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Copper A comparison of dissolved-copper concentra- tions at the fanhest downstream site in the upper basin, Portland (fig. 7). and the first site downstream from Pueblo Reservoir, Below Pueblo Reservoir (fig. 18), indicates that dissolved-copper was trans- ported conservatively through the reservoir. Dissolved-copper concentrations in the lower Arkansas River generally were largest during the post- snowmelt runoff regime; the maximum median concentration of 4 ~gIL occurred during the post- snowmelt runoff regime at Highway 227 (fig. 18). This substantial increase cannot be accounted for by inflow from Fountain Creek, whicb typically had concentrations of 2 ~g/L (Dash and Ortiz, 1996). Nonpoint sources in the Pueblo area or resuspension of colloidal copper, which would be included in the dissolved fraction, are possible sources for the increase. Downstream from Avondale. dissolved- copper concentrations typically decreased. Total- recoverable copper concentrations were largest during the post-snowmelt runoff regime and increased down- stream to Catlin Dam; the maximum median total- recoverable copper concentration (42 ~gIL) occurred at Catlin Dam during the post-snowmelt runoff regime (fig. 18). Total-recoverable copper concentrations at Catlin Dam (fig. 18) typically were larger than concen- trations at any main-stem site in the upper basin (fig. 7). Resuspension of copper with fluvial sediment and unsampled tributary inflow are likely sources of the elevated total-recoverable copper concentrations at Catlin Dam. Two water samples that were collected during two stonn-runoff events from the Apishapa River (pI. 1), which is a tributary to the Arkansas River 2.3 miles upstream from Catlin Dam, had total- recoverable copper concentrations of 300 J.lgIL and 260 J.lg/L (Dash and Ortiz, 1996). These concentra- tions are 200 to 230 percent larger than the highest instantaneous total-recoverable copper concentration that was measured in any lower Arkansas River sample. Anotber stonn-runoff sample, which was collected from the Purgatoire River just upstream from John Manin Reservoir, had a total-recoverable copper concentration of 200 J.lgIL (Dash and Ortiz, 1996). Based on these three samples, it seems likely that the tributary inflow to the lower Arkansas River, espe- cially during stonn-runoff events, probably represents a major copper source to the lower Arkansas River. Total-recoverable copper concentrations dec.reased substantially downstream from John Manin Reservoir, possibly because of deposition in the reservoir. Stream water-quality standards for dissolved copper were not exceeded at any site in the lower Arkansas River. Iron A comparison of dissolved-iron concentrations at Portland, the upstream site closest to Pueblo Reser- voir (fig. 9), and the site immediately downstream from the reservoir, Below Pueblo Reservoir (fig. 19), indicates nonconservative transport of dissolved iron through Pueblo Reservoir. Deposition within the reser- voir is a probable cause of decreased concentrations. Concentrations of dissolved iron generally were similar at all sites downstream from Pueblo Reservoir to Las Animas (fig. 19). During low flow, median dissolved-iron concentrations increased substantially at the site just downstream from John Manin Reser- voir; the maximum median concentration of dissolved iron (15 ~g/L) occurred at this site. It is suspected that the water in John Martin Reservoir becomes anoxic during the winter; tberefore, dissolution of iron from reservoir bottom sediments and release to the water column would be a possible cause of increased dissolved-iron concentrations downstream from the reservoir. Temporally, total-recoverable iron concen- trations generally did not vary substantially at any main-stem site downstream to Avondale (fig. 19). Concentrations did, however, increase substantially between Avondale and Las Animas during the snow- melt-runoff and post-snowmelt runoff regimes (fig. 19), presumably because of res us pension and trib- utary inflow. In the two samples that were collected from the Apishapa River during stonn-runoff events, total-recoverable iron concentrations were 270,000 and 180,000 ~gIL (Dash and Ortiz, 1996); these concentrations were about 200 to 300 percent larger than the highest instantaneous total-recoverable iron concentration in any lower Arkansas River sample. A stonn-runoff sample collected from the Purgatoire River had a total-recoverable iron concentration of 250,000 ~gIL (Dasb and Ortiz, 1996). These tributary stonn-runoff samples indicate that inflow from tribu- taries, especially during stonn events, represents a substantial iron source to the lower Arkansas River. Only one sample collected in the lower Arkansas River had a dissolved-iron concentration that exceeded the chronic water-quality standard for dissolved iron (table 22). Conversely, about 37 percent 40 Water.Quallty Assessment 01 tho Arksns.s River B.sln, Southe.stern Color.do, 1991Hl3