reservoir sediment ground water. The lowest concentrations 

 were found in the northern portion of the study area, 

 reflecting high-quality recharge water from the Blackfoot 

 River (Woessner et al. 1984). 



The authors concluded that the distribution of metals in 

 the ground water and ground water flow patterns proved that 

 reservoir sediments were a likely source of contaminants to 

 the alluvial ground water system. The sediments contain very 

 high concentrations of heavy metals that are extremely 

 enriched above natural levels and rival many severely 

 contaminated sediment systems. Because the reservoir 

 contains approximately 120 million cubic feet of sediment, 

 it represents a huge source of metals to the surface and 

 ground water systems (Woessner et al. 1984) . 



In January 1987, the Montana Power Company installed 

 three monitoring wells within ten feet of each other on the 

 containment side of Milltown Dam. The wells were completed 

 in three different lithologic units at depths of 45, 30, and 

 15 feet (Hydrometrics 1987) . The wells were sampled in 

 February and March of 1987. Results of these water analyses 

 are summarized in Table 3-28. 



TABLE 3-28. RESULTS OF MFC SAMPLING OF MONITORING WELLS 

 AT MILLTOWN DAM (FEB. - MARCH 1987)^ 



^ Range of values from three samplings. 

 Source: Montana Power Company 1987b. 



Some of these values are quite high relative to drinking 

 water standards and aquatic life toxicity criteria and 

 provide further evidence of the effect of contaminated 

 sediments on the ground water system in this area. 



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