290 TYPHOID FE'STIR IX DISTRICT OF COLHMBL^. 
reducing action of tiie organic impurities of the water and soil. In 
this manner the organic matter originally contained in the water is 
oxidized and in the presence of the dissolved oxygen naturally present 
in surface waters these processes would occm over and over agaiu 
imtil all the organic matter originally present in the water had been 
oxidized and all of the dissolved oxygen consumed. Hence it is that 
by acting as an oxygen carrier the iron accomplishes both the oxida- 
tion of the organic impurities originally present in the water and the 
removal of the dissolved oxygen. In this way we can satisfactorily 
accoimt for the freedom of the water of deep wells from organic 
pollution.^ 
SHALLOW WELLS. 
At the present time the waters of 63 shallow wells are in use for 
drinking purposes in the city of TTashington and the District of 
Columbia. They are \videly scattered over the city and are probably 
used for drinking purposes by a considerable niunber of persons, 
especially among the poorer classes. Twenty-nine of the shallow 
wells now in use are located in the northwest section of the city, 13 in 
the northeast section, 20 in the southeast, and 1 m the southwest. 
As a general tiring the waters of the shallow wells are very attractive 
so far as their appearance is concerned. They are clear and sparkling 
and even during the hottest part of the summer their temperature was 
rarely found to exceed 20° C. So that among a great number of peo- 
ple the use of these waters for drinking purposes does away rvith the 
necessity for the use of ice during the summer months. Unfortu- 
nately, however, nearly all of these waters show unmistakable evi- 
dences of pollution, either unmechate or remote, so that their use for 
drinking purposes must always remam a menace to the health of the 
community. It will be seen from an examination of the results of 
the analyses given in Tables 9, 10, 11 , and 12 that as a rule the waters 
of the shallow wells contain considerable amounts of total solids, con- 
sisting for the most part at least of perfectly soluble salts. These 
waters are also characterized m most instances at least by the presence 
of large amounts of chlorme and nitrates, indicative of remote sewage 
pollution, and in a fewer number of instances by the presence of rela- 
tively large amounts of nitrites and free and albuminoid ammonia. 
In many of the wells shoving large amounts of chlorine and nitrates, 
the waters show only small amounts of nitrites and free and albimii- 
noid ammonia, indicating that the pollution is remote, and that suffi- 
cient time has elapsed between the pollution of the water by sewage 
and its entrance into the well to permit of the complete oxidation of 
the polluting substances. lYliile as vdth the other impurities the 
greatest difference of opinion exists as to the significance of nitrates in 
Q See also ‘"'Tlie Filtration of Public Water Supplies,” by Hazen (1900), p. 187. 
