polution. In this connection it may be pointed out again that Frost, et al, 

 '(1921a) found no undoubted effect of the sewage of Cincinnati on the nitrogen 

 content of Ohio Pdver when the discharge exceeded 50,000 second-feet, that 

 is, when the number of contributing persons per second-foot was approximately 

 the same as in Detroit River. Obviously the failure to find definite evi- 

 dence of pollution in the analyses for nitrogen near the mouth of Detroit 

 River is explained by the great excess of river water over sewage. 



In view of the slight evidence of increase in nitrogen in Detroit 

 River, what explanation can be offered for the sharp increase in chloride 

 in the lower river (Table U7)? This increase is believed to be due in 

 large part to natural causes. There are no available data on the chloride 

 content of the lower river before the river became subject to pollution, 

 but, as pointed out on page 228, there are numerous sources of saline ground 

 waters in this region, particularly on Grosse Isle, and in the rocks under- 

 lying the Livingstone channel. l'\/hile it is not possible to determine how 

 much of the increase was due to natural causes and how much to pollution, in 

 the light of the data on nitrogen, it seems not imreasonable that most of it 

 should be assigned to natural causes. This statement should be qualified by 

 saying that some may have been derived from wastes of salt works on the bank 

 of the river, but it is not probable that, much of it came from domestic 

 sewage. The large variations shown in Table li6 may be explained on the 

 basis of incomplete mixing of the incoming saline waters with the water of 

 the river. 



Conclusions regarding pollution 



Although Detroit River receives sewage from municipalities aggre- 

 gating nearly two ;nillion persons, determinations of oxygen and nitrogen 

 near the mouth of the river yield no definite evidence of pollution. The 

 explanation for this fact lies in the great volume of discharge of the 

 river in relation to the number of persons contributing sewage. Doubtless 

 the nitrogen content of the river has been increased as a result of pol- 

 lution, but in all probability the increase has been too small to have 

 an appreciable effect on the oxygen content of the water (\rith the probable 

 exception of water immediately in contact with polluted bottom) <■ The 

 reductions in oxygen content noted at Station 126 (Table U2) probably 

 resulted principally from natural causes. It may be concluded that, at 

 the depths studied, pollution in Detroit River has had no harmful chemical 

 effect on the water of Western Lake Erie, 



Chemical conditions 

 near the west shore in winter 



During the warm months of the year decomposition of organic matter 

 in the water proceeds at a high rate, and dissolved oxygen is rapidly con- 

 sumed. But the water is exposed to the air, and as it is churned by waves 



13U 



