 

 of four ways: first, by the self-purification which is con- 

 stantly going on in streams and lakes; second, by coagula- 

 tion and sedimentation; third, by a chemical treatment; 

 and fourth, by filtration. 



Self Purification of Water. It is a matter of common 

 observation that streams, although they may be heavily 



