Stream Pollution 25 



purification of a small sewage polluted stream in Massachusetts. 

 Of one article (Turner, 1918) discussing the results of this work, a 

 brief outline may be given to indicate the character of the changes. 



As the effluent from sand filter beds reaches the stream, a little 

 creek of pure water and very moderate flow, the acquatic life is 

 promptly changed. A gelatinous growth of Crenothrix, the iron 

 bacterium, covers the entire bottom of the stream, which a little 

 further down is black with a " pollution carpet " or " false bottom/' 

 as he terms it. In this material bacteria abound and on it one finds 

 gray woolly masses of Carchesium, while burrowing in it are seen 

 colonies of red worms (Tublfex tubifex), a typical pollution species, 

 and Midge larvae (Chironomus decorus), the so called "blood 

 worm/* which is a most important factor in the removal of organic 

 materials. At the place where the effluent enters, higher plants are 

 lacking, but only three-quarters of a mile below they grow rankly, 

 choking the stream and affording food and shelter to a multitude of 

 smaller organisms, snails, isopods, daphnids, etc. 



Chemical changes in the stream are rapid in this short distance, 

 the total organic nitrogen is greatly reduced, and the dissolved 

 oxygen, which was richly present in the clear stream and nearly 

 absent from the polluted water at the start, has been increased 

 despite the rapid processes of oxidation which have gone on during 

 the interval. At three-quarters of a mile from the original pollu- 

 tion the false bottom no longer persists and the stream is nearly 

 normal, though even one or two miles further down stream chemical 

 tests demonstrate the existence of pollution. Turner summarizes 

 this work thus (p. 45) : 



Importance of Biologic Factors 



" It is obvious that the biological factors of stream purification 

 are much more important than the strictly chemical and physical 

 factors * * * Certain organisms are characteristic of an unpol- 

 luted stream. Others are characteristic of pollut on and by their 

 presence and numbers indicate the intensity of biological activity. 

 Some forms like rotifers and certain green algae may be present in 

 either polluted or unpolluted water, and their correlation with each 

 other and various plants and animals must be understood to appre- 

 ciate their significance." 



The changes by which animal and plant wastes are transformed 

 and utilized are universal in nature and are applied most profitably 

 in agriculture generally. The concentrated wastes in domestic sew- 



