Fifty-six samples were collected from 50 wells (Figure 

 3-31) completed in a variety of geologic formations. The 

 dominant ions in the ground water sampled were calcium and 

 bicarbonate. Twenty-seven of the samples from 21 of the 

 wells contained at least one constituent value (or charac- 

 teristic) that equalled or exceeded either the primary or 

 secondary drinking water standards established by the EPA 

 (198 6a, b) . Constituent concentrations that exceeded these 

 standards include sulfate, dissolved solids, iron, manganese, 

 and nitrate. One well had a pH value outside the acceptable 

 range. Exceedences for iron and manganese were most common 

 in water from wells less than 50 feet deep, and exceedences 

 for sulfate and dissolved solids were most common in water 

 from wells more than 50 feet deep. Most of the wells sampled 

 are located near the mainstem Clark Fork. Therefore, the 

 general water chemistry derived in this study may not be 

 representative of the Clark Fork Valley as a whole. 



Clark Fork streamflow was measured at 16 sites from Warm 

 Springs to Turah in October 1986. No significant losses in 

 streamflow were measured throughout the reach. However, 

 gains in streamflow, presumably from ground water inflow, 

 were measured from Racetrack to Deer Lodge. A final report 

 on this study will be published in 1989. 



Mi 11 town Area 



The principal ground water system in the vicinity of the 

 Milltown Reseirvoir is the unconfined valley fill alluvial 

 aquifer, composed of well-sorted sand, gravel, and boulders. 

 The aquifer thickens from about 40 feet near the reservoir to 

 over 100 feet north of Milltown. Ground water flow direction 

 is generally parallel to the Blackfoot River and the Clark 

 Fork. Recharge to the system is derived from the Clark Fork 

 and the Blackfoot River just above the reservoir and from the 

 reservoir itself. Discharge is to the Clark Fork below the 

 dam (Woessner et al. 1984). 



Woessner et al. (1984) conducted a study of the ground 

 water in the Milltown area to identify the source of arsenic 

 contaminating wells in Milltown (discussed earlier in this 

 chapter) and to locate a new water supply. Many of the 

 existing wells sampled before this project was started 

 (August-September 1983) were contaminated with arsenic, iron, 

 and manganese, and nearly all other constituent concentra- 

 tions exceeded background levels. Samples collected in 

 November and December 1983 from project monitoring wells, 

 sand point wells in the reservoir sediments, and selected 

 existing wells showed high levels of arsenic, iron, man- 

 ganese, and TDS at a number of sites. The highest concentra- 

 tions occurred in the southern Milltown area and in the 



3-110 



