SANITARY SURVEY OF DRAINAGE BASIN OF POTOMAC RIVER. 251 
The dilution of the sewage at the points w^here it is discharged is, 
almost vdthout exception, sufficient to prevent an}^ local nuisance. 
In its course dovui the river the dilution increases progressively and 
by the time it reaches Great Falls it is very great. The degree of 
this dilution will be clearly comprehended b}^ considering the dis- 
charge of the river in relation to the total pollution. 
In Table No. 1 is given the mean monthly discharge at Point of 
Rocks, Md., 33 miles above Great Falls. With 45,000 as the total 
population contributing direct sewage, the discharge of the river 
during April was 449 cubic feet a second per 1,000, during May it was 
123 cubic feet a second per 1,000, and during elune 156 cubic feet a 
second per 1,000. This dilution would be considerable, even if no 
change in the character of the pollution during its progress toward 
Great Falls took place; but when it is considered that conditions such 
as shoals, rapids, natural pools and dams favoring the processes of 
purification by aeration and sedimentation are found on the Potomac 
and its tributaries, and when, in addition, it is remembered that the 
other factors generall}^ recognized as important agents in the natural 
process of purification of polluted waters also exist and are probably 
at work here, the dilution may properly be regarded as, at times, very 
great. 
Table No. 5. — Population of Potomac basin which contributes direct sewage pollution. 
Distance 
above 
Great 
Falls, in 
miles. 
Popula- 
tion (cen- 
sus 1900). 
Estimated 
population 
contribut- 
ing direct 
sewage, 
pollution. 
Per cent 
of total 
direct 
pollution. 
(a) More than 175 miles above Great Falls: 
Cumberland, Md 
176 
17, 128 
17,000 
Keyser, W. Va 
202 
2,536 
2,200 
Piedmont, Westernport, Luke 
207 
4,623 
4,000 
Harrisonburg, Va 
211 
3,521 
2, 800 
Staunton, Va 
223 
7,289 
6, 000 
Other places 
3,000 
Total 
35,000 
78 
(5) More than 50 and less than 175 miles above Great Falls: 
Hagerstown, Md 
76 
13, 591 
500 
Martinsburg, W. Va 
86 
7, 564 
600 
Waynesboro, Pa 
87 
5,396 
1,200 
Winchester, Va 
121 
5,161 
500 
Chambersburs:, Pa 
127 
8,864 
1,200 
Other places 
3, 800 
Total 
7,800 
17 
(c) More than 19 and less than 50 miles above Great Falls: 
Leesburg, Va 
19 
1,513 
200 
Point of Rocks, Md 
33 
364 
150 
Brunswick, Md 
39 
2,471 
450 
