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r::' 0' ".! r; Chapter 3 CAUSES AND IMPACTS OF SALINITY CAUSES OF SALINITY Nearly half of the salinity in the Colorado River System is from natural sources. Saline springs. erosIOn of saline geologic fonnations. and runoff all contribute to this background salinity. Irngation. reservoir evaporalion. and municipal and industrial (\1&1} !'.ources make up the balance of the salimty problem in the Colorado River Ba!'.in. Figure:! show!'. the relative amount!'. each source contribute!'. to the salinity problem. The Environmental Protection Agenc)" (EPA) estimated the natural salinil)' in the Lower Colorado River Basin was 334 milligrams per liter (mg/l) (U.S. Environmental Proteclion Agenc)". 1971). In 1997. the salinity at Imperial Dam was reported to be 7CJ..l. mg/L. a 370-mg/L increase. Salinity of the Colorado River has been Increased by the development of water resources in tWO major ways: (1) the addition of saIts from water use and (:!) the consumption (depletion) of water. The combined effects of water use and consumption have had a significant impact on salinil)" In lhe Colorado River Basin. Concern over the damaging levels of salinit)' prompted the Basin Stales and the Federal Government to adopt salinil)' standards and an implementation plan to limit further increases in salinity that are discussed later in chapters 4 and 5. Agricultural Sources 01 Salinity Irrigated agriculture is the largest user of water in the Colorado River Basin and a major contributor to the salinity of the system. lom<; found that irrigated lands in the Upper Colorado River Basin contribute about 3.4 million tons of salt per year (37 percent of the salinity of the river) (Iorm;, et al.. 1965), Irrigation increases salinity (figure 3) by consuming water and by dissolving salts found in the underlying saline soils and geologic fomlalions. usually marine shales. Sources of Salinity Natur3l(.1%l \ 1.1&1(.%) FIgure 2.-Sources 01 sallntty In the Colorado RIver BaSin. Irrigation Sources of Salinity Colorado RN9r lrrigabon Oit_ ~- , .,"~ " Saline I Soils--.J lrrgated F.. lUlU I w- -- . Saltne Manoe Shales FIgure 3_ SchematIC 01 agricultural sail pIckup. 11