210 ANNUAL REPORT OF THE Off. Doc. 



to the fact that the well stands ou higher ground than either. He 

 forgets the fact that it is underground not surface drainage that has 

 polluted the well. While it is true that the movements of the ground 

 water frequently follow more or less closely the course of the surface 

 water yet such is by no means always the case. It is entirely unsafe 

 to attempt to draw any conclusion with regard to the movements of 

 the ground water in a small area by observing the surface drainage. 

 Oftentimes the movement of the underground water is in exactly the 

 opposite direction from the surface water. Sometimes very good 

 evidence can be obtained as to the probable course taken by the sub- 

 soil water, from an inspection of the geological formation in the 

 neighborhood of the well. Frequently, however, soil and vegetation 

 completely conceal all evidence as to probable course of the under- 

 ground water. Iti some localities where the geological formation 

 and the dip of the strata are uniform, an opinion can be formed as to 

 the course of the underground water, though nothing can be seen 

 of this formation in the immediate vicinity of the well. On the other 

 hand, in some localities^ as for example in the vicinity of West Ches- 

 ter, the structure of the earth is so complex that frequently very 

 little information can be gained in regard to the movements of the 

 subsoil water by surface inspection. The country about West Ches- 

 ter is composed either of igneous rocks or very highly metamorphosed 

 sedimentary rocks, without evident stratification and cut in different 

 directions by numerous dikes and fissure veins. 



In order to illustrate how a well may be contaminated by a neigh- 

 boring source of pollution, let us imagine a cesspool six feet deep 

 placed within forty feet of a well, say fifty feet deep. The ground 

 slopes from the well toward the privy the difference in elevation 

 being possibly five feet or more. 



Under such circumstances as these the bottom of the cesspool will 

 be about forty feet above the level of the water in the well and con- 

 sequently above permanent ground water. If the subsoil is homo- 

 geneous and permeable in character the water from the cesspool will 

 drain downward and outward from this point in every direction. 

 The earth will gradually become saturated with organic matter and 

 polluted water will be carried to the well before it has descended to 

 the general level of the ground water. If in addition to this the move- 

 ment of the ground w^ater is toward the well, still more serious pol- 

 lution will result. Should the dip of the rock be toward the well 

 or should it be impermeable and filled with fissure veins, the danger 

 of pollution is still greater. In many cases where the earth is com- 

 posed of compact rock cut by fissure veins or other cracks it is diffi- 

 cult to tell where the water of a well or spring may come from. In 

 such cases pollution may be carried a long distance. 



At this writing I have in mind a certain farm, the water supplies 



