CHAPTER IX 

 GROUND-WATER REACHING SEWERS 



THE amount of rain-water entering a sewer has been dis- 

 cussed, and also the amount of water from domestic uses, the 

 latter including the amount used for manufacturing and other 

 municipal purposes. It now remains to determine the amount 

 likely to come from the ground-water through which the sewer- 

 line passes. This amount will depend on the material of which 

 the sewer is made, on the kind of joints, on the method used in 

 making them, and on the distance and head of ground-water 

 in which the sewer is exposed to the infiltration. The last 

 condition has considerable variation even in the same line, 

 both because of irregular variation due to rain and because of 

 periodic seasonal changes. In constructing the filter-beds at 

 Brockton, it was found that there was a seasonal variation 

 of 4 feet in the height of the ground-water, the height being 

 greatest in May and least in November. Such a rise in the 

 elevation of the ground-water might increase the length of 

 sewer exposed to ground-water by some miles, especially if 

 the hydraulic grade of the underground stream followed nearly 

 parallel to the sewer-grade. 



In his report on the Sewerage of Ithaca, Mr. Hering says: 

 " In addition thereto [60 gallons per capita per day, assumed 

 for average water-supply], 10 per cent of this quantity has 

 been added to allow for ground-water which will probably find 

 its way into the sewers in spite of the most careful workman- 

 ship." 



In the report on the Sewerage of the Mystic and Charles 

 rivers, January, 1889, the engineer, Mr. F. P. Stearns, has 



collected the following information: 



142 



