good water for many years does not supply good water to-day. 

 The answer in many cases is that when the well was first dug 

 the buildings which it was designed to supply had only just 

 been constructed, and little or no sewage had ever been de- 

 posited upon the ground in the region about the well. As soon 

 as the premises were occupied the seepage from the privy, sink 

 drain and cesspool, which were commonly located at no great 

 distance from the well, began percolating into the ground in its 

 neighborhood. The drawing of water from a well naturally 

 draws the ground water at that point to a lower level than 

 elsewhere in the neighborhood, and induces a flow of water 

 through the ground in its direction, and where sewage is dis- 

 charged upon or into the ground in the region influenced by the 

 draft of water from the well, seepage therefrom would naturally 

 have a tendency to flow toward the well. 



It is probable that in the beginning, and perhaps for many 

 years, polluted matter from privies, cesspools, etc., thus per- 

 colating through the ground was thoroughly purified by oxida- 

 tion and nitrification in its passage through the soil before 

 reaching the well, just as the sewage of villages and towns is 

 now purified by passing it slowl}^ and intermittently through 

 sand filter beds; but after a long period of continuous passage 

 of water containing organic matter through the ground with air 

 constantly excluded, the efficiency of the purification becomes 

 less and the effect of the pollution upon the ground water more 

 serious. It is also probable that the finer matters in the soil are 

 gradually washed out, and the passage of the water through 

 the ground thus allowed to become more rapid. 



The indications are that polluting matter from a privy or 

 cesspool percolating toward a well does not spread out over a 

 very large section of soil, but where the soil is fairly homo- 

 geneous is confined to a section of about the same area as that 

 of the privy or cesspool from which it comes. It is sometimes 

 practicable to determine quite definitely the area affected by 

 seepage from a privy or cesspool when excavations are made at 

 a point where such receptacles for sewage have been located. 

 An example of this was once seen by the writer, where a reser- 

 voir, upon one shore of which were located several privies and 

 cesspools, was drawn down, exposing a steep slope from which 



