PRELIMINARY INTERPRETATION OF THE WATER QUALITY ASSOCIATED WITH THE BUTTE MINE FLOODING

J. L. Sonderegger and T. E. Duaime
Montana Bureau of Mines and Geology
Butte. MT 59701
and
Sam Stephenson
 The Anaconda Minerals Company
Butte,MT 59701

Abstract--Data collected by sampling at the Kelley Shaft show reasonable uniformity in chemistry until late 1983, at approximately the time that the water level reached the elevation of the bottom of the open pit. Since that time, samples collected from near the surface of the water in the shaft have been quite variable in composition, suggesting that during some periods water may have moved from the pit into the underground workings.    The water samples collected from April 1982 to late fall of 1983 were consistently metal rich (iron concentrations of about 1000 mg/L; copper concentrations of about 100 mg/L), of low (~2.8) pH and with Fe ranging from about 0.1 to 0.3 of the total dissolved iron. In 1984, both copper and iron concentrations erratically fell to below 1 mg/L (and the pH rose to as high as 5.7), but reverted to near their former levels by the end of 1984.

    Two sets of multiple level samples collected in the Kelley Shaft during July and November 1984 depict quite different conditions. The July samples showed a loss of iron (91 mg/L; pH = 4.2) near the top of the water column, but iron increased to over 3000 mg/L and pH fell to 3.6 at depths of 400 and 750 feet below the static water level. However, the November sampling results are essentially uniform (Fe ~ 1600 mg/L; pH = 3.6) for samples to a depth of 900 feet below the static water level. The November results could have been caused by a bailer "hangup"; conversely, the July results could represent water moving from the pit, whereas the November samples represent reestablished ground water flow to the pit. The pit and shaft water levels were nearly identical at these points in time and the measurement error is great enough for such reversals to be undetected.

    The data collected since April 1982 do permit the following observations and interpretations to be made:

1. The initial rate of acid production and consumption has decreased. In the early stages of flooding the pH averaged 2.8, whereas it has averaged 3.6 during the last 18 months.

2.  Plots of water chemistry data show that the deep shaft samples are currently saturated with respect to kaolinite and that the water is approaching saturation with respect to montmorillonite.

3. The deep shaft water chemistry is still far from approaching a stable equilibrium condition. The stable pH will be above 4.5 as the acid production rate decreases.

4. The one set of pit water samples shows increasing metal content with depth. The depth of oxygenation and whether turnover occurs will control the pit water chemistry.

5. Wells installed in old mine waste along Silver Bow Creek suggest that greater oxygen levels and buffering may reduce most metal levels to below earlier predicted concentrations.