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SUMMARY N .&:0. . .&:0. CO The purpose of the Grand Valley Unit of the Colorado River Basin Salinity, Control Project is to decrease salt loading to the Colorado River. ;The unit was authorized for construction by the Colorado River Basin S4linity Control Act of June 24, 1974 (Public Law 93-320). The Grand V4lley Unit area is located along the Colorado River in western Mesa Coubty in west-central Colorado. The area includes mostly the irri- gated pqrtion of the Grand Valley, consisting of approximately 70,000 acres a~dinvolving about 200 miles of canals and about 500 miles of laterals~ The: Colorado River at its headwaters in the mountains of north- central cj:olorado has a salinity concentration of about 50 milligrams per liter (mg/L). The concentration progressively increases downstream as a result o~ water diversions and salt contributions from a variety of sources. : In 1983, the total dissolved solid concentration averaged ahout 710 mg/L:at Imperial Dam, the last major diversion point of the Colorado River in;the United States. In 20 of the last 23 years, average annual total diSsolved solids exceeded 800 mg/L with a maximum of 896 mg/L in , . 1970. Uj:lless control measures are undertaken, the concentrations are expected :to increase as the Colorado River Basin States continue to de- velop the~r compact-apportioned waters.l/ . ,~ The ~alt loading to the Colorado River from the unit area is esti- mated at:approximately 580,000 tons annually, which is 7.0 percent of the avera~e annual load at Imperial Dam. The salt loading is attributed to the Mapcos Formation which underlies the Grand Valley and consists of a sequenc~ of thick beds of gray calcareous marine shale that contains a high percentage of salt. At the land surface, the Mancos Formation exhibits ~ thin, weathered, and fractured zone that acts as an aquifer which. alll>ws water from conveyance system seepage and irrigation deep percolati&n to dissolve salts from the shale and convey them to natural drainages 'and ultimately the Colorado River. ". Deve~opment of the Grand Valley Unit waS planned in stages. Stage One, encompassing about 10 percent of the unit area and completed in April 1983, consisted of the concrete lining of 6.8 miles of canal, con- solidatin~ 34.2 miles of open laterals into 29.7 miles of pipe, and in- stalling ~n automated moss and debris removal structure. The Stage One area is designated by the cross hatching on the Frontispiece Map. The Stage Two area involves most of the remainder of the valley outside the Stage One area. ",,,, . ,.,-- 1/ TIle waters of the' Colorado River are divided by the Colorado River-ComP4ct of 1922, agreed on by the seven Colorado River Basin States (Arizona, California, Colorado, Nevada, New Mexico, Utah, and Wyoming). S-l i -j:.[ -