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21 LOWER GUNNISON BASIN SALINITY CONTROL UNIT, COL 1_ November, 1993 t\) 0') l\J c:o The Lower Gunnison Basin Unit Delta and Montrose Counties. Uncompahge and Gunnison River is located in western Colorado in The land to be treated is in the valleys. Backqround: The Lower Gunnison Basin in Colorado illustrates the geo-hydro- chemical process, where a salinity control unit is underlain by a massive Mancos Shale Formation with an unlimited salt supply. Soils are derived from the Mancos shale Formation and generally have a high clay and silt content. Irrigation water percolating below the root zone picks up salt from an infinite source. The Gunnison and Uncompahgre Rivers provide an adequate water supply for irrigating 171,000 acres of irrigated land through 685 miles of canals and laterals, and 4,092 miles of onfarm ditches. Crops are surface irrigated with furrow systems. The major crop is alfalfa. Some small grain, corn, pasture, and orchard crops are grown. The present USDA plan is to improve irrigation systems and management on about 80 percent of the area or 135,000 acres and line or pipe 104 miles of off-farm laterals. The Gunnison River contributes about 1.4 million tons of salt annually to the Colorado River. About 600,000 tons corne from natural resouces and about 800,000 tons are attributed to irrigated agriculture. About 440,000 tons is picked up from onfarm deep percolating water, 140,000 tons from onfarm ditch seepage, and 220,000 tons is picked up by seepage from off-farm canals and laterals. Salinity control practices in this unit focus on improving surface irrigation systems by land leveling, lining and placing earthen ditches in pipeline, and installing surge systems on gated pipe. Micro-jet and other specially designed systems are being installed on orchards, vineyards, and vegetable crops. Salt Load Factor: Irrigation water delivered to the farms averages 260 mg/L or 0.35 tons per' acre foot. Drainage water returning to the river averages 2,650 mg/L or 3.6 tons per acre foot. (p.31, USDA 1981 report). The salinity concentration of base return flow varies from less than 1 ton/acre/foot to over 6 tons/acre foot. The weighted average salt load pickup value used by SCS for the Lower Gunnison is 3.2 tons/acre foot. ':60 mg/L inflow 2650 mq/L outflow 2390 mg/L Salt Pickup .35 tns/_c ft 3.60 tns/ac ft 3.20 tns/ac ft USBR 1982 feasibility report used an average salt loading factor of 3.23 tons/acre foot for their study of the Lower Gunnison;