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MISCELLANEOUS PUBLICATION NO. 10 65, U.S. DEPARTMENT OF AGRICULTURE 



tation of the salts, even though precipitation de- 

 creases the total salt concentration. The slightly 

 soluble carbonate and sulfate of calcium precipi- 

 tate as the water volume is reduced by evapotran- 

 spiration, thus increasing the proportion of 

 sodum in the effluent. High proportions of sodium 

 in the irrigation waters are generally damaging 

 because they cause deterioration of soil properties 

 and specific toxic effects of sodium in some fruit 

 crops and other woody plants. For municipal and 

 many industrial uses, an increased proportion of 

 sodium is, however, not harmful. 



The increase in total salinity of return flows 

 from irrigation projects will generally render the 

 water unpalatable even before its utility for agri- 

 culture is completely lost. Thus, public health 

 standards for potable water have been set at 250 

 mg./liter of chloride, a level which may damage 

 only some sensitive fruit crops. In fact, waters 

 containing several thousand mg./liter of total salts 

 may still be of value for agriculture, especially 

 when favorable soil conditions prevail and tolerant 

 crops are grown, but such waters cannot be used 

 for municipalities or for many industrial processes. 

 The upper recommended limit of total salts for 

 potable water is 500 mg./liter, although waters 

 with twice this salt content are actually being used. 

 Highly saline waters also have considerable reuse 

 potential for recreational purposes. The drainage 

 waters of the Imperial and Coachella Valleys, con- 

 taining about 2,000 mg./liter of total salts, flow 

 into the Salton Sea, which affords fishing, boating, 

 and other water sports for many hundreds of peo- 

 ple. However, since the Salton Sea has no outlet, 

 continued evaporation will ultimately render the 

 sea so saline that the recreational use pattern will 

 undoubtedly have to be adjusted. 



The trend toward intensified use of limited ir- 

 rigation water resources foreshadows more severe 

 problems in the future. Irrigation projects must 

 generally maintain salt balance. Salt cannot ac- 

 cumulate in the soil without damaging crops, so as 

 much salt must be carried out in drainage water as 

 is applied in irrigation water. The decrease in vol- 

 ume of water with each use results in a propor- 

 tional increase in salt concentration. Thus, if 20 

 percent of the total water flow in a river is con- 

 sumptively used by each of four successive projects 

 along the river, the relative salt concentration of 

 the river will increase from 1.0 to 1.25, 1.67, 2.50, 



and 5.00 following each stage of use and return 

 flow. It is obvious that as the river flow is depleted, 

 the salt concentration of the remaining flow must 

 increase inversely with the volume of water if salt 

 balance in all projects is to be maintained. It is 

 further obvious that, under salt balance conditions 

 and with return flow of drainage water, complete 

 allocation of the river flow may give the last proj- 

 ect a water with prohibitively high salt content. 

 Alternative procedures for handling return 

 drainage flow along some rivers may have to be 

 devised, especially if downstream users are to have 

 potable water as well as irrigation water. One solu- 

 tion may be to purify the drainage flow by suitable 

 desalination process, returning the purified frac- 

 tion to the river and disposing of the concentrated 

 salines in other ways, such as outfall drains to the 

 sea, collection in evaporation ponds, or percolation 

 into the ground where usable ground water would 

 not be affected. Such solutions would be practical 

 only when return flows are readily recoverable, as 

 from tile drains or pumped wells, or can be readily 

 intercepted before the underground flow returns 

 to the river. 



Organic Wastes 



It is in the nature of things that man's domesti- 

 cated animals produce wastes ; that his fields and 

 forests produce residues and trash; and that his 

 food- and fiber-processing plants produce useless 

 byproducts. 



There are instances wherein wastes of this cate- 

 gory have emitted odors into the atmosphere; 

 have contaminated the water of streams, ponds, 

 lakes, and reservoirs; have provided dangerous 

 fire hazards ; have disseminated infectious agents ; 

 and have been the very spawning ground of a 

 whole array of vermin. 



Animal Wastes 



Time was when animal wastes were considered a 

 tremendous asset in providing fertility to the Na- 

 tion's soils. The 1938 Yearbook of Agriculture 

 {121, p. 44:5) carries this statement : 



One billion tons of manure, the annual product 

 of livestock on American farms, is capable of 

 producing $3,000,000,000 worth of increase in 

 crops. The potential value of this agricultural 

 resource is three times that of the Nation's wheat 

 crop and equivalent to $440 for each of the coun- 

 try's 6,800,000 farm operators. The crop nutrients 



