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Table 23. Number of samples that exceeded chronic manganese water-quality standards in the lower Arkansas River, April 199O-March 1993 . Site name (see table 1) Number of samples analyzed Below Pueblo Reservoir Pueblo Highway 227 Avondale Carlin Dam Las Animas Below John Martin Reservoir ICo]orodo Department of Health, 1994. 250 micrograms per liter. 31,000 micrograms per liter Nickel Median dissolved-nickel concentrations at the four main-stem sites in the Pueblo area (fig. 22) were less than 3 ~glL, which is within the typical range of dissolved-nickel concentrations in unpolluted surface water (Moore and Ramamoorthy, 1984). Median total- recoverable nickel concentrations varied minimally and ranged from 2 to 7 ~gIL at the four main-stem sites in the Pueblo area (fig. 22). Six water samples were collected at the Catlin Dam, Las Animas, and Below John Martin Reservoir sites and were analyzed for nickel. Concentrations of dissolved nickel ranged from I to 7 ~glL at these sites (Dash and Ortiz, 1996), thus indicating no apparent downstream change in concentration. Total-recoverable nickel concentrations were elevated at Catlin Dam and Las Animas, where median concentrations were 23 ~glL and 25 ~g/L, respectively (Dash and Ortiz, 1996). Resuspension and tributary inflow are possible causes of these increases. The median total-recoverable nickel concentration between Las Animas and Below John Martin Reservoir decreased substantially to 4 ~glL, presumably due to deposition in the reservoir. Nickel concentrations in the lower basin did not exceed stream-water-quality standards at any site. Selenium Selenium was analyzed in 16 to 24 samples collected at most main-stem sites in the lower Arkansas River Basin. Only three samples were collected and analyzed for selenium at the Below 46 26 26 24 24 2S 24 24 Number of samples that exceeded standard I Chronic Chronic standard for standard for dIssolved total-recoverable manganese2 manganese3 o 0 o 0 o 0 o 0 o 2 S 0 16 6 Pueblo Reservoir and Highway 227 sites; therefore, these two sites are not included in the selenium- concentration plots (fig. 23). The downstream temporal patterns in dissolved and total-recoverable selenium concentrations were similar (fig. 23) because more than 90 percent of the total-recoverable selenium was in the dissolved phase. Median concentrations of dissolved selenium typically were largest during the low-flow regime. The maximum median dissolved- selenium concentration (13.5 ~gIL) occurred at Las Animas during low flow (fig. 23). In general, dissolved- and total-recoverable selenium concentra- tions increased downstream to Las Animas; concentra- tions decreased substantially between Las Animas and Below John Martin Reservoir (fig. 23). The temporal and spatial patterns in selenium concentrations are a function of local geology, exten- sive agricultural irrigation, and chemical processes. Cretaceous marine shales and limestones are exposed extensively within the study area (Mueller and others, 1991); concentrations of selenium can be elevated in these shales and limestones compared to concentra- tions in other rock types (Turekian and Wedepohl, 1961; Hatten, 1969; Schultz and others, 1980). There are about 300,000 acres of irrigated agriculture in the lower Arkansas River Basin, where irrigation water is derived from the Arkansas River and from pumped ground water. Irrigation drain water in tbe lower Arkansas River Basin flows to the Arkansas River via tributary streams, drainage ditches, and the alluvial aquifer. Cain (1985) reported that downstream from La Junta, much of the streamflow in the Arkansas Water-Quality Assessment of the Arkansas River Basin, Southeastern Colorado, 1990--93