CHAPTER XXI. 

 COMPOSITION AND TESTING OF WELL WATERS. 



Purity of Rain Water. Rain, which is the ultimate source of 

 practically all waters reached by wells, is essentially pure when it 

 falls upon the surface. Although it is true that a certain amount 

 of dust and minute quantities of gas, including carbonic, sulphuric 

 and nitric acid, are absorbed in the vicinity of cities by the rain 

 drops in falling, and that near the sea a small amount of salt is 

 brought down with the rain, the amount is very small. The gas 

 in a cubic foot of rain water would hardly fill a half-inch cube, 

 while the amount of mineral matter is insignificant compared to 

 that dissolved in the passage of the water through the soil and 

 rocks, and is, for the most part, negligible. Bacteria in rain are 

 few in number and commonly of harmless types. 



Source of Mineralization. As the rain falls on the surface it 

 commonly, in all but desert regions, sinks through a thin layer of 

 humus or vegetable mould where it becomes charged with certain 

 organic acids. These, together with the gases previously ab- 

 sorbed from the air, attack certain of the minerals with which 

 they are brought in contact. The feldspars, which are com- 

 ponents of granite and several other rocks, afford sodium and 

 potassium; calcium and magnesium are obtained from many 

 minerals and rocks, especially from limestone; while iron, alumi- 

 num and silica are derived from a variety of sources. Most of 

 the substances mentioned are in the form of carbonates, sulphates, 

 nitrates or chlorides. Many other substances, including some 

 rare elements, are present in ground waters in small amounts. 



The amount of mineral matter dissolved by the ground 

 waters depends to a considerable extent upon the character of 

 the materials through which the waters have passed. In sands 



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