CHEMICAL EXAMINATION OF THE WATEK SUPPLY. 285 
and fluctuating character of Potomac water. During a large part of 
the month of November, 1906, the river was low and the water 
relatively clear and free from mud; during the months of July and 
August, on the other hand, the river was quite high and the water 
muddy and extremely turbid. The purification of the Potomac 
water supply presents much the same problem that has been encoun- 
tered in the purification of the waters of our southern rivers, which, 
at certain seasons of the year, contain large amounts of clay in sus- 
pension, thereby causing sudden and extreme variations in turbidity.® 
petrification of THE POTOMAC WATER SUPPLY BY SUBSIDENCE AND 
FILTRATION. 
As the water of the Potomac Kiver passes through the several 
reservoirs on its way to the sand filters, the flow of the water becomes 
less rapid and certain chemical and bacteriological changes occur, 
all tending toward a considerable purification of the water. A fur- 
ther purification of the water is effected by sand filtration. That 
such is the case may be seen from the general summary of the results 
of our analyses of the waters of the reservoirs and storage basin given 
in Table 16, and also from the summary in Table 17, giving the 
results of the analyses made by Mr. Longley of 20 weekly samples of 
the waters of the reservoirs and storage basm covering a period from 
February 13 to June 26, 1906. These two sets of analyses show a 
reasonable agreement, especially when it is borne in mind that they 
were made at different seasons of the year and that the water of the 
Potomac is subject to considerable variation. It will be seen from 
Table 16 that during the period covered by our examination of the 
waters of the reservoirs, viz, from July 30 to September 27, 1906, the 
average turbidit}^ gradually diminished in passing from the inlet of 
the Dalecarlia reservoir to the filter beds from an average of 221 at 
Dalecarlia inlet to 46 in the Washington Citv reservoir. There was 
also a gradual falling off of about one-third in total solids and 
mineral matter, and also about the same diminution in the quanti- 
ties of free and albuminoid ammonia. The oxygen consumed also 
showed a falling off of about one-fourth, the nitrates and dissolved 
oxygen remaining practically the same as at the inlet, whereas the 
nitrites were found to increase gradually from 0.0031 parts per 
million at the Dalecarlia inlet to 0.0065 parts per million in the 
Georgetown reservoir, and then to decrease to 0.0056 parts per million 
in the Washington City reservoir. The chlorine was also found to 
diminish slightly in passing through the series of reservoirs, so that the 
general effect of the slow passage of the w’ater through the reservoirs 
“See statement of G. W. Fuller in “Purification of the Washington Water Supply,” 
by Charles Moore, pp. 41-42. 
