638 



ELECTRICAL METHODS 



IChap. 10 



A comparison of analyses of water from different sources with the results 

 of theoretical calculations has shown, however, that for waters of less than 

 1000 ohm-cm, the resistivity can be determined closely enough from their 

 chlorine content alone. The curves in Fig. 10-3 show resistivities of 

 NaCl solutions as a function of concentration. 



tuooo 



2oim 



mm 



MOO 



600\ 

 % 



I 



^ 400 



^ 300 



200 



100 







50 



0JO3 om 



05 o.e 



Per Cent Chlorine 



K 15 



Fig. 10-3. Resistivities of salt solutions as a function of chlorine content (after 



Sundberg). 



The composition of natural waters varies widely, depending on origin 

 and on geologic occurrence. They may be classified as follows: 



1. Meteoric waters, derived from precipitation: p = 3000 — 100,000 

 ohm-cm. 



2. Surface waters (lakes, rivers, and the like) vary from 300,000 ohm-cm 

 for very pure water to as little as 10 ohm-cm for salt lakes. Surface 

 waters in districts of igneous rock are estimated to range from 3(X)0 to 

 50,000 ohm-cm; surface waters in areas of sedimentary rock from KXK) 

 to 10,000 ohm-cm. 



3. Soil waters (discharged into the atmosphere by evaporation) may be 

 as low as 10 ohm-cm, but their average is around 10,000 ohm-cm. 



4. Normal ground water in areas of igneous rock is of the order of 3000 

 to 15,(X)0 ohm-cm and in areas of sedimentary rocks as low as 100 ohm-cm. 



5. Subsurface (connate) waters (Na, K, Ca, and Mg chlorides) are 

 generally good conductors and are between 3 and 10 ohm-cm. 



6. Mine waters (usually copper, and zinc, and so on, suKates) are like- 

 wise of low resistivity, generally not exceeding 30 ohm-cm. 



