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<br />178n <br /> <br />Lead <br /> <br />, Dissolved-lead concentrations in the upper Arkansas River exhibited very different spatial and temporal vari- <br /> <br />ability than the other trace elements of concern (fig. 12). To a large degree, these patterns were a function of the large <br /> <br />frequency of occurrence of dissolved-lead concentrations that were equal to or less than the detection limit (0.5 1Jg/ <br /> <br />L). Seasonally, the median dissolved-lead concentrations were typically largest during snowmelt runoff; the largest <br /> <br />median concentrations occurred at Malta (l.llJgIL) and at Nathrop (11JgIL) (fig. 12). Moore and Ramamoorrhy <br /> <br />(1984) reported that the dissolved-lead concentrations in unpolluted freshwater is typically less than 3 IJgIL. During <br /> <br />pre-snowmelt and snowmelt runoff dissolved-lead concentrations increased substantially at Malta. During snow- <br /> <br />melt runoff and post-snowmelt runoff, concentrations increased substantially at Nathrop (fig. 12). Concentrations of <br /> <br />dissolved lead were not affected by changes in the partitioning oflead between the dissolved and particulate phases, <br /> <br />because the dissolved-lead fraction remained relatively constant (10 to 20 percent) throughout most of the upper <br /> <br />basin (fig. 13). Total-recoverable lead concentrations were largest during snowmelt runoff and the maximum median <br /> <br />concentration (SllJgIL) occurred at Nathrop (fig. 12). There was little variability between total-recoverable lead <br />'oncentrations during the other three seasonal flow regimes and concentrations were much smaller than during <br />snowmelt runoff (fig. 12). The elevated concentrations of dissolved and total-recoverable lead at Malta (fig. 12) are <br /> <br />attributable to mine drainage, primarily from California Gulch. The elevated lead concentrations at Nathrop cannot <br /> <br />be accounted for by inflow from the two major tributaries in the reach, Cottonwood Creek and Chalk Creek (plate <br /> <br />. 'I), because their concentrations were generally less than those in the main stem of the river. Resuspension of Iead- <br /> <br />enriched fluvial sediment and non-point source contributions are likely sources of the elevated lead concentrations <br /> <br />at Nathrop. <br /> <br />---',,,- .~~.."...". ~lf:" .;, '" ~..." .'. ... <br />.''t'" '.' ....""~ '.,,' .-,.: ~,; Ii J; ~:i " ._'J ,:,., !/,:;;. <br />~z'~"a, ~~;""').."'Id"Q~ i2a~a- ]j <br />Subiect to Ravlsion <br />i.';<) NOT QUOTE OR REtEAS;~ <br />?'5nding Approval by Directcr, <br />U 5 G I . IS <br />...-.-:....... a~ . . <br />.0. __.....'.o~,_tJ ur'fe:l <br />'.- , <br /> <br />I <br /> <br />29 <br />