DisTRiHi'Tiox OP Copper Compounds 151 



have brought copper from a concentration of possibly less than 

 0.02 per cent in the original porphyry through every degree of 

 ricluK^ss to the condition in some cases of pure copper. 



Processes by which Copper is Added to the Water-Supply 



To a slight extent, drainage waters from the ore deposits and 

 from the mines, containing considerable amounts of copper in 

 solution, find their way downstream. But by far the larger part 

 of the copper which gets into the irrigating supply is derived 

 from the ores and tailings which, in the concentrators, on the 

 dumps, and finally in the river itself, are subjected to the action 

 of atmospheric oxygen, and water containing carbon dioxide and 

 various salts in solution. The residual chalcocite in tailings from 

 sulphide .ores thus reacts with oxygen from the air and yields 

 copper sulphate in solution. This, in turn, reacts with the excess 

 of bicarbonate of lime ordinarily contained in the waters of tlie 

 San Francisco and Gila rivers. The resulting basic carbonate of 

 copper is notably soluble in water containing carbon dioxide and 

 certain of the various salts commonly found in river waters. The 

 residues of carbonates of copper in oxidized ores are directly 

 dissolved in waters containing carbon dioxide and certain soluble 

 salts. 



Along with minute quantities of copper thus dissolved and 

 carried forward, pass the solid residues discharged from the 

 concentrators — solid wastes which find their way, unchanged, 

 downstream and finally upon the soils of irrigated fields. At this 

 point begins another and very important series of reactions be- 

 tween dissolved copper compounds and the soil, tending in 

 general to withdraw copper from its solutions and precipitate 

 it in the form of less harmful solid compounds. These are briefly 

 referred to above and will be discussed more in detail further 

 on in this paper. Opposing these precipitations of copper 

 are those solvents which tend to maintain this metal in soluble 

 form in small quantities in the soil. Chief of these is carbon 

 dioxide, which is always present in agricultural soils in 

 significant quantities. Of interest in this connection is the fol- 



