1917] Forbes: Irrigation Effects of Copper Compounds Upon Crops 399 



district. Recent investigations by the companies indicate a pos- 

 sibility that with copper at 15 cents a pound these stored tailings, 

 which average about 0.85 per cent copper, may be profitably 

 reworked. 



In the long run, therefore, it may be found that an adjust- 

 ment based upon a complete and impartial statement of facts 

 relating to the tailings situation is beneficial both to the agri- 

 cultural and to the mining interests concerned. 



Soluble Copper Compounds 



Following the disposition of mining detritus, there remained 

 the problem of soluble copper compounds which, in small but 

 continuously appreciable quantities, find their way with waste 

 waters into the stream-flow of the region. These compounds 

 originate in the ores of the district and are, as in the case of the 

 carbonates, directly soluble to a slight extent in drainage waters, 

 especially in the presence of carbon dioxide. In other cases, the 

 original ores are changed through the action of air into soluble 

 substances which then escape downstream. Sulphide ores are 

 thus oxidized in the presence of air into soluble copper sulphate. 

 Inasmuch as it is well known that minute amounts of copper in 

 solution are extremely toxic to plant roots directly exposed to 

 them, some apprehension naturally existed as to the effects of 

 these small amounts of copper salts escaping into the water- 

 supply of an irrigated district. 



In some respects, conditions were especially favorable here 

 to the successful prosecution of a study of the foregoing ques- 

 tion. The irrigated lands are at a distance of twenty miles or 

 more from the smelters, so that injurious gases could not com- 

 plicate effects upon irrigated crops. There are, also, only traces 

 of other toxic metals to be found within the district — more par- 

 ticularly, arsenic, antimony, and zinc. Injurious effects due to 

 the possible toxic action of compounds originating in the mines 

 are therefore limited to copper. 



Scientific study relating to toxic effects of copper upon plants 

 under varying conditions has thoroughly established not only 

 the fact that copper compounds are extremely toxic to plants 

 when they obtain entry to their tissues, but also that various 



