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Table 7. Results of Wilcoxon rank-sum test (f-tailed) of pre-treatment (April 199O-March 1992) and post-treatment (April 1992-March 1993) instantaneous streamflow and copper concentrations at selected sites in the upper Arkansas River Basin [fi3/s. cubic feel per second; j,tg/L. micrograms per liter; <, less than;:5, less than or equal to; n, lest not perfonned; NA, not applicable] Streamflow Dissolved copper Total-recoverable copper Site name Pre-/post- Pre-/post- Pre-/post- (see table 1) treatment p-value treatment p-value treatment p-value median 1 median 1 median 1 (ft3IS) ("glL) ("g/L) Leadville Mine Drainage Tunnel 2.3/2.4 0.38 __l NA __l. NA California Gulch \.8/1.8 .50 1017.5 .11 130129. .03 Leadville 32/58 .32 III .49 2/2 .25 Mall. 95/134 .26 . ill> <.01 " ..613' <.01 Granite 228/392 .18 2/2 .26 5/3 .11 Buena Vista 253/456 .22 2/2 .47 . ....... 4/2." .02 Nalhrop 4331701 .2\ 2/1.5 .23 4i2. .02 Wellsville 460/690 .12 2/2 .32 4/2 .07 Parkdale 476/788 .\0 1/1.5 .11 4/1.5 .10 Portland 470/694 .08 III .48 4/3.5 .28 1 Significant differences (p:SO.05) are shaded. 2TesI could not be perfonned because more lhan 50 percent of the dala were less than the reponing limit. The incidence of dissolved-copper concentra- tions that exceeded stream-water-quality standards was small (tahle 8). No samples exceeded the acute copper standard, whereas only one sample collected at Nathrop in May exceeded the chronic copper standard. It is possible that more exceedances of the copper standards occulTed during the snowmelt- runoff regime, but this determination cannot be made because most of these samples were not included in the analyses due to quality-assurance concerns associated with the use of the D74 sampler. Table 8. Number of samples that exceeded acute and chronic copper water-quality standards in the upper Arkansas River, April 199o-March 1993 Site name Number of samples Number of samples that exceeded standard I (see table 1) analyzed Acute Chronic standard" standard3 Leadville 24 0 0 Malta 25 0 0 Granite 22 0 0 Buena Vista 22 0 0 Nathrop 21 0 I Wellsville 22 0 0 Parkdale 21 0 0 Ponland 21 0 0 1 Acute and chronic copper standards are based on disso]ved-<:opper concentration (Colorado Depanment of Health, 1994). 2Acute standard. in microgramo:; per liter = efO 9422[ln(hardnml]-I.4634). 3Chronic standard. in micrograms per liter = e(O 8~45[ln(hardnc:s5)].1.46S). 22 Water-Quality Assessment of the Arkansas River Basin, Southeastern Colorado. 199~3