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geology and mining activity. In the lower basin, the trace elements selected for analysis are affected by O?fll>\1 and agricultural land use and local geology. '"i'Wenty-four to 28 water samples were collected and analyzed for trace-element concentrations at most of the main-stem Arkansas River sites. Tributaries were sampled less frequently. Trace-element concentra- tions in and contributions from tributaries are discussed in less detail unless a tributary represented a substantial trace-element source to the main stem of the river. This discussion of trace elements focuses on downstream temporal variations in concentrations in the main stem of the Arkansas River. For the purposes of graphical presentation and statistical analysis in this report, trace-element concentrations reported as less than the reporting limit were assumed to equal 0.7 times the reporting limit (Kimball and others, 1995; Clark and Lewis, 1997). Water samples collected in the Arkansas Ri ver Basin were analyzed for dissolved and total-recover- able trace-element concentrations. Samples analyzed for dissolved trace-element concentrations were filtered through 0.45-~m membrane filters. Kimball and others (1995) reported that colloids, which are solids with effective diameters that range from I ~m to I nm, affect the occurrence and transport of trace elements in the upper Arkansas River. Because colloids may pass through a 0.45-~m filter, trace- element concentrations reported as dissolved might include a substantial percentage of colloidal-size particles. A review of dissolved-trace element concentra- tions in field-equipment quality-assurance blanks indicated that water samples collected with a D74 nonsolenoid, brass sampler were unacceptably contaminated with dissolved copper. Therefore, all dissolved- and total-recoverable copper concentration data that were collected with the D74 sampler were not included in the analyses presented in this report. All other trace-element analytes appeared to be rela- tively insensitive to sampler contamination or had only minor contamination; therefore, they were judged to be acceptable and are included in the interpretations presented in this report. Trace-element concentrations were compared to the stream-water-quality standards that have been established for the Arkansas River by the Colorado Water Quality Control Commission (Colorado Depart- ment of Health, 1994). The standards were established for individual reaches of the Arkansas River and reflect the stream classification for the individual reaches. Stream classifications include aquatic life, domestic water supply, and recreational and agricul- tural use. Aquatic-life standards are further divided into cold-water fisheries (upper basin) and warm- water fisheries (lower basin). Aquatic-life standards generally are calculated values that require a bardness value in the algorithm. For the purpose of this report, instantaneous hardness values were used when calcu- lating acute and chronic aquatic-life standards. Upper Basin In the upper basin, historical mining of ore deposits has yielded large quantities of many valuable. metals, including copper, gold, iron, lead, silver, and zinc (Tweto, 1968). Water flowing through the aban- doned mines and mine tailings in the upper basin bas contributed substantial quantities of cadmium, copper, iron, lead, manganese, and zinc to the Arkansas River (Moran and Wentz, 1974; Wentz, 1974). In this study, these six trace elements were selected as the primary trace elements of potential concern in the upper Arkansas River Basin. Samples analyzed for concen- trations of these primary trace elements were collected from the following main-stem sites: Leadville, Malta, Granite, Buena Vista, Nathrop, Wellsville, Parkdale, and Portland (table 1). Additionally, water samples collected five times during the study at four of the eight sites were analyzed for concentrations of arsenic, chromium, mercury, nickel, selenium, and silver for the purpose of confirming that their concentrations were relatively insignificant from a water-quality perspective. Trace elements were analyzed in water samples collected from all major tributaries in the upper basin. In this study, the major tributaries in the upper basin were defined as those tributaries that contribute at least 5 to 10 percent of the annual flow in the upper Arkansas River or those that are known or suspected to be substantial sources of some trace elements, primarily owing to localized land use in the tributary drainages. Where reported, trace-element loads were computed by multiplying the trace-element concentra- tion of the respective sample and the instantaneous streamflow during sampling. Trace-element loads were not computed for samples with constituent concentrations that were less than the reporting limit. No load estimates are presented for cadmium because of the large number of censored total-recoverable WATER QUALITY 15