front has been moving very slowly down through the tailings. 

 Geochemical modeling of the pond system has predicted that 

 the oxidizing zone will reach the bottom of the tailings 

 ponds in 10,000 to 20,000 years. This oxidizing zone could 

 serve as a source of solutes to ground water for a long time. 

 However, if there is a sufficient thickness of unsaturated, 

 calcareous alluvium beneath the tailings to neutralize the 

 acidity they release, most of the metals would likely be 

 attenuated rapidly. The model predicted that worst-case 

 future ground water concentrations (thousands of years from 

 now) at a distance of 1,000 meters downgradient of the ponds 

 are expected to be 3 ppb cadmium, 34 ppb copper, <1 ppb lead, 

 4 ppb zinc, and 80 ppb arsenic. Although sufficient data 

 were not available to accurately predict the effect of 

 tailings leachate on the Clark Fork, a preliminary analysis 

 indicated that future low-flow solute concentrations in the 

 Clark Fork might be: 



Arsenic 16-20 ppb 



Cadmium <1-1 ppb 



Copper 24-61 ppb 



Lead <2 ppb 



Zinc 32-33 ppb 



Sulfate 230-330 ppm 



These concentrations are only slightly higher than 

 existing concentrations in the Clark Fork below the Warm 

 Springs Ponds (Tetra Tech 1986b) . 



Floodplain Mine Wastes 



As discussed earlier in this chapter, mine wastes are 

 deposited in the channels and floodplains of Silver Bow 

 Creek, Warm Springs Creek, the Mill-Willow Bypass, and the 

 Clark Fork. These materials are found in large quantities 

 for over 100 miles and have significant potential to 

 contaminate the ground water resource. Sulfide oxidation of 

 these wastes may release soluble metals into the ground 

 water, and preliminary modeling indicates the possibility 

 that the deposits could contribute significant amounts of 

 trace metals to local ground water during a wet season. 



Warm Springs to Milltown Data 



In 1987, the USGS initiated a study of the shallow 

 aquifers along the Clark Fork between Warm Springs and 

 Milltown, Montana. The project was designed to assess the 

 physical and chemical characteristics of ground water, 

 seasonal changes in the systems, and ground water-surface 

 water interrelationships. 



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