WASTES IN RELATION TO AGRICULTURE AND FORESTRY 



39 



Stout and Burau 4 studied nitrate accumulation 

 in ground water within the 10-square-mile basin 

 occupied by Grover City and Arroyo Grande, 

 Calif. Concentrations of nitrate at 90 to 130 p.p.m. 

 in water percolating to the underground reservoir 

 were not uncommon. The sources appeared to be 

 nitrification of sewage effluent and natural nitri- 

 fication processes in the surface soils that are fos- 

 tered by the climate and the nature of the soil. 



The Soils Department (59) at "Washington State 

 University studied nitrate content of well water 

 in areas where various amounts of fertilizer had 

 been used, and found that high nitrate in the well 

 water was not a direct consequence of heavy fer- 

 tilizer use. 



At a recent American Association for the Ad- 

 vancement of Science symposium, Smith 5 re- 

 ported on his extensive studies of the nitrate 

 content of 6,000 rural water supplies. He 

 found that animal wastes, improperly con- 

 structed shallow wells, and septic tank drainage 

 were the main sources of water contamination. His 

 evidence showed that leaching of fertilizer nitro- 

 gen was an insignificant source of nitrate in the 

 well waters of Missouri. 



The foregoing presents conflicting evidence. 

 Conclusive evidence is lacking that chemical fer- 

 tilization of fields results in high nitrate levels 

 in well water. Natural nitrification processes in 

 soils and nitrification of sewage effluent and animal 

 wastes do seem to be major contributors when 

 nitrate is found in ground water. The general 

 problem is one of growing importance and much 

 more definitive information is needed. 



Inorganic Salts and Minerals 



Of the water diverted from massed supply ami 

 consumptively used, 90 percent is used in irriga- 

 tion. Thus it is that agriculture is sometimes 

 referred to as ''the greatest water hog of all" [26). 



Because of this high consumptive use through 

 the processes of evapotranspiration, salts present 

 in the irrigation water supply become more con- 

 centrated in the drainage effluent. Consequently, 



1 Stout. P. R., and Eurau, It. Extent and Significance of 

 Fertilizer Buildup in Soils. Presented at Aiuer. Assoc. Adv. 

 Sci. Symposium, Washington, D.C., Dec. 28, 196G. 



5 Smith, G. E. Fertilizer Nutrients as Contaminants in 

 Water Supplies. Presented at Amer. Assoc. Adv. Sci. Sym- 

 posium, Washington, D.C., Dec. 27, 19G6. 



frequent claims are made that irrigation agricul- 

 ture seriously impairs the quality of water. 



Irrigation agriculture provided the very spawn- 

 ing ground for the beginnings of civilization in 

 the valleys of the Nile, the Indus, and the Tigris- 

 Euphrates. The benefits that have arisen from ir- 

 rigated farming have not been fully appreciated. 



The irrigation water brought onto a field always 

 carries some dissolved salt — usually within the 

 range of 25 to 8,000 p.p.m. Plants extract water 

 from the irrigated field for transpiration, but most 

 of the salt is excluded by the roots. Water evapo- 

 rating from the soil surface is pure water. The 

 residual salt accumulates in the soil. In arid cli- 

 mates where Nature has left an accumulation of 

 salt in the soil, the application of irrigation water 

 will fortify this salt concentration. 



For irrigation agriculture to survive, this proc- 

 ess of salt accumulation in the soil must contin- 

 ually be countered by leaching with excess 

 applications of water. The salt moves to deep hori- 

 zons, the ground water, or the drainage system. 

 Tims, the salt appearing in drains is that brought 

 in by the irrigation water plus that which may 

 have been naturally present in the soil. The irriga- 

 tion farmer adds but little to this salt burden by 

 his application of chemical fertilizers. These fer- 

 tilizer salts barely meet crop needs. Under the 

 natural conditions of crop production, evapotran- 

 spiration depletes the vehicle-water that trans- 

 ports salt to the drainage system. This natural sys- 

 tem imposes an increase in salt burden in the 

 drainage water from an irrigated field compared 

 with that in the water supply. 



In arid regions, it is a first criterion for the sur- 

 vival of an irrigated area that there be favorable 

 salt balance; salt leaving the project by drainage 

 or deep percolation must equal or exceed that re- 

 ceived in the water supply. In the initial years of 

 a project, when there is residual salt in the soil, 

 more salt will have to be removed than is brought 

 in. 



Thus, the irrigation farmer does not actually 

 produce a waste in the form o( d\>>tA\cA salts, he 

 merely transfers the waste in a more concentrated 

 form. Nevertheless, drainage water frequently 

 warrants the label of "impaired quality" because 

 of its burden of dissolved solid-. 



The quality of the return flow from irrigation 

 projects may also be impaired by partial preoipi- 



