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001333 water. Two wells were sampled monthly-(me in Mancos Shale residuum and one in alluvium overlying Mancos Shale--to describe the seasonal effects of irrigation on trace-element concentrations in ground water. Historical ground-water-quality and ground- water-level data were compiled by the BOR to describe the long-term effects of irrigation on ground water in the study area. Hydraulic data from historical aquifer tests performed by the BOR also were compiled by the BOR. Ground-Water Hydrology Ground water in the study area affected by irrigation occurs in shallow systems consisting of (I) Mancos Shale residuum, (2) alluvium overlying Mancos Shale, (3) the cobble aquifer in eastern Grand Valley, and (4) terrace deposits in the western Uncompahgre Project area. The hydro- geologic settings, general lithologic descriptions, and general hydrogeologic characteristics of these ground-water-f1ow systems are described in table 4. Although the shallow systems might not have contained appreciable quantities of water in their natural (preirrigation) state, irrigation now provides substantial quantities of water for the existence of a water-table system in the shallow unconsolidated materials. The shallow ground- water systems generally discharge to local drains, streams, and rivers (fig. 10). Hydrogeologic characteristics differ greatly over short distances in the shallow system (table 4) because of the differences in composition of alluvium (which include paleochannels consisting of gravels and cobbles, fig. 10) and the differences in weathering and fracturing of the Mancos Shale. Table 4. Hydrogeologic senlngs of the ground.water-IIow systems affected by irrigation In the Uncompahgre Project area end in the Grand Valley (Fm, Formation; f1, feet; n. number of data points; <, less than; ftId, feet per day; hydraulic-conductivity data wert provided by the Bureau of Reclamation; DC, Dry Creek 'ypc. Brea; RW, Reed Wash type area; 51... Sweitzer Lake type area; --, llydrogeologic setting not studied as a type area] 1Vpe 0..0 Hydrogaologlc letting Alluvium on Mancos Shale RW Source rock(a) Mancos Shale, Mesaverde Fm (Grand Valley); Mancos Shale, Dakota Fm, San Juan Tuff, and gneisses of the Black Canyon of the Gunnison and Cimrnaron Ridge areas (Uncompahgre Valley) Cobble aquifer (Grand Valley) Grand Mesa basalts, Mesaverde Fm, Green River Fm, Wasatch Fm, Mancos Shale Terrace deposits (western part of the Uncompahgre Valley) Mancos Shale residuum DC Glacial outwash and aHuvium from lhe San Juan Mountains and lhe Uncompahgre Plateau SL Mancos Shale Genefalllthologlc description and hydrogeologic characteristics Clay. silt. sand, and gravel deposits of Holocene age. Grand Valley: Erosional detritus of clay, sand, and silt; mixture of Mesaverde Fro (indicated by Inoceramus and Pychnodantes fossils) and shale- derived clays (smectite and illite) thai might be derived from mudflows. Thickness ranges from 5 to 40 ft. Eastern part of Uncompahgre Valley: Erosional detritus from Precambrian gneisses. Oligocene tuffs, and the Dakota Fm of Cretaceous age. Thickness ranges from 5 to 2S ft. Hydraulic conductivities range from 8 to 140 ftId and average 74 ftld (n=3). Valley-fill deposits in an ancient valley of the Colorado River. Well-sorted pebbles, cobbles. and boulders in sandy matrix; alluvium and eolian sand might locally cover gravels. Late Holoce.ne to Pleistocene age. Thickness ranges from 10 to 65 ft. Hydraulic conductivities range from 3810 510 ftId and average t 90 ftld (n=26). Poorly sorted materials ranging from fine rock flour through sand, gravel, cobbtes, and boulders (Meeks. 1950). Glacial outwash and alluvium of Pleistocene age locally covered by recent alluvium. Thicknesses range from 5 to 60 ft. Hydraulic conductivity from one hydraulic lest was 650 ftld. Clayey, silty residuum consisting of smectite, illite. and kaolinite clays. Abundant secondary gypsum occurs in shale partings and fractures; pyrite concretions altered to limonite; ash layers weathered 10 kaolinite and quanz containing some jarosite. Thickness ranges from <lID 100 ft. Hydraulic conductivities range from 4.5 to 300 ftld and average 88 ftld (n=17). GROUNO-WATER INVESTIGATIONS 25