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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 formations, and runoff all contribute to this background salinity. Irrigation, reservoir evaporation, and municipal and industrial (M&I) sources make up the balance of the salinity problem in the Colorado River Basin. Figure 2 shows the relative amounts each source contributes to the salinity problem. EPA[I] estimated the natural salinity in the lower basin was 334 mgIL.' In 1995, the salinity at Imperial Dam was reported to be 803 mg/L, a 469-mg/L mcrease. Salinity of the Colorado River has been increased by the development of water resources in two major ways: (1) the addition of salts from water use and (2) the consumption (depletion) of water. The combined effects of water use and consumption have had a significant impact on salinity in the Colorado River Basin. Concern over the damaging levels of salinity prompted the Basin States and the Federal Government to adopt salinity standards and an implementation plan to limit further increases in salinity that are discussed later in chapters 5 and 6. Sources of Salinity M&I (4%) Figure 2.-Sources of salinity in the Colorado River Basin. Agricultural Sources of Salinity Irrigated agriculture is the largest user of water in the Colorado River Basin and a major contributor to the salinity of the system. Iorns[2] 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). Irrigation increases salinity (figure 3) by consuming water and by dissolving salts found in the underlying saline soils and geologic formations, usually marine shales. Irrigation Sources of Salinity Colorado River Irrigation Ditches Irrisated Field .-. ... ~;~~;j~~!~;~;~i~~~~;j;~~j~m!~ij)j~W;~;~;T;;;;";~;;;~~;;;;;;;;;";;;;;';nili~;li!ilH1j1i1Hj1!ijijij' ,..,.....-........-..,...,.."'.,.,.,.,15 alme Marine 5 halesi,_,.'i'.'...................... ~111~[~lm~m!lmm[~~1111Ir~ill~~~111~1~1~1!llljl;1~1~;!j1~11~1~i~;;;; ;"l~lllllll!ljlllll~l~l!lj. Figure 3.-Schematic of agricultural salt pickup. 9