the surface soil had been removed when the reservoir was built. 

 In this case the indications of the areas through which the 

 liquid percolating from the privies and cesspools to the reser- 

 voir was passing were very clearly marked by the iron rust which 

 settled out of the water on its coming to the air at the bank of 

 the reservoir, and the traces of percolation from these eight or 

 ten receptacles were in each case very clearly marked. In these 

 cases it was evident that the path of the polluted water was 

 quite direct, and that the area of cross section through which 

 it passed was but little greater in any of these cases than the 

 area of the privy or cesspool itself. The path of the waste had 

 evidently remained the same for many years, indicating that 

 the passage of the polluted liquid did not have a tendency to 

 clog up the ground, but that rather the ground offered less 

 resistance to the passage of the polluted water as time went on. 

 The soil in this case was but slighly porous, being what is 

 sometimes called a gravelly hardpan. 



The rate of percolation of water through the soil varies 

 greatly with the character and porosity of the material, and 

 liquid percolating from a cesspool toward a well may require a 

 long time in its passage through the ground before it materially 

 affects the character of the water of the well, but where the soil 

 is porous sand or gravel, and much water is drawn from the 

 well, percolation may be rapid. 



Under the general geological conditions in Massachusetts, the 

 water which enters a well is derived from the rainfall which 

 falls upon the ground immediately about it and percolates 

 through the ground to the well. The area from which ground 

 water is influenced to flow toward a well depends upon the 

 porosity of the soil, the quantity of water drawn, and the gen- 

 eral trend of the ground water of the locality in which the well 

 is situated. The area from which a well derives its supply can- 

 not always be determined very definitely, but it can usually be 

 determined with sufficient accuracy for practical purposes. 



An average family of six persons would ordinarily use — 

 with the water required for animals, etc., and allowing a small 

 amount for irrigation of gardens about the house in the summer 

 season — ^ about 600 gallons per day. Assuming an average 

 rainfall of 42 inches per year, about 8,500 square feet of land 



