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. r. on z.~~l" ~C.. and Qt (soils derived from terrace and alluvial-fan deposits) (table 2). Only 13 percent of the tolal variation in selenium concentration in alfalfa was attributed to differences among geologic unils. About one-third of the tolal variation resulted from concentration differences within sections, indicating high variability between samples collected in close proximity within the same geologic unit. Similar large variations in selenium concentrations in alfalfa collected in close proximity on the same parent geologic unit were reported for alfalfa in the Kendrick Reclamation Project area in Wyoming (See and others, 1992) and in native vegetation from seleniferous soils in South Dakota (Moxon and others, 1938; Olson and others, 1942). The results indicate that selenium is mobilized from Mancos Shale and redistributed downslope in the Uncompahgre Project area by irrigation. The ANOVA results and the high correlations of selenium in alfalfa with water-extractable soil selenium, especially in alluvial soils, indicate that the fonus of selenium have changed from immobile selenides incorporated in sulfide minerals and selenites sorbed to other iron minerals in the residual shale (Lakin, 1961; Elrashidi and others, 1989) to a higher proportion of the more mobile selenate in the alluvium (Geering and others. 1968; Balistrieri and Chao, 1987). This change in chemical species might occur because of increased exposure to ground water and of changing redox conditions in geographically lower elevations and could be expected to occur under the alkaline soil conditions that are present in the project area. Displacement of selenium downslope from a source area and an increase in content of dissolved species in contact with ground water was reported for the San Joaquin Valley in California (Tidball and others, 1989) and for the Kendrick Reclamation Project area in Wyoming (See and others, 1992). Because the Uncompahgre Project area generally is well drained, selenium does not accumulate to the levels that were reported in soils and alfalfa from the Kendrick Reclamation Project area (See and others, 1992), where irrigation drainage was evaporated in a closed basin. Other trace constituents of environmental interest that had higher concentrations in soils of the Uncompahgre Project area than in soils in the Western United States were arsenic, molybdenum, uranium, and zinc. Concentrations of these trace constituents were highest in soils from alluvium overlying Mancos Shale (unit Qm), and the geometric means for unit Qm are listed in table 3. Alfalfa from three of the five geologic units had mean copper to molybdenum ratios of less than 2:] . A minimum ratio of2:] in livestock feed is considered necessary to prevent symptoms of molybdenum- induced copper deficiency in callie (Miltmore and Mason, ] 971; Erdman and others, 1978). Table 3. Geometric mean concentrations of arsenic, molybdenum, uranium, and zinc in soils from alluvium overlying Mancos Shale in the Uncompahgre Project area and in soils from the Western United States [Concentrations. in micrograms per gram; data for Western United Stales soils from Shacklette and Boemgen (1984)] AI'8II Arsenic Molybdenum Uranium Zinc Uncompahgre Projecl- 9 5 5 110 Alluvium from Mancos Shale (unitQm) Western United Slates 6 .9 3 55 GROUND-WATER INVESTIGATIONS The objectives of the ground-waler investiga- tions were to describe concentrations and distribution of selenium and other trace constituents in ground water in the Uncompahgre Project area and in the Grand Valley and 10 describe the hydrochemical processes affecting selenium release and mobilization in ground water in irrigated areas on Mancos Shale terrain. The approach was collection of geologic and ground-water-quality data from type areas that were representative of the hydrogeology of the whole area. Selection of type areas was based on the availability of historical ground-water data, existing wells, and geologic information. The type areas include: ]. Sweitzer Lake area in the eastern part of the Uncompahgre Project area-representing terrane on Mancos Shale residuum (fig. 8); 2. Dry Creek Basin in the western part of the Uncompahgre Project area-representing terrane on Quaternary terrace deposits (fig. 8); and, 3. Reed Wash Basin in the Grand Valley- representing terrane on Mancos Shale residuum and alluvium overlying Mancos Shale (fig. 9). Hydrogeologic characteristics of the type areas are described in the "Ground-Water Hydrology" section. GROUND-WII.TER INVESTIGII.T,oNS 11