The distribution of metals indicates that both vertical 

 and lateral migration have occurred. During high-evaporation 

 and low-precipitation periods, metals and sulfate in solution 

 migrate to the surface and are precipitated as metal-enriched 

 sulfate salts. Subsequent intense precipitation and rapid 

 surface runoff results in the instantaneous dissolution of 

 these salts, causing an abrupt lowering of pH and mobilizing 

 metals to surface waters. Also, during flood conditions, 

 metals can be incorporated into bed sediment and surface 

 waters where tailings deposits are directly exposed to the 

 active channel (Brooks 1988) . 



Downward vertical migration within the stratigraphic 

 profile is indicated by the highly elevated concentrations of 

 metals in organic-rich clayey silt directly underlying the 

 tailings deposits. Complexation of metals in this unit is 

 highly enhanced by the abundance of organic material, the 

 proximity of the redox boundary, and the fine-grained nature 

 of the sediment. Consequently, these factors prevent 

 movement into the underlying coarse sand and gravel aquifer. 

 Any small-scale downward mobilization of metals into the 

 aquifer would likely be masked by dilution from ground water 

 (Brooks 1988) . 



Ray (1983) conducted an investigation of metals-enriched 

 fluvial sediments in the upper Clark Fork. Samples were 

 collected from the floodplain near Rocker, Racetrack, 

 Garrison, and Drummond (Figure 3-8) . A check site was 

 sampled in the Tin Cup Joe Creek drainage, and a control site 

 was sampled in the Blackfoot River drainage. Results of this 

 study are summarized in Table 3-11. 



control site 



^ Arithmetic means 

 Source: Ray 1983. 



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