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<br />i:;~ <br />~~ <br />"l <br />c.u <br />~ <br /> <br />Of all man's activities, irrigation is responsible for the <br />largest increase in salinity. Water is 'removed through <br />evaporation and consumption by plants, but nearly all of <br />the salt is returned to the river, concentrating the salts <br />in a smaller volume of water. At the same time the return <br />flows leach salts from the soils and rocks which add to the <br />River's salt load. <br /> <br />Reservoir evaporation also contributes to increasing sa- <br />linity because evaporation removes water but leaves the <br />salts to be concentrated in a smaller volume of water. <br />Out-of-basin exports and in-basin uses that do not return <br />salt to the system ordinarily cause higher salt concentra- <br />tions down stream. Salt loads contributed by municipal and <br />industrial use are of minor significance in this Region. <br /> <br />.,.... <br />:{;~~;i.1 <br /> <br />The Federal Water Pollution Control Act Amendments'of 1972 <br />(P.L. 92-500) require the adoption of water quality stan- <br />dards for the Colorado River System. The seven Colorado <br />River Basin States have adopted an objective that calls for <br />maintenance of salinity in the lower mainstem of the river <br />at or below the average value found during 1972 while the <br />Basin States continue to develop their compact-apportioned <br />waters. <br /> <br />Consistent with the foregoing policy and with regulations <br />of the Environmental Protection Agency, recommended flow- <br />weighted average annual numeric salinity criteria [emphasis <br />added] have been adopted by the affected Scaces for three <br />locations in the lower main stem of the Colorado River Sys- <br />tem as follows: <br /> <br />Below Hoover Dam <br />Below Parker Dam <br />Imperial Dam <br /> <br />Salinity in mg/l <br />723 <br />747 <br />879 <br /> <br />, <br /> <br />The plan of implementation comprises Federal and non-Federal <br />projects and measures to maintain these conditions. The <br />principal components of the plan are: (1) prompt construction <br />of four authorized salinity control units of the Colorado <br />River Salinity Control Project (Salinity Control Act, Public <br />Law 93-320); (2) construction of 12 other units or their <br />equivalent after receipt of favorable planning reports; <br />(3) the placing of effluent limitations on industrial dis- <br />charges; (4) reformulation of previously authorized, but <br />unconstructed, federal water projects to reduce salt loading <br />effect; and (5) use of saline water for industrial purposes <br />whenever practical, programs by water users to cope with <br />river's high salinity, studies to minimize salinity in muni- <br />cipal discharge, and studies of future possible salinity con- <br />trol programs. <br /> <br />3-17 <br />