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c...J (.;'1 i-" Uf of Reclamation - 5) estimates that 500,000 tons of salt, or nearly one half of the total subbasin salt load, are .produced annually by the G1enwood and Dotsero Springs. The effect of irrigation returnf10w on the salt load from groundwater during the base flow period (winter) is very small for most subbasins, especially those at high elevations. Soils being irrigated are the alluvial, relatively well-drained deposits along streams. The distances groundwater (irrigation returnflow) must travel before entering a stream chan",j are, in most cases, relatively short. Therefore, water quality data collected in December or January should reflect a minimal influence from irrigated agriculture. However, the larger blocks of farmland located on fine-textured, poorly drained soils, and/or at long distances from streams can have a great influence on groundwater. Examples of , these would be the Uinta and Price River Basins in Utah,the lower 'Gunnison and Grand Valley Projects in Colorado, and the Eden-Farson Project in Wyoming. . .Wa ter quality data were collected on dra i nagechanne 1 sin the Lower Gunnison and Grand Valley areas thought to be yielding only irrigation returnf10w. The annual salt loads from these channels were subtracted. from salt loads estimated to be contributed by natura1.groundwater. All of the base flow salt load contributed by the Price .River was believed. to be from irrigation returnf10w and was subtracted from the total. An average value of 50 percent of the base flow salt load was arbitrarily set for irrigation returnf10w on the San Rafael, Duchesne, and Blacks Fork Rivers, and Piceance Creek. A value of 20 percent was similarly estimated for irrigation returnf10w salt load from the Big Sandy River. These were all subtracted from computed total salt loads for the sub- basins. Table 2 shows the results of this exercise. ...... It is estimated that the contribution of groundwater from natural sources to the total salt load of the Green and COlorado Rivers above their confluence is 38 percent. This is a reduction from the 55 percent measured as groundwater-contributed salt load (41) . The remaining 17 percent of salt load waul d theoretically be from i rri gation returnf1 ow. during the;base flow period. This 38 percent from natural groundwater sources is in contrast to the 58 percent estimated by the Environmental Protection Agency to be from natural diffuse and point sources (16). The EPA also estimates that the total annual yield to the salt load from irrigation returnf10w is 41 percent and 1 percent from industrial and municipal sources. BLM (3) estimated that surface runoff fr.omthe pub1 ic lands yielded a total of 8 percent of the total salt load of the Upper Basin. Surface runoff from similar lands owned or administered by private individuals, states, Indian tribes, and other federal agencies probably yields an additional 7 percent nf the Upper Basin salt load. These estimates account for 95 percent of the total salt load. . The 5 percent unaccounted for is well within the error of these calculations. 13 f ., , ;} ., I ',~ "1 , ,>'"