half the time in Silver Bow Creek downstream of the ponds, 

 and annual average values were not much higher than the 

 criteria. Thus, copper criteria exceedences tended to be 

 frequent but slight. Zinc toxicity criteria were not exceeded 

 in FY 85 or FY 87 and were only infrequently exceeded in FY 

 86. The worst water quality occurs in winter due to lower pH 

 and decreased efficiency of the treatment ponds caused by 

 channeling, ice cover, and colder water temperatures. 



The Pond 2 discharge was the largest contributor of 

 contaminant loads to the Clark Fork during the Phase I RI and 

 significantly degraded water quality with sulfate, copper, 

 zinc, iron, and lead. This may have been due in part to the 

 low-flow conditions that occurred in 1985. The Mill-Willow 

 Bypass discharge also contributed elevated concentrations of 

 sulfate, copper, zinc, iron, and cadmium (MultiTech 1987a) . 

 This has also been documented by WQB sampling, which shows 

 that metal concentrations in the bypass (when Silver Bow 

 Creek is not bypassing) are highest during snowmelt runoff 

 and after heavy rains. Presumably, the tailings deposits in 

 the bypass are the source of these metals. During FY 1985-87 

 WQB sampling, the bypass had the highest arsenic concentra-r 

 tions of the stations monitored, and the federal drinking 

 water standard was exceeded periodically. However, federal 

 drinking water standards for arsenic, cadmium, copper, iron, 

 lead, and zinc generally were not exceeded during the Phase I 

 RI, neither in discharges from the Warm Springs Ponds to the 

 upper Clark Fork, nor within the ponds. The four-day 

 (chronic) aquatic life criteria for cadmium, copper, lead, 

 and zinc, and the one-hour (acute) aquatic life criteria for 

 zinc were exceeded occasionally throughout the ponds. The 

 acute aquatic life criteria for copper were usually exceeded 

 within the pond system, but were not exceeded in discharges 

 to the upper Clark Fork. Waters of the Mill-Willow Bypas^ 

 exhibited chronic aquatic life toxicity with respect to 

 copper and zinc concentrations and acute aquatic life 

 toxicity with respect to copper- concentrations. Silver Bow 

 Creek and the Opportunity Ponds surface discharges are the 

 principal sources of contaminants for the pond system (CH2M 

 Hill 1987d) . 



Upper Clark Fork . Some general observations of the 

 geographic and hydrologic variations in trace element 

 concentrations can be made from USGS data collected in the 

 upper river (Figures 3-17, 3-18, and 3-19). Differences in 

 height between the dissolved and total or total recoverable 

 bars on the graphs represent the concentration of trace 

 elements transported in suspension. 



3-70 



