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179:'1 I Total-recoverable copper concentrations decreased significantly during the mine-drainage post-trealIl1ent eriod at the LMDT and in California Gulch; dissolved-copper concentrations at the LMDT exhibited a significant decrease during post treatment (table 8). Post-tre:llment dissolved-copper concentrations decreased at Malta, imme- diately downstream from the Leadville mining district, while total-recoverable copper concentrations exhibited sig- nificant decreases at Malta, Buena Vista, and Nathrop (table 8). This pattern in post-treatment copper concentrations is opposite the pattern for cadmium, which exhibited more decreases in dissolved cadmium than in total-recoverable cadmium (table 4). The lack of significant decreases in dissolved copper is probably a statistical anomaly caused by the large frequency of occurrence of dissolved-copper concentrations that were equal to or just slightly larger than the detection limit for dissolved copper (I Ilg/L). The incidence of dissolved-copper concentrations that exceeded stream water-quality standards was extremely small (table 9). No samples exceeded the acute copper standard, while only one sample, which was co 1- lected at Nathrop in May, exceeded the chronic copper standard. It is possible that more exceedances of the copper standards occurred during the snowmelt-runoff period, but because most of these samples were not included in the 'alyses, due to quality-assurance concerns, this determination cannot be made. Table 8 near here. Results of Wilcoxon rank-sum test (I-tailed) of pre-treatment (AprilI990-March 1992) and post-treatment (April 1992-March 1993) instantaneous streamflow and copper concentrations in the Arkansas River Table 9 near here. Number of samples that exceeded the acute and chronic copper water-quality standards in the npper Arkansas River, April 1990-March 1993 '. .~.-c~;~iI":~~,."",a"'iI- . ,..,. "" . "":''''o;!',JJ!;;;l!,\"""!",, n~.,.&,,, ~ -......... ~ "v!:.o.~~ U!JiJ'-.;~' ~ Subject to Rsvision J-;) NOT QUOTE OR REl:=h~:"; ;';~!;ding Aooroval by DI'r-e-~;:~ I I ..,\,;., U.S. G~c:og1cal Survey , I 25