individual toxicities) are an important concern that has yet 

 to be quantified for the Clark Fork. Arsenic is also present 

 in the system, and while the federal drinking water standards 

 are occasionally exceeded at some locations, aquatic life 

 criteria are rarely surpassed. 



A variety of analytical techniques for heavy metals 

 analysis is used by the agencies that monitor water quality 

 in the basin. These are summarized in Table 3-17. Because 

 some techniques are more rigorous than others and yield 

 higher values, it is often difficult to make comparisons 

 among data sets. 



The current EPA metals toxicity criteria for the 

 protection of freshwater aquatic life give threshold levels 

 in terms of total recoverable concentrations. Although the 

 WQB monitors for total recoverable metals, it should be noted 

 that the EPA and USGS total recoverable analysis method 

 differs from the WQB total recoverable method in that a soft 

 digestion is performed prior to sample analysis. This 

 process releases a certain quantity of sediment-bound metals 

 that may be present in the sample. The WQB method consists 

 of field acidification of the sample followed by analysis. 

 This method is comparable to the EPA acid-soluble method, 

 which is compatible with nearly all available data concerning 

 toxicity and bioaccumulation of metals by aquatic organisms. 

 The EPA criteria are based on total recoverable concentra- 

 tions instead of acid-soluble or other forms, because 

 sediment-bound metals in a wastewater discharge can 

 eventually become bioavailable in a receiving stream as the 

 chemical and physical properties of the wastewater change 

 upon mixing. The WQB total recoverable method is suitable 

 for surface waters but could underestimate the toxicity 

 potential of metals present in wastewaters. 



Silver Bow Creek . MultiTech (1987a) reported that the 

 Metro Storm Drain (MSD) was the most severely contaminated 

 part of its study area, which extended from the Weed 

 Concentrator outfall in Butte to near Garrison, Montana. 

 Total cadmium and zinc concentrations regularly exceeded 

 federal drinking water standards. Other contaminants 

 exceeded the standard less frequently. During a storm event 

 in May 1985, all the measured total metal concentrations 

 exceeded federal drinking water standards at most of the 

 Silver Bow Creek (SBC) stations sampled. Aquatic life 

 criteria for copper and zinc were regularly exceeded at most 

 SBC stations. An organic contaminant of concern, penta- 

 chlorophenol, or PCP, was detected at a site below the 

 Montana Pole Treatment site and exceeded the drinking water 

 lifetime health advisory for adults (0.22 milligrams per 

 liter [mg/1]) on one occasion (MultiTech 1987a). Major 



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