MUNICIPAL WATER SUPPLIES. 109 



begin. If the free ammonia and nitrite determinations are 

 relatively high, it indicates active decomposition. If, on the 

 other hand, it is the nitrates that are relatively high, it may 

 be assumed that the source of contamination is more distant 

 and that the water is well able to care for its impurities. 

 Naturally, the more remote the contamination of the water, 

 the less danger there is of its carrying pathological bacteria. 

 If all four of these determinations are high, albuminoid 

 ammonia, free ammonia, nitrites and nitrates, then there is 

 evidently a large amount of contamination present at some 

 point, a conclusion which would be confirmed by the deter- 

 mination of required oxygen being also probably high. It 

 must be borne in mind, however, that a large oxygen con- 

 suming capacity in itself maj' not be at all indicative of 

 danger, as the greater amount of the impurity thus indicated 

 ma}' be due to harmless vegetable matter. 



Chlorine, if persistently high, indicates grave danger of 

 sewage contamination or barnyard drainage. A water with 

 high chlorine content must always be looked upon with great 

 suspicion as very likely contaminated by sewage. If it 

 happens that with high chlorine the nitrates are high, while 

 the albuminoid ammonia, free ammonia and nitrites are rela- 

 tively low, the indications might point to a distant sewage 

 pollution which the water had been amply able to dispose of. 



As stated above, the required oxygen is by no means indi- 

 cative of dangerous contamination at all times. For instance 

 streams flowing from heavily wooded, swampy districts will 

 be found largely charged with vegetable impurity giving a 

 high required oxygen, and yet the animal pollution, the dan- 

 gerous part, ma}' be very small. 



The determination of total solids is not of so much prac- 

 tical importance beyond indicating after ignition the total 

 mineral sediment in the water and furnishing a check on the 

 total organic solids. 



The Bacteriological Examinatioii . — The examination of a 



