The 100-year flood was estimated by CH2M Hill (1988b) to 

 be 4,000 cfs for Silver Bow Creek, and the pond structures 

 would probably withstand a flood of that magnitude. However, 

 during floods slightly larger than the 100-year flood, risk 

 of pond failure increases significantly. At flows greater 

 than 4,000 cfs on Silver Bow Creek, the diversion structure 

 at the upper pH shack would no longer function reliably, and 

 the full flood would possibly enter the Mill-Willow Bypass 

 through the diversion ditch (lECO 1981) . This flood probably 

 would cause failure of at least one of the pond berms and 

 loss of the contents of that pond (U.S. Army Corps of 

 Engineers 1978) . Pond 3 could fail directly when its outflow 

 reached 5,600 cfs, and a flow of 7,000 cfs would overtop both 

 Ponds 2 and 3, causing their failure (lECO 1981). Failure of 

 the ponds also could occur if a large magnitude (6.9 Richter 

 scale) earthquake weakened the pond embankments. Failure of 

 the Warm Springs Ponds embankments would release large 

 amounts of mining and milling wastes to the Clark Fork. 

 Under those conditions, the Warm Springs Ponds would become a 

 major source of contamination. 



An evaluation of the remaining useful life of the Warm 

 Springs Ponds treatment system indicates that incoming 

 sediment loads are the principal controlling factor and 

 suggests that the life of the pond system could exceed 100 

 years under existing operating conditions (this calculation 

 assumes no major changes in pond design or operation for the 

 next 100 years) . However, the pond sediments have some of 

 the highest concentrations of toxic metals found anywhere in 

 the area, and they pose a long-term potential threat to the 

 water quality of the Clark Fork (MultiTech 1987a) . 



Lands Affected by Aerial Deposition 



Nearly 100 years of smelting activities at the Anaconda 

 Smelter resulted in the migration of a large burden of heavy 

 metals, arsenic, and sulfur compounds to soils in the area. 

 The main mechanisms were smelter stack emissions and fugitive 

 dust from various waste deposits in the Anaconda area. 



Studies conducted for the Stage I Superfund investiga- 

 tion of the Anaconda Smelter site included a soils investi- 

 gation to determine the extent and severity of soil con- 

 tamination from smelter stack emissions. Soil profiles (0- 

 2", 2-10", 10-25" intervals) were sampled at 23 sites along 

 four transects emanating from the smelter stack in four 

 directions (Figure 3-5) . Where possible, adjacent tilled and 

 untilled fields were sampled to determine if there was a 

 difference in the vertical distribution of metals in the 

 soils. Such pairs were sampled at seven of the sample sites. 



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