WATER SUPPLY AND SEWAGE DISPOSAL 157 



habit of disposing of rubbish by dumping it into streams. 

 The most important source of bacteria in many of our streams 

 is the sewage of the cities on their banks. The bacteria from 

 this source are particularly objectionable, because the sewage 

 includes human wastes and therefore commonly contains the 

 germs of such infectious diseases as are present in the com- 

 munity. As the water of the river flows onward many of the 

 bacteria settle to the bottom along with other sediments 

 or are killed by exposure to air and sunlight, but the 

 water is not likely to be wholly purified by the time it 

 reaches the next city or town. Excepting near the source, 

 very few rivers are suitable for domestic use in their natural 

 condition. 



The larger lakes are fed by many rivers and therefore 

 receive large numbers of bacteria from this source. The con- 

 tamination is increased by refuse dumped alongshore and by 

 the sewage of adjoining cities. It should be noted that all the 

 principal sources of contamination of lake waters are located 

 near shore. The waters of the center of the lakes are polluted 

 only as bacteria and other sediments are carried out into the 

 lake by currents in the water. The movements of lake water 

 are so slow that bacteria and other sediments usually settle 

 to the bottom before they have traveled many miles from land. 

 For this reason the water in the central part of large lakes 

 is usually of excellent quality. As cities increase in size, how- 

 ever, the zone of pollution alongshore extends farther into 

 the lake. 



175. Methods of securing pure water. The cities on the Great 

 Lakes have ordinarily taken their water supplies from the lakes, 

 and they have sought to secure pure water by constructing 

 tunnels in the lake bottoms which make it possible to secure 

 water from points at a distance of from one to six miles from 

 shore. Even this distance is not great enough to guarantee 

 pure water in all cases, and filtering or other means of 

 purification is desirable. 



