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fish and wildlife or (2) may limit the suitability of water for beneficial uses. The duration of reconnaissance investigations is about 2 years. Detailed studies are ini- tiated if reconnaissance investigations indicate that potentially serious water-quality problems are related to irrigation drainage. The purpose of the detailed stud- ies is to gather sufficient information to provide the sci- entific understanding needed for development of reasonable alternatives to mitigate or resolve identified problems. The purpose of planning is to develop coor- dinated action with appropriate Federal, State, and local agencies for cleanup. The final phase involves implementation of corrective actions developed during planning. A reconnaissance investigation of the Uncompahgre Project (irrigated area outlined in fig. I and pI. I), which is a BOR irrigation project, and of Sweitzer Lake documented elevated concentrations of selenium in some water, bottom-sediment, and biota samples collected in 1987-89 (Butler and others, 1991). Because of the limited scope of reconnaissance inves- tigations, source areas and distribution of selenium in irrigated areas of the Uncompahgre Project were not determined nor were effects on biota documented. Also, trace-element data were not collected in other irrigated areas in the lower Gunnison River Basin that are adjacent to or upstream from the Uncompahgre Project. Based on results of the reconnaissance inves- tigation, a detailed study of irrigation drainage in the Uncompahgre Project was initiated in 1991. The detailed study of the Uncompahgre Project (fig. 1) was expanded to include other irrigated areas in the lower Gunnison River Basin either adjacent to or upstream from the Uncompahgre Project. Therefore, the "Uncompahgre Project area" includes the irrigated area served by the Uncompahgre Project and selected drain- age basins adjacent to or upstream from the Uncompahgre Project. The BOR identified irrigation drainage from the Grand Valley (fig. I) as a large source of salt to the Colorado River (U.S. Bureau of Reclamation, 1978). Irrigated land in the Grand Valley (fig. 1) is served by Federal and private irrigation systems. The geologic unit that is the source ofthe salt in the Grand Valley is a marine shale (the Mancos Shale), and selenium prob- lems associated with irrigation on marine shales have been documented in other irrigated areas in the Western States (See and others, 1992; Stephens and others, 1992). Also, historical selenium data for the Colorado and Gunnison Rivers indicate that the Grand Valley may be contributing substantial quantities of selenium to the Colorado River. There are similarities of cli- mate, physiography, and geology between the Grand Valley and the Uncompahgre Project area; therefore, the Grand Valley was included in the detailed study. Purpose and Scope This report presents physical, chemical, and bio- logical data collected for the detailed study of the Uncompahgre Project area (pI. I) and of the Grand Val- ley (pI. 2) during 1991-92. Additional water-quality data collected by the USGS at six streamflow-gaging stations on the Gunnison, Uncompahgre, and Colorado Rivers during 1991 and 1992 also are included in this report. Tables are presented that include data for onsite measurements done at surface-water and ground.water sites and concentrations of major constituents and trace elements in surface-water, ground-water, bottom-sedi- ment, bedrock, aquifer.sediment, and various biologi- cal samples. Based on results of the reconnaissance investigation in 1987-89 (Butler and others, 1991), selenium was the primary trace element of concern in irrigation drainage in the study area. Data also are pre. sented for stable isotopes, pesticides, clay and bulk mineralogy of material from the Mancos Shale, sele- nium speciation, and polycyclic aromatic hydrocar- bons. Data collection primarily was in the irrigated areas served by the Uncompahgre Project and in irri. gated areas in the Grand Valley. Water and biota sam- ples also were collected in irrigated areas in the lower Gunnison River Basin that are adjacent to or upstream from the Uncompahgre Project. In addition, historical water. level and dissolved-solids data for two wells and selected aquifer-test data collected by the BOR in the Grand Valley are presented. Soil and alfalfa samples were collected in June 1991 in irrigated areas in the Uncompahgre Project by the USGS. Soil samples were analyzed for total sele- nium, for selected trace elements, and for selected water-extractable constituents, including water- extractable selenium. Alfalfa samples were analyzed for total selenium. The chemical results for the water- extractable constituents in soils are listed in Stewart and others (1993), and the chemical results for total trace elements in soils and total selenium in alfalfa are listed in J.G. Crock and others, (U.S. Geological Sur- vey, written commun., 1994). Acknowledgments The authors thank numerous property owners in the Uncompahgre Project area and in the Grand Valley for allowing access to their property for collec- tion of samples. The authors thank Larry M. Fukui of 2 Physical, Chemical, and Biological Data lor Detailed Study 01 Irrigation Drainage In the Uncompahgre Project Area and In the Grand Valley, West-Central Colorado, t991-92