698 BACTERIOLOGY OF THE SOIL, WATER AND AIR 
organic matter by bacterial activity in the upper layers of the soil, 
so essential to promote fertility, is of paramount importance in the 
purification of water and sewage. Indeed, the essential features of 
the nitrogen cycle are involved in both instances. 
WATER AND SEWAGE. 
The very general distribution of bacteria in the superficial layers 
of the soil makes it almost inevitable that waters which wash the 
surface of the earth shall receive some bacteria, consequently rivers 
and smaller streams, lakes and other surface waters always contain 
bacteria and other microorganisms. The number of bacteria per 
unit volume, however, is far less in water than upon the land, unless 
floods carry large amounts of soil with adherent organisms directly 
into water courses. Then the bacterial content of the water is greatly 
increased. 
The bacterial flora of surface waters is normally -considerably 
reduced by the action of sunlight— which is germicidal at a depth of 
several feet in quiet, clear water— by dilution, sedimentation, oxida- 
tion, and by the activities of predatory aquatic animals. The average 
soil pollution of water by surface contamination in sparsely populated 
drainage areas is not harmful to man, and such waters would ordinarily 
be suitable for domestic use. 
Unfortunately water courses are convenient channels for the removal 
of human waste, including excreta, and such waste is potentially 
dangerous because it may contain pathogenic bacteria. Extensive 
epidemics of water-borne excrementitious disease— as typhoid and 
cholera— have focused attention upon the potential dangers attending 
the use of unpurified surface water for domestic purposes, and the sta- 
tistical evidence of a reduction in the incidence of intestinal diseases 
when water supplies have been purified by filtration or by other 
methods is conclusive proof of the occasional transmission of excre- 
mentitious diseases through polluted water. 
Ground water— from deep wells and from springs^— is usually rela- 
tively free from bacteria unless surface pollution occurs. The water 
which feeds these sources is filtered free from bacteria during its 
passage through the deeper layers of the soil. Ground water is not 
extensively used for municipal supplies at the present time. Surface 
waters furnish the principal available sources of this commodity for 
domestic use, and in thickly settled areas it has been found necessary 
to purify the water before it is safe for human consumption. 
The objects of water purification are: To eliminate pathogenic 
bacteria, and to reduce the dissolved and suspended organic matter 
to a state of complete oxidization and mineralization. It will be 
' C. W. Stiles: Principles Underlying Movement of B. Coli in Ground H2O Resulting 
in Pollution of Wells. Public Health Reports, 1923, 38, 1350. H. W. Streeter: Report 
on Studies on Efficiency of H2O Purifying Processes, Public Health Bull., 1927, No. 172. 
