CHAPTER IX 



PURIFICATION OF RIVER AND LAKE SUPPLIES 



The composition of river-water is influenced by the 

 geological nature of the region through which it flows, 

 and by the surface drainage it receives. Streams pass- 

 ing through limestone regions carry in solution con- 

 siderable quantities of lime. Their waters are, there- 

 fore, called hard waters. Those that pass through heavily 

 wooded regions and through swamps and meadows 

 acquire large quantities of organic matter, often to 

 such an extent as to give their water a distinct color. 

 Streams that .pass over strata rich in iron may, similarly, 

 be noted for their high content of iron salts, while others 

 may be distinguished for their high content of various 

 salts, as is true of rivers that constantly receive the 

 seepage of alkali soils. 



The differences in the composition of river-water 

 are not without important influences on the bacteria 

 inhabiting them. It is well known, for instance, that 

 the proportion of lime in the soil affects intimately the 

 character of its bacterial flora. It would be but reason- 

 able to expect analogous relations in surface waters. 

 It still remains to be determined to what extent the 

 varying proportion of lime in water affects the character 

 of its bacteria. 



(77) 



