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0232 secondary Trace Elements Most water samples collected in the upper basin were not analyzed for concentrations of the secondary trace elements arsenic, chromium, mercury, nickel, selenium, and silver. The secondary trace elements were assumed to occur at concentrations that would not constitute a water-quality concern. To vali- date this assumption, water samples from four main- stem Arkansas River sites (Buena Vista, Wellsville, Parkdale, and Portland) were collected five times during the study period and analyzed for concentra- tions of dissolved and total-recoverable secondary trace elements. These data are reported in Dash and Ortiz (1996). All samples (20 of 20) analyzed for dissolved arsenic and 90 percent (18 of 20) of the samples that were analyzed for total-recoverable arsenic had concentrations that were less than the reporting limit (lllg/L). One sample collected at Parkdale and one at Portland had total-recoverable arsenic concentrations of I Ilg/L. Moore and Ramamoorthy (1984) reported that arsenic concentrations in most unpolluted fresh- water are less than I Ilg/L. Ninety-five percent (19 of 20) of the samples analyzed for dissolved chromium and 70 percent (14 of20) of the samples analyzed for total-recoverable chromium had concentrations that were less than the reporting limit (lllgIL). The median concentration of the six samples that contained total- recoverable chromium concentrations larger than the reporting limit was I IlgIL. In unpolluted lakes and rivers, dissolved-chromium concentrations typically range from I to 2 IlgIL (Moore and Ramamoorthy, 1984). Ninety-five percent (19 of20) of the samples that were analyzed for dissolved and total-recoverable mercury had concentrations that were lcss than the reporting limit (0.1 Ilg/L). Dissolved and total-recov- erable nickel concentrations were less than the reporting limit (1Ilg/L) in 60 percent (12 of20) of the samples. The occurrence of nickel concentrations that was larger than the reporting limit increased down- stream; the median concentration of those samples was 3.5 Ilg/L. At Portland, 80 percent (4 of 5) of the total-recoverable nickel concentrations were larger than the reporting limit; the median dissolved-nickcl concentration of those four samples was 2.5 Ilg/L. Snodgrass (1980) reported that dissolved-nickel concentrations in unpolluted freshwater usually range from I to 3 Ilg/L. About 90 percent of the samples analyzed for total-recoverable and dissolved selenium had concentrations less than the reporting limit (1IlgIL). Concentrations of dissolved and total- rccoverable silver were less than the reporting limit (lllgIL) in all 20 samples that were collected and analyzed. The analytical reporting limit was too large to indicate if sil ver occurred at concentrations that might be toxic to aquatic life. Based on these results, arsenic, chromium, mercury, nickel, and selenium do not occur in large enough concentrations to pose a concern for the quality of water in the upper Arkansas Ri ver. Lower Basin Trace-element sources in the lower basin include inflow from the mining-affected upper basin, point and nonpoint sources associated with urban land use, irrigation return flows associated with agricultural land use, and tributary inflow. Lewis and Edelmann ( 1994) reported that a substantial percentage of many trace elements that enter Pueblo Reservoir from the upper basin was removed from the water column between tbe upper and lower ends of Pueblo Reservoir because of deposition within the reservoir. These find- ings were bascd on data collected during 1985-89. Although Lewis and Edelmann (1994) did not measure the release of trace elements from the bottom sediments, periods of prolonged anoxia in Pueblo Reservoir could cause dissolution of substantial quantities of some trace elements from the bottom sediments to the water column, to the reservoir outflow, and eventually to the lower Arkansas River. No periods of prolonged anoxia were measured in the reservoir during 1985-89, and dissolution of sedi- mented trace elements did not occur. In the reach downstream from Pueblo Reservoir to Avondale (pI. I), the trace elements of potential concern include cadmium, chromium, copper, iron, lead, manganese, nickel, selenium, silver, and zinc. These trace elements were selected based on the potential for trace-element transport from the upper basin, urbanized land use in the Pueblo area, use of the Arkansas River for domestic water supplies by the city of Pueblo and the St. Charles Mesa Water Users Asso- ciation, and tributary inflow from Fountain Creek. The diversion point for the city of Pueblo water supply is located between the main-stem sites Below Pueblo Reservoir and Pueblo (table I); the diversion point for the St. Charles Mesa Water Users Association water supply during low flow is located at the main-stem WATER QUALITY 37