CHAPTER X 
BACTERIA IN WATER 
This subject is of great importance in the relation it bears to the 
water supplies of cities, and most of its important phases concern 
only the city water-supply. So far as relates to farm life the sub- 
ject has interest in two directions: (1) The purity of the drinking- 
water. (2) The pollution of streams. 
ABUNDANCE OF BACTERIA IN WATER 
All surface waters contain bacteria. We shall find them when- 
ever we examine the water of the ocean, the brook, the pool or the 
reservoir. Even rain water contains them, doubtless washed from 
the air, and the same is true of snow and hail. 
The number of bacteria in water is not exactly what would be 
expected in accordance with our ideas of pure water. The water 
in the running brook is commonly thought of as purer than that of 
the stagnant pond. But this is certainly not true; the brook con- 
tains more bacteria than the pond, and the supply streams always 
contain more bacteria than the water of the lake or reservoir. 
The reason for this is evident. The brooks form the drainage 
system of the country. The rains wash the whole surface of the 
land, and all the dirt and dust is carried into the brooks. In this 
dust will always be hosts of bacteria which are thus carried by the 
streams into the lake in great numbers. In the lake many of 
them soon die; others settle to the bottom; the water in the res- 
ervoir rapidly becomes purified, and it always contains fewer 
bacteria than the water brought into it by its supply streams. 
The number of bacteria in a body of water will depend upon the 
extent of the contamination which it receives from sources of active 
bacterial growth. The actual number is quite variable, ranging 
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