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(.L) C..,'1 00 CO" and plan of augmentation. The State has also approved pumping and well testing as stipulated in the existing well permits. Grand Valley Unit (Reclamation and USDA). The Grand Valley Unit in western Mesa County in west-central Colorado includes about 71,000 acres and involves about 200 miles of canals and 500 miles of laterals. Grand Valley contributes an estimated 580,000 tons of salt annually to the Colorado River. Most of the salts are leached from the soil and underlying Mancos Formation by ground water that receives its recharge from canal, lateral, and onfarm seepage. The Mancos Formation is a thick sequence of gray marine shale varying locally from 4,000 to 5,000 feet thick. Salts present in the shale are mostly calcium $plfate with smaller amounts of sodium chloride, sodium sulfate, and magnesium sulfate. Calcium sulfate (gypsum) is commonly found in crystal form in open joints and fra~tures i~ the upper portion of the shale. Below the soil derived from the shale, the weathered zone of Mancos shale transmits water along open joints, fra~tures, and bedding planes. Percolating water from irrigation and conveyance system seepage dissolves salts from the weathered shale zone. The excess percolating water and seepage contribute to saline ground water flows that return to the river. Development of the Grand Valle:-- Unit, by Reclamation, was planned in stages! Stage One, encompassing about 10 percent of the unit area, consisted of concrete lining 6.8 miles of canal, consolidating 34 miles of open laterals into 29 miles of pipe laterals, and installing an automated moss and debris removal structure. This work was completed in April 1983. To test the effects of Stage One improvements on ground water flows and quality, a hydrologically isolated sub-basin, the Reed Wash study area, was instrumented to monitor surface and ground water inflow and outflow. Salt loading reduction in Stage One from the canal and lateral improvements was determined to be 21,900 tons. Detailed information on surface and ground water inflow and outflow to other selected sub-basins ,within the unit were collected and used to develop water and salt budgets. In addition, an intensive drilling and aquifer testing program was conducted in both the areas underlain by cobble deposits and in the weathered Mancos shale areas. The purpose of this program was to determine aquifer characteristics, such as hydraulic conductivity, as well as to identify quality and direction of ground water flow. The plan for Stage Two, essentially the remainder of Grand Valley, provides for replacing existing open earth laterals with buried pipe and lining three reaches of the Government Highline Canal with membrane liners. The supplement to the Definite Plan Report and Final Environmental Impact Statement was filed with the Environmental Pfotection Agency on May 23, 1986. -37-