PROBLEMS FROM LAND OCCUPANCY 201 



resources would be rendered unsafe for use in a few days if 

 dilution alone were the factor of protection. But other factors 

 are at work, as summarized by Stead: 35 



Sewage bacteria applied to the surface of the ground or slightly 

 below it may never leave the top few feet of soil, they may be com- 

 pletely removed by downward percolation through porous soil, 

 or, even though they reach water table they may be stranded by a 

 falling water table, travel only in narrow bands in the direction 

 of ground water flow and the length of the band is limited to the 

 distance the water moves in two or three weeks. Ground-water 

 velocities vary but are usually in terms of not more than a few feet 

 per day, so that travel horizontally of sewage bacteria for distances 

 greater than a few hundred feet is extremely rare. Furthermore, 

 the general pollution of deep water strata with sewage bacteria to 

 any alarming degree is virtually an impossibility. 



The above discussion applies to liquid sewage discharged to the 

 surface of the ground or the shallow leaching devices in soils or 

 fine sands. Disposal to coarse uniform-sized gravels, cavernous 

 limestone formations, lava with underground channels, or by 

 direct discharge to coarse water-bearing formations would permit 

 much greater travel of pollution, and therefore no such disposal 

 can be considered safe. 



Stead also points out that although a very few sewage bac- 

 teria can survive for months in ground water, more than 99 

 per cent will die in ground water within two or three weeks. 

 The minimum limits of safety from a polluting source are 

 therefore determined in large part by the velocity of down- 

 ward percolation and of lateral ground-water movement. The 

 permeability of the materials through which the water must 

 move is thus a critical factor. It has been found that as little 

 as 10 feet of downward percolation in fine sand is capable 

 of removing all bacteria from water. 36 Wells along the banks 



35 Stead, F. M., "A Discussion of Factors Limiting the Bacterial Pollution of 

 Underground Waters by Sewage," Report of Interior Fact-finding Committee 

 on Water Pollution to the Assembly of the State of California, pp. 138-139, 1949. 



36 Caldwell, Elfreda, Studies of Subsoil Pollution in Relation to Possible 

 Contamination of Ground Water from Human Excreta, Jour. Infectious 

 Diseases, vol. 62, no. 3. May-June 1938. 



