CLASSIFICATION OF WATERS 341 



earth further contamination occurs and it is a well-known fact 

 that some of the filthiest water used for domestic purposes comes 

 from rainwater tanks. This is due both to the methods of collect- 

 ing and of storing which pollutes but usually does not infect it. 



2. Surface waters include rivers, .creeks and smaller streams 

 and are immediately exposed to contamination. They vary 

 greatly in composition, depending upon the nature of the catch- 

 ment basin. Waters flowing through rock, gravel or sand forma- 

 tion are better than are those which flow over or drain loam or 

 swamps. But even the waters from sand and gravel regions may 

 be polluted or even infected, depending upon the relationship borne 

 by the drainage basin to animal life, and especially to human beings. 

 In the thickly settled portions of the country and as the new dis- 

 tricts build up these waters must be more carefully protected. 

 Sanitary workers are being forced to the conclusion that it is 

 impossible to protect such waters against contamination, and as 

 far as possible such waters should be purified before they are used. 



3. Stored waters include lakes and large ponds. These, when 

 fresh and kept free from the pollution with the wastes of human 

 life and industry make admirable sources of water. On account 

 of the limited area of the drainage basins they are more easily 

 protected than large streams. Moreover, the natural agencies 

 for purification time, sedimentation and enormous dilution play 

 a great part in freeing the water from any accidental foreign material 

 which may find its way into the water. 



4. Ground waters are of two classes: (a) Deep springs and 

 wells, from which most bacteria and other suspensoids have been 

 removed by filtration. Such waters in passing through the soil 

 take up large quantities of carbon dioxid which has been set free 

 by the decay of organic matter. Water heavily charged with 

 carbon dioxid has a great solvent action for lime and other inor- 

 ganic constituents. Hence, while such waters are usually safe they 

 are hard and carry large quantities of organic material. (6) Shal- 

 low springs and wells correspond more nearly to surface waters 

 and are often polluted and at times infected. 



Waters are also classified as polluted and infected. A good 

 water is one of high standard quality, as determined by physical 

 inspection, sanitary survey of the watersheds, clinical experience, 

 bacteriological and chemical analysis. 



A polluted water is one containing organic waste of either animal 

 or plant origin. A polluted water is not necessarily a dangerous 

 water but is always looked on by the bacteriologist with suspicion. 



An infected water is one which contains the specific micro- 

 organism which causes disease and is always dangerous. The 

 bacteriologist in examining seldom proves that a water is infected, 

 but draws his conclusions from indirect evidence. 



