320 



L. A. ROGERS 



Above the city where there was no sewage pollution all of the 

 cultures isolated were B. aerogenes. After flowing through the 

 town, by which it was badly polluted, there was a preponderance 

 of cultures of the B. coli type. Two miles below the city sewer 

 there was only 1 B. aerogenes culture in 11 isolated. This ratio 

 changed rapidly however, and B. aerogenes soon outnumbered 

 B. coli though the latter type was still present ten miles below 

 the sewer. 



Rock Creek, the second stream investigated, runs into the 

 Potomac river between Washington and Georgetown. It is not 

 so well adapted to this study as Wolf Creek because it receives 

 sewage at various points and the self purification cannot be so 



TABLE 3 

 Relative numbers of B. aerogenes and B. coli in Wolf Creek 



satisfactorily observed. In its upper course it flows through an 

 agricultural country and receives no direct sewage. A few 

 miles above the District of Columbia line the untreated sewage 

 of the village of Kensington is emptied into the stream. There 

 are probably some private sewers before it enters Rock Creek 

 Park in which it is protected from contamination with the 

 exception of two small tributaries, Broad Branch and Piney 

 Branch, both of which are evidently polluted. 



The results of the study of Rock Creek are given in table 4. 

 At a point about 10 miles above the district line a sample was 

 taken from which 10 cultures were isolated. All of these were 

 B. aerogenes. Two and one-half miles below where the stream 

 passes the small village of Garrett Park nearly one-half of the 



