2. Study ground water effects on metals loading. 



Conduct a comprehensive study of the contribution of 

 ground water to metals loading problems in the upper 

 Clark Fork. The study should use existing wells (and 

 possibly some new wells) and should focus on the 

 headwaters and Deer Lodge areas. This may be partially 

 addressed through the Silver Bow Creek RI/FS and the 

 Clark Fork screening study. 



3 . Document the extent of the carbonate zone and ground 

 water flow patterns in the vicinity of the Anaconda and 

 Opportunity ponds. Again, this may be addressed when the 

 geohydrologic and geochemical conditions in the vicinity 

 of the Anaconda and Opportunity ponds are further inves- 

 tigated during future RI/FS activities at the Anaconda 

 Smelter site. 



a. Determine the actual thickness of the alluvial 

 deposits underlying the tailings contained in the 

 Anaconda and Opportunity ponds. Two distinct 

 source zones for solutes have been identified in 

 the tailings — a saturated zone just above the 

 alluvium and an oxidizing zone in the upper part of 

 the tailings that will slowly move downward. 

 Modeling has predicted that many thousands of years 

 from now, oxidation of sulfides to sulfuric acid 

 could lower the pH at the bottom of the tailings 

 and cause the release of metals such as arsenic, 

 cadmium, copper, lead, and zinc (Tetra Tech 

 1986b) . If there is sufficient thickness of 

 carbonate-rich alluvium beneath the tailings, the 

 acidity may be neutralized and the metals atten- 

 uated before reaching the ground water. The 

 unconsolidated alluvial deposits are estimated to 

 range from more than 100 feet thick in the western 

 portion of the site to about 20 feet thick east of 

 the Opportunity Ponds. However, a detailed study 

 should be conducted in the vicinity of the ponds to 

 document the actual thickness and percentage of 

 carbonate in the alluvium to determine if it will 

 afford adequate ground water protection in the 

 future. More modeling efforts may be required to 

 make this determination. 



b. It is also important to determine ground water flow 

 patterns through the carbonate zone of the 

 alluvium. It may be that only a portion of the 

 carbonate mass is available to attenuate the 

 metals. An investigation should be initiated to 

 address this question. 



5-22 



