The median concentrations of total arsenic were not 

 significantly higher than the dissolved phase at most sites 

 (Figure 3-17) , which indicates that much of the arsenic was 

 dissolved in the waters during most flows. The highest 

 median concentration of total arsenic among the six stations 

 was 17 ug/1 at Deer Lodge, which represents a 5 ug/1 

 difference between the median dissolved and total phases. 



In contrast, a greater proportion of copper was present 

 in the suspended fraction (Figure 3-18) , which illustrates 

 the greater affinity of copper to the sediments. The highest 

 median concentration of copper also occurred at Deer Lodge, 

 with a total recoverable value of 59 ug/1. 



Similarly, zinc also is transported primarily in 

 suspension (Figure 3-19) . As with arsenic and copper, the 

 median concentration of zinc was highest at Deer Lodge, with 

 a total recoverable value of 80 ug/1. 



Samples collected during the February 1986 snowmelt 

 represented the maximum concentrations measured by the USGS 

 from 1985 to 1987. Total or total recoverable concentrations 

 of arsenic, copper, and zinc during this event were substan- 

 tially higher than median values. Arsenic concentrations 

 during the February snowmelt were highest at Deer Lodge, with 

 a total arsenic concentration of 130 ug/1, compared with a 

 median of 17 ug/1. The maximum concentration of total 

 recoverable copper was 630 ug/1 at Deer Lodge, compared with 

 a median of 59 ug/1, which represents more than a tenfold 

 increase during runoff. More than 95 percent of the copper 

 at Deer Lodge was transported in the suspended phase. 



Maximum zinc concentrations were also measured in the 

 mainstem, but the highest total recoverable value of 1,100 

 ug/1 occurred at Turah Bridge. ' The total recoverable zinc 

 concentration at Deer Lodge was 770 ug/1. Arsenic, copper, 

 and zinc concentrations in the tributaries during this period 

 were only slightly to moderately higher than median con- 

 centrations. 



A general observation from the median and maximum 

 measured concentrations is that the sampling station farthest 

 upstream, Clark Fork at Deer Lodge, typically has the highest 

 concentrations, presumably due to its proximity to the major 

 headwater tailings sources. Flint Creek also has relatively 

 high trace element concentrations, probably as a result of 

 historical and current small-to-moderate-scale mining in its 

 basin. Lower trace element concentrations are typical of 

 the Little Blackfoot River and Rock Creek. These tributaries 

 aid in diluting the concentrations of trace elements in the 

 Clark Fork mainstem, which has generally lower concentra- 

 tions downstream at Turah Bridge compared with Deer Lodge. 



3-71 



