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(!:> en o o ~ In summary, this EA has been prepared to develop and analyze the (~ incremental impacts of modifying the design, lining methods and associated land requirements and policies from those set forth in the FEIS as they affect Reaches lA, lB and 2 of the Government Highline Canal. The FEIS analyzes the entire Unit and is available for inspection at Reclamation's office in Grand Junction. Puroose and Need The primary purpose of the salinity control project is to decrease salt loading to the Colorado River. The Grand Valley Unit was authorized for construction by the Colorado River Basin Salinity Control Act of June 24, 1974 (Public Law 93-320). The Colorado River, at its headwaters in the mountains of north-central Colorado, has a salinity concentration of about 50 milligrams per liter (mg/L). The concentration increases downstream as a result of water use and salt contributions from a variety of sources. In 1989, the salinity concentration averaged about 680 mg/L at Imperial Dam, the last major diversion point from the Colorado River in the U.S. In 20 of the last 23 years, average annual salinity concentrations exceeded 800 mg/L with a maximum of 896 mg/L in 1970. The salt loading to the Colorado River from the Unit area is estimated at approximately 580,000 tons annually, which is about 7 percent of the average annual load at Imperial Dam. The salt loading is primarily due to the Mancos Formation, a geological formation underlying the Grand Valley, that consists of marine shale with a high percentage of salt. Water from irrigation system seepage and irrigation deep percolation dissolves salts from the shale and conveys the salts to the Colorado River. The Unit is designed to reduce this seepage and deep percolation and thus the salt loading. As described in the FEIS, the lining of canals and piping of laterals in the Unit is expected to reduce the salt loading by 161,400 tons per year and reduce the salinity concentration at Imperial Dam by 14.7 mg/L. The inclusion of the Stubb and Price Ditches in the Unit in 1990 increased the overall salinity reduction to 172,000 tons per year (Reclamation, 1990). Because of the land acquisition needs and impacts associated with implementing the membrane lining plan for Reach 1, preparation of the construction contract has been delayed pending results of this EA. Analyses and decisions needed include a determination of the cost effectiveness of alternative design and lining methods for each reach, and consideration of impacts related to any purchase of fee land and/or temporary ROW. This includes the proposal to purchase, as fee land, all ROW currently reserved under the 1890 Canal Act. 7