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0229 relatively little streamflow for dilution (Dash and Ortiz, 1996). During the other three flow regimes, dissolved-manganese concentrations were much smaller, and they were not substantially different from one another (fig. 13). Dissolved-manganese concentra- tions showed consistent increases for all streamflow regimes between Leadville and Malta, probably due to mine drainage from California Gulch, before decreasing downstream from Malta (fig. 13) largely because of increased partitioning of manganese to the particulate phase (fig. 14). Upstream from Buena Vista, total-recoverable manganese concentrations typically were largest during early-snowmelt runoff; therefore, the largest median total-recoverable manga- nese concentrations in the upper basin coincided with the timing of the largest median dissolved-manganese concentrations (fig. 13). Downstream from Granite, total-recoverable manganese concentrations were largest during snowmelt runoff. In the reach between Leadville and Granite, most of the total-recoverable manganese occurred in the dissolved phase (fig. 14). Concentrations of total-recoverable manganese decreased between Malta and Granite, probably because of dilution by Lake Creek and deposition of particulate manganese. Downstream from Granite, a larger percentage of manganese existed in the particu- late phase than in the dissolved phase (fig. 14). There- fore, downstream from Granite, total-recoverable manganese concentrations might have been affected 100 ZW o~ -'" !;(~ 80 a:w 1-> 158 Uw Za: O~ 80 u" wo- "'0 WI- Z"- "0 "'w 40 z'" "" "0- oZ ww >u ~cr: 20 Ow gjo. -z 0- by deposition during much of the year and by resus- pension during the elevated streamflow of snowmelt runoff(May-June). Manganese loading from tribu- taries, with the exception of California Gulch, Lake Fork above Halfmoon Creek, and Lake Creek, gener- ally was insubstantial (<10 percent) compared to main-stem loads (table 15). During the mine-drainage post-treatment period, dissolved and total-recoverable manganese concentra- tions decreased significantly at the LMDT, California Gulch, and Leadville (table 16). Dissolved-manganese concentrations also decreased significantly at Malta, Granite, Nathrop, and Wellsville during the post- treatment period (table 16). Water-quality standards for manganese were exceeded in a relatively small number of samples collected at four sites in the upper basin (table 17). The chronic standard for dissolved manganese (50 J.1glL) was exceeded in a total of l3 samples that were collected at Granite, Buena Vista, and Nathrop (table 17). Eighty-five percent of the exceedances occurred during the mine-drainage pre-treatment period. About 62 percent of the chronic exceedances occurred during snowmelt runoff, and about 31 percent occurred during early-snowmelt runoff. Only one sample, which was collected at Parkdale following an August thunderstorm, exceeded the chronic standard for total-recoverable manganese (1,000 J.1g!L) (table 17). o .~0 ",'" ,,0 ~...~ ~ .,0 cJ'lt'~ ...:i.,<}'b- ~~ '1>,,0 <-,<11 ,,~ ,f> ~o~ r,,4*-0 ~~ ~~~o Figure 14. Median percentages of dissolved manganese in percentage of total-recoverable manganese concentrations in the upper Arkansas River, April 199O-March 1993. WATER QUALITY 31