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<br />00507 <br /> <br />,^,A TER ()[J,~L ITY <br /> <br />The mean-annual discharge-weighted, dissolved-solids concentrations <br />for the Colorado River at three sites for the 1942-75 water years are <br />shown on Figure 1. Prior to 1969 daily water samples were composited <br />into three or more samp 1 es per mon th for chemi ca 1 ana 'Iys is whi ch a 11 owed <br />for the computation of annual weiohted-averaqe concentrations. Since <br />1969, average annual concentrations have been computed from a dail.y <br />specific conductance versus dissolved solids linear regression analysis. <br />The values noted as Historical represent actual concentrations computed <br />for these sites. The values shown as With 131,000 acre-feet diversion <br />represent the concentrations computed for these sites if 131,000 acre- <br />feet of water with a total dissolved-solids (Tds) concentration of <br />100 milligrams per liter (mgl) was diverted from Dillon Reservoir each <br />year. <br /> <br />The average increase of dissolved-solids concentration of 11 mg/l <br />at the station Colorado River near Cameo for the period 1964-73 as <br />indicated in the Fianl Environmental Statement for the Proposed Foothills <br />project substantially agrees with the average of 15 mg/l determined by <br />the Geological Survey for the period 1942-75 at the same site. This <br />increase is less than the normal variation in the annual dissolved- <br />solids concentration of the river at this station. <br /> <br />For comparative purposes, water with total dissolved solids lower <br />than 500-600 milligrams per liter (mgl) is considered preferable for <br />drinking water, although many western communiti.es use water of 1,000 <br />mgl or more. <br /> <br />Guidelines for salinity in irrigation water for arid regions. <br />published by the Federal Water Pollution Control Administration (now <br />E.P.A.h contain the following data: <br /> <br />Crop Response <br /> <br />IDS ,mg!l <br /> <br />Water for which no detrimental effects will <br />usually be noticed. <br /> <br />500 <br /> <br />Water which can have detrimental effects on <br />sensitive crops <br /> <br />500-1,000 <br /> <br />Water that may have adverse effects on many <br />crops and requiring careful management <br />practices <br /> <br />1 ,000-2,000 <br /> <br /> <br />Water that can be used for Salt-tolerant <br />plants on permeable soils with careful <br />management practices <br /> <br />2,000-5,000 <br /> <br />8 <br />