Blackfoot River. Fine-grained bed sediments were collected 

 in areas of low-flow velocity and were separated into mud 

 and sand fractions in the laboratory. 



The authors reported that mean concentrations of 

 cadmium, copper, manganese, and zinc in mainstem samples were 

 well above those in tributary samples. All four metals 

 showed general decreases in concentration downstream (this 

 trend was more pronounced in the mud fraction) and varia- 

 bility among sites was high. Brook and Moore attributed 

 these results to the downstream decline in frequency of 

 metals-laden floodplain deposits and speculated that dilution 

 by uncontaminated tributary sediments might also be a factor. 

 They also found that more of the bulk metals concentrations 

 were derived from the sand fraction than from the mud 

 fraction (Brook and Moore, unpublished manuscript) . 



Using the data on bank sediments from Moore's 1985 EPA 

 study (discussed in the previous section), Moore et al. (in 

 press) examined the controls exerted by sediment particle 

 size on metals concentrations in the Clark Fork system. The 

 traditional view of metal-sediment association is that most 

 of the metals are carried in the fine fraction. Moore et al. 

 (in press) found that this relationship held true in the 

 tributaries, where there were significant correlations 

 between most of the metals and the percentage of clay. 

 However, in the mainstem, most or all of the size fractions 

 were found to be important contributors to the high metals 

 concentrations. The Clark Fork is a high-gradient, coarse- 

 grained system that commonly carries coarse sand in suspen- 

 sion during spring runoff. Some of this coarse sand is 

 actually extremely metal-rich mine and smelter tailings. The 

 authors also suggested that the coarse-grained floodplain 

 sediments may reside in an oxygenated environment longer than 

 fine sediments and may have more time to accumulate oxide 

 coatings and associated trace metals. 



Moore et al. (in press) concluded that distribution of 

 metals in a complex system such as the Clark Fork is more 

 likely to be based on chemical associations than on grain- 

 size parameters. Application of traditional methods to 

 correct for grain size effects may lead to erroneous 

 conclusions about metal trends in the Clark Fork and other 

 contaminated systems. 



Researchers with the U.S. Geological Survey (USGS) are 

 conducting investigations in the Clark Fork using sediments 

 to determine the fate and distribution of trace metals in 

 river systems. They are also using aquatic insects as 

 indicators of biologically available metals. The Clark Fork 

 has been selected for these investigations because of the 

 predominance of mine waste metals and the lack of other major 



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