WATER 35 



time. Salt may be found in sea-spray; it may enter a supply 

 from the sea ; it may be present in natural salt beds, and may have 

 percolated the soil from top-dressing the land with salt. 



Nitrates. The nitrates themselves found in water are harm- 

 less. They may or may not be an indication of pollution. Rain 

 water contains traces of nitrates which are formed in the air by 

 electric action during thunderstorms. Nitrates are chiefly derived 

 from animal organic matter by oxidation, the animal matter being 

 sewage, manure or the buried carcases of animals. The nitrogen 

 comes from the decomposition and splitting up of the animal 

 protein; this becomes converted into nitric acid in the soil, and 

 in turn unites with the soil carbonates to form nitrates. The 

 sequence is as follows : Protein, Ammonia, Nitrites, Nitrates. 



Nitrifying organisms in the soil bring about the formation of 

 nitrates. In deep well waters from chalk, nitrates may be plentiful 

 owing to the nitrification and filtration that has occurred in its 

 passage through the soil. But Rideal points out that filtration and 

 nitrification may be so rapid that pathogenic organisms may still 

 be present in the water though nitrates have been formed. In 

 general, the presence of nitrates indicates a previous pollution, 

 and that the water has undergone purification. 



As to the limit of nitric nitrogen allowed in drinking water, 

 this depends a good deal on other factors. Probably 5 parts 

 per 100,000- may be taken as the maximum that should be allowed. 

 No arbitrary standard can, however, be laid down. 



Nitrites. These may be formed by the reduction of nitrates 

 in the soil, with this exception, their presence always points to recent 

 contamination of the water with animal organic matter, and that 

 it is undergoing oxidation or purification. A reducing metal, lead, 

 zinc or iron, in the water will bring about reduction of the nitrates 

 to nitrites, and absence of either of these when nitrites are present 

 means that they are of animal origin. 



Ammonia. Ammonia is estimated as Free and as Albuminoid 

 Ammonia. Nearly all waters contain ammonia but, with the 

 exception of rain-water, water containing reducing metals which 

 reduce the nitrates, and some deep-well waters, the presence of 

 ammonia in anything above a trace is strongly suspicious of very 

 recent pollution with sewage or other animal matter. Absence of 

 ammonia, on the other hand, is no indication that the water is 

 pure. Water containing nitrates passing through ferruginous 

 sands has the nitrates reduced to ammonia in the same way as the 

 iron of iron pipes reduces it. The presence of ammonia in such 

 a circumstance is not, therefore, to be regarded as evidence of 



