WSP10284 - Laserfiche WebLink

Home Browse Search

0223 During the study period, the incidence of dissolved-cadmium concentrations that exceeded stream-water-quality standards was relatively small (table 5). Only one sample exceeded the acute stan- dard (Leadville), whereas two samples at Leadville, three samples at Granite, and two samples at Buena Vista exceeded the chronic standard (table 5). All exceedances occurred during the pre-treatment period (April 199o-March 1992). There was no obvious temporal pattern associated with these exceedances of water-quality standards. Table 5. Number of samples that exceeded acute and chronic cadmium water-quality standards in the upper Arkansas River, April 199O-March 1993 Number Number of samples that Site name exceeded standard1 (see table 1) of samples Acute Chronic analyzed standard} standard] Leadville 27 1 2' Malta 26 0 0' Granite 27 0 3 Buena Vista 27 0 2 Nathrop 27 0 0 Wellsville 27 0 0 Parkdale 27 0 0 Portiand 26 0 0 I Acute and chronic cadmium standards are based on dissolved- cadmium concentration (Colorado Department of Health, 1994). 2 Acute standard, in micrograms per liter = e{l 128[ln(hardness)]-2.90S). 3Unless nmed: Chronic standard. in micrograms per liler = e(O.78S2[In(harciness>i.3 490) . 4Chronic standard = 1.4 micrograms per liter. 5 Chronic standard = 2.3 micrograms per liter. Copper Median dissolved-copper concentrations increased between Leadville and Malta during the early-snowmelt, snowmelt-, and post-snowmelt- runoff regimes because of substantial copper loading from California Gulch and nonpoint sources (fig. 7). The median dissolved-copper concentration in Cali- fornia Gulch was 8 flglL (Dash and Ortiz, 1996); in unpolluted freshwater, concentrations typically range from 0.5 to I flg/L (Moore and Ramamoorthy, 1984). During snowmelt runoff, median dissolved-copper concentrations increased /Tom 2 flg/L at Leadville to 4 flg/L at Buena Vista. Downstream from Buena Vista, they decreased substantially (fig. 7) probably because of increased partitioning of copper to the particulate phase. Median concentrations were equal to or less than 2 flglL during the other three regimes. The percentage of copper in tbe dissolved phase ranged from about 50 to 75 percent between Leadville and Parkdale but decreased to almost 30 percent at Portland (fig. 8). Dissolved-copper concentrations were not diluted hetween Malta and Granite by the substantial tributary inflow from Lake Creek because of somewhat elevated dissolved-copper concentrations (2 to 3 flglL) in Lake Creek (Dash and Ortiz, 1996). Median dissolved-copper concentrations were largest during snowmelt runoff (fig. 7), whicb is somewhat unusual because dissolved trace-element concentra- tions typically are inversely proportional to stream- flow and are diluted by increasing streamflow. It is likely that colloidal copper, which will pass through a 0.45-flm filter, increased during snowmelt runoff (Kimball and others, 1995). Median total-recoverable copper concentrations were highest during the snowmelt-runoff regime (fig. 7). During the other three streamflow regimes, there was little difference among the median total- recoverable copper concentrations, which ranged from I to 5 flglL (fig. 7). During snowmelt runoff, the median total-recoverable copper concentration increased from a minimum of 4 I!glL at Leadville to a maximum of 12.5 flg/L at Parkdale (fig. 7). Although most of the measured copper load in most of the main- stem reaches was accounted for by measured tributary and main-stem loads, this was not the case in the reach from Buena Vista to Nathrop (table 6). During July- August and October, the percentage of the measured copper load at Nathrop that was accounted for by main-stem loading at Buena Vista or by measured tributary loading in the reach was 57 and 40 percent, respectively. The unaccounted copper load may be from the resuspension of sediment or nonpoint sources within the reach. Downstream /Tom Buena Vista, copper loading from the tributaries was relatively small (<10 percent) compared to main-stem load contributions (table 6). Total-recoverable copper concentrations decreased significantly during the mine-drainage post-treatment period at California Gulch. Post- treatment dissolved-copper concentrations decreased at Malta, which is immediately downstream from the mining-affected Leadville area, and total-recoverable copper concentrations decreased significantly at Malta, Buena Vista, and Nathrop (table 7). WATER QUALITY 19