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CHAPTER I PURPOSE AND NEED • It is unlikely that all of the authorized salinity control measures will be fully implemented. To meet the adopted salinity criteria, addi- tional salinity control, water augmentation, or other management steps will be necessary. Thus, beneficial use, weather modification, vegeta- tion management, watershed improvements, and possibly other measures remain to be considered and studied in detail. Salt Loading from the Grand Valley Unit Area As previously mentioned, the salt loading from the Grand Valley Unit area is estimated at 580,000 tons annually. The origin of the salt is the Mancos Formation, which underlies the Grand Valley and consists of a sequence of thick beds of marine shale containing salt. At the land surface, the Mancos Formation exhibits a thin, weathered, and frac- tured zone characterized by open joints and bedding planes which often contain gypsum and other minerals. This zone acts as an aquifer which allows water from conveyance system seepage and irrigation deep percola- tion to dissolve salts from the shale and convey them to natural drain- ages and ultimately the Colorado River. Table 4 summarizes the salt pickup and water inflow and outflow in the unit area. Water quality itself is discussed in more detail in Chapters II and IV. Table 4 • Water and salt budg et l./ Water Salt volume load (acre-feet) (tons) Inflow Colorado River near Cameo 2,636,000 1,423,000 Plateau Creek .near Cameo 118,000 52,000 Gunnison River near Grand Junction 1,628,000 1,260,000 Ungauged inflows 3,000 16,000 Municipal imports 8,000 1,000 Precipitation 82,000 _ 0 Total 4,475,000 2,752,000 Consumptive use Irrigation 146,000 Other (phreatophytes, municipal and industrial, etc.) 101,000 Outflow Colorado River near Colorado-Utah State Line 4,228,000 3, 334,000 Salt pickup (rounded) 580,000 1/' Source: Reclamation 1983. Grand Valley Salt Pickup Calcula- tions, Draft Report. I 11