SANITAKY SUKVEY OF DEAINAGE BASIN OF POTOMAC RIVER. 253 
or more miles from the intake of the Washington Aqueduct at Great 
Falls, about 15 per cent enters it at points between 50 and 175 miles 
above Great Falls, and 5 per cent, contributed by about 2,200 of the 
population, enters at points between 19 and 50 miles above the intake. 
9. Practically no direct pollution of the Potomac takes place within 
19 miles of the intake; but because this portion of the watershed is 
inhabited, the possibility of its occurrence can not be ignored. 
10. The pollution of the Potomac undergoes more or less natural 
purification and in addition becomes very greatly diluted before it 
reaches Great Falls. 
