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021'1 Water-Quality Assessment of the Arkansas River Basin, Southeastern Colorado, 1990--93 By Roderick F. Ortiz, Michael E. Lewis, and Mary Jo Radell Abstract This report describes the spatial and temporal variations in water-quality conditions in the Arkansas River Basin between April 1990 and March 1993. Discussions focus on the site-to-site variability of water quality in the main stem of the river in the upper basin (upstream from Pueblo) and in the lower basin (from Pueblo to the Colorado-Kansas State line). Water-quality data are presented by streamflow regime. In general, the data were separated into three stream- flow regimes that are defined as: low flow (October-April), snowmelt runoff (May-June), and post-snowmelt runoff (July-September). For trace elements in the upper basin, data are further separated into an early-snowmelt runoff period in April; therefore, the low-flow regime for trace elements in the upper basin is defined as October through March. The primary cause of elevated trace- element concentrations in the upper Arkansas River is metal-laden drainage from abandoned mines and mine tailings. The spatial and temporal variability of trace-element concentrations at main-stem sites in the upper Arkansas River probably was influenced by (I) distance from the Leadville area, (2) dilution by tributaries, (3) degree of partitioning between the dissolved and particulate phases, and (4) deposition and resuspension of fluvial sediment. Overall, dissolved trace-element concentrations in the upper basin generally decreased from Leadville to Portland. Temporally, dissolved-cadmium, iron, manganese, and zinc concentrations generally were largest during early-snowmelt runoff; total- recoverable cadmium, copper, iron, lead, manga- nese, and zinc concentrations generally were largest during snowmelt runoff when resuspen- sion of fluvial sediment occurred. Trace-element concentrations decreased significantly at several downstream main-stem sites following the completion of water-treatment facilities at the Leadville Mine Drainage Tunnel and Yak Tunnel. Although stream-water-quality standards for trace elements were exceeded in several samples collected at a number of locations in the upper basin, the vast majority of exceedances occurred during the pre-treatment period. An analysis of trace-element loads indicated that most tributaries in the upper Arkansas River Basin, particularly downstream from Granite, generally did not contribute substantial loads to the river. Trace-element concentrations were substantially lower in the lower basin than in the upper basin. Temporally, concentrations in the Arkansas River downstream from Pueblo Reser- voir generally were highest during snowmelt runoff and post-snowmelt runoff. During the low- flow regime, substantial increases in dissolved- manganese and dissolved-iron concentrations were measured immediately downstream from John Martin Reservoir. Water in John Martin Reservoir may become anoxic during the winter months, resulting in the dissolution of iron and manganese from bottom sediments. Chronic stream-water-quality standards for most trace Abstract