DUG WELLS 81 



hands and others coming from manured fields, hogpens or barn- 

 yards. 



The "Safety Distance" Factor in the Location of Dug Wells. - 

 By "safety distance" is meant the distance from a source of 

 pollution at which a well may be sunk with a fair degree of safety. 

 Some writers have spoken of a "cone of safety," by which is 

 meant an inverted conical section of earth with its apex at the 

 bottom of the well and its base a circle of some fixed radius on the 

 surface. The radius taken by some is the depth of the well, by 

 others twice the depth of the well, but such limits are usually 

 fixed without taking into consideration the nature of ground- 

 water movements or the character of the passages in which it 

 moves. The distance of safety also depends to a considerable 

 degree on the quantity and concentration of the pollution enter- 

 ing the ground water. Where coming from the surface the 

 amount is commonly not large, but where entering at a con- 

 siderable depth, as from cesspools sunk in limestone or in porous 

 sands which also supply water to wells, it may reach the water 

 stratum almost undiluted. It follows that no absolute radius can 

 be laid down, each case demanding individual consideration. 

 Certain generalizations, however, may be made as to conditions 

 in materials of different types and under different topographic 

 conditions, some of which are indicated below. 



In ordinary clay and in the pebbly or boulder clay known as 

 "till" the water circulates in part by general seepage through the 

 mass, in part through relatively thin sandy layers and in part 

 along more or less open but irregular tubular passages. Seepage 

 through the body of the clay or till is very slow and polluting 

 matter is rarely carried for any great lateral distance; 100 feet 

 from the nearest source of pollution may perhaps be regarded as 

 a safe limit. The clay offers even more resistance to the passage 

 of water directly downward, a 5-foot bed as a rule effectively 

 shutting off polluting matter from the underlying water beds, un- 

 less such matter obtains access along the break made in sinking 

 a well or other excavation. When the water follows sandy layers 



