Danger from Wells 91 



is largely due to the carbon dioxid contained in it, the 

 latter being derived from the decomposition of organic 

 matter. Soils rich in organic substances, particularly 

 those contaminated with sewage, produce more carbon 

 dioxid than those poor in organic matter, and the 

 brightness of well-water is, therefore, far from being 

 a guarantee of its purity. 



Soil may be regarded as a good filter. It holds back 

 undissolved organic materials including bacteria, and, 

 to a marked extent, also materials in solution. The 

 water of springs and deep wells is, at times, almost 

 germ-free, largely because of the filtering power of the 

 soil. With that much granted, however, it still remains 

 true that the soil cannot be entirely relied upon to pre- 

 vent the passage of typhoid or other disease germs 

 from cess-pools and privy-vaults into shallow wells 

 located a short distance from them. This fact has long 

 since forced itself upon the attention of municipal au- 

 thorities, and has led to the establishment of regulations 

 calling for a minimum distance of 20 to 50 feet between 

 wells and cess-pools or privy-vaults. Unfortunately, 

 the distance of 25 feet, frequently allowed by the munici- 

 pal regulations, is a very uncertain security against the 

 entrance of disease bacteria into wells. 



The soil is not a solid mass of material, but an aggre- 

 gation of particles of various sizes, separated by pores, 

 or spaces filled with air. The rain that falls on the soil 

 makes its way downward past the solid particles until 

 it reaches a point where the pores are filled with water 

 instead of air. The depth in the subsoil beyond which 

 the ground is thus saturated with moisture is known 



