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probably was due to dilution by Lake Creek and increased partitioning to the particulate phase. Dissolved-cadmium concentrations increased at Buena Vista. Dissolved-cadmium concentrations typically were highest during early-snowmelt runoff (fig. 6). During early-snowmelt runoff and snowmelt runoff, median total-recoverable cadmium concentra- tions increased substantially between Leadville and Malta (fig. 6) but were subsequently diluted by inflow from Twin Lakes (fig. I). The median total-recover- able cadmium concentration during snowmelt runoff increased downstream from Buena Vista to a maximum of 2.5 J.Ig/L at Parkdale. The downstream increase probably was caused by the resuspension of trace-element-laden sediment during high streamflow. Three cbaracteristics of main-stem and tributary cadmium concentrations support this conclusion. First, total-recoverable cadmium concentrations increased and dissolved-cadmium concentrations decreased downstream from Buena Vista, thus indicating that particulate cadmium concentrations increased. Second, concentrations of total-recoverable cadmium in the tributaries downstream from Buena Vista were smaller than those in the main stem (Dash and Ortiz, 1996), thus indicating that the increase in main-stem concentrations of total-recoverable cadmium prob- ably was not caused by tributary inflow. Third, Kimball and others (1995) stated that resuspension of colloids in the main stem of the river is a "likely source of colloidal iron load during runof!" and that cadmium, copper, lead, and zinc all occur in substan- tial concentrations in the iron colloids. These charac- teristics also support measured increases in other total- recoverable metal concentrations during the high-flow condi tions of snowmelt runoff. Cadmium concentrations decreased signifi- cantly at several sites after the mine-drainage treatment plants came on line at the LMDT and Yak Tunnel (California Gulch) (table 4). The median dissolved-cadmium concentration in the effluent from the LMDT and the Yak Tunnel (via California Gulch) decreased from 12 to 0.3 J.Ig/L and from 48 to 5.2 J.Ig/L, respectively (table 4). Post-treatment (April 1992-March 1993) dissolved-cadmium concentrations also decreased significantly (p:>0.05) at Leadville, Malta, Granite, Buena Vista, and Nathrop (table 4). Post-treatment total-recoverable cadmium concentrations decreased significantly during post- treatment at the LMDT and at California Gulch (table 4). Table 4. Results 01 Wilcoxon rank-sum test (Hailed) of pre-treatment (April 199Q--March 1992) and post-treatment (April 1992-March 1993) inslanlaneous streamflow and cadmium concentrations at selected sites in the upper Arkansas River Basin [ftJ/s. cubic feet per second; )J.gfL, micrograms per liter; <, less than; S,less than or equal to; --, tcst nor performed; NA. not applicable] Streamflow Dissolved cadmium Total-recoverable cadmium Site name Pre-/post- Pre-/post- Pre-/post- (see tsble 1) treatment p-yslue treatment treatment median 1 medl.n' p-value median' p-vslue (!t3/a) (~g/L) (~g/L) Leadville Mine Drainage Tunnel 2.3/2.4 0.38 . .: '. 1210.3,. . <0.01 18.~<1 <0.01 California Gulch 1.8/1.8 .50 .48/5.2 .' <.01 ., '62/10 <.01 Leadville 70/58 .50 . . .':.O,s/oj <.01 j NA Malta 99/134 .35 . ... <:1.210.5 <.01 -- 2 NA Granite 277/470 .45 .': (l.45/0.20 .02 -- 2 NA Buena Vista 400/462 .45 ...0.410:3. <.01 2 NA N alhrop 5631717 .45 od./<o.i .01 -- 2 NA Wellsville ...,.. 2 524/742 .38 0.1/<0.1 .24 n NA Park dale 582/843 .41 2 NA -- 2 NA Portland 512/800 .18 -- 2 NA -- 2 NA I Significant differences (p:sO.05) are shaded. 2Test not perfonncd because more than 50 percent of the data were less than the reporting level. 18 Water-Quality AalieSament 01 the Arkansss RIver Besln, Southeastern Colorado, 1991Hl3