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<br />2 <br /> <br /> <br />W <br />N <br />00 to the Imperial Dam. Ilere the present modified ~ average concen- <br />~ <br />tration is 865 mg/l. This increase arises as a result of both <br /> <br />natural processes and the activities of man. Wherever rain falls, <br /> <br />natural solute erosion occurs. This process embraces the geochemical <br /> <br />reactions that take place as water moves through the hydrologic cycle. <br /> <br />The pathways and some of the important reactions involved in this <br /> <br />cycle are depicted in Figure 1. The process has been active over <br /> <br />geologic time. Even with the extensive developments by man, the <br /> <br />natural processes are still the principal source of salinity in the <br /> <br />Colorado River. <br /> <br />While the geochemical processes add a large variety of dissolved <br /> <br />matter to the water, only 10 elements make ~ 99 percent or so of <br /> <br />the dissolved constituents. These are hydrogen, sodi\w, magnesium, <br /> <br />potassium, calcium, silicon, chlorine, oxygen, carbon, and sulfur. <br /> <br />The elements occur in solution as various ions, moleolles, or radi- <br /> <br />cals. The major part of the dissolved constituents in the Colorado <br /> <br />River are made ~ of the cations calcium, magnesiIW, and sodium, and <br /> <br />the anions sulfate, chloride, and bicarbonate. These, plus minor <br /> <br />amounts of other dissolved constituents, are commonly referred to as <br /> <br />salinity. <br /> <br />2/ Present modified refers to the historic conditions (1941-1968) <br />modified to reflect all upstream existing projects in operation for <br />the full period. <br />