CHAPTER VIII 



CONTAMINATION OF STREAMS AND LAKES 



Bacteria find their way into drinking-water from 

 numerous sources. The dust particles floating in the air, 

 and the bacteria attached to them, are washed down by 

 the rain. The winds that pass over the land carry away 

 with them not inconsiderable numbers of bacteria, and 

 deposit a part of them in the water over which they 

 pass. The surface washings and drainage waters that 

 reach the rivers and lakes contribute to their water a 

 goodly number of bacteria. 



Soils contain bacteria. — All soils contain large num- 

 bers of bacteria, usually several hundred thousands per 

 gram. In the case of fertile lands, the number may reach 

 several millions per gram of soil. Hence, the rainwater 

 that drains off the surface of the fields, or percolates 

 downward and escapes as subsoil drainage, comes in 

 contact with enormous numbers of bacteria and removes 

 many of them to the water-courses. Anything, there- 

 fore, that tends to increase the number of bacteria in 

 the soil will also tend, indirectly, to increase their num- 

 bers in the neighboring streams. The applications of 

 heavy dressings of animal manure, the plowing under 

 of green crops, the turning under of sod, or the appli- 

 cation of lime and fertilizer, stimulate bacterial develop- 



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