A Survey of the Geochemistry of Flooded Mine Shaft Water in Butte, Montana

Abstract

This paper outlines general trends in the geochemistry of the more than 10,000 km of flooded underground mine workings in the Butte mining district. The waters in question range in pH from 4 to 8, are all moderately to strongly reducing, and show a huge range in concentration of dissolved metals such as Al, As, Fe, Mn, and Zn. Metal concentrations and total acidity are highest in the Kelley mine shaft, which was the main dewatering station used to pump ground water from the underground mine complex during active mining operations. In contrast, metal concentrations are much lower in the outer portions of the district where many of the mines contain hydrogen sulfide formed by sulfate-reducing bacteria. In comparison to the other heavy metals, concentrations of Pb and Cu are quite low in the flooded mine shafts. An interesting inverse correlation between pH and water temperature is noted, which may be partly caused by exothermic pyrite oxidation reactions in the central portion of the district.

Long Term Changes in the Limnology and Geochemistry of the Berkeley Pit Lake, Butte, Montana

“…Several of the mine shafts closest to the pit lake are even warmer (e.g. Kelley >30GC), which may be due to pyrite oxidation, a strongly exothermic reaction (Gammons et al 2006b;Pellicori et al 2005). The fact that the colder water in the top 50 m did not sink is most likely due to the salinity gradient in the top 50 m of the lake.…”

“…Annual and semi-annual monitoring of ground water in flooded mine shafts and bedrock monitoring wells by the MBMG has shown only one station out of several dozen that consistently has a pH below 5 (Gammons et al 2006b). The lone exception is the Kelley shaft, which was also the location of the main dewatering pumps for the Butte mining district.…”

Summary of Deepwater Sediment/Pore Water Characterization for the Metal-laden Berkeley Pit Lake in Butte, Montana

“…Because we were unable to obtain in situ measurements of the temperature of the sediment pore waters, the calculations were performed at 25GC, i.e., the same temperature at which the pore water pH and E H were measured. Although the deep pit lake waters are cold (less than 8GC, Gammons and Duaime 2006), the surrounding fractured bedrock and groundwater are warm (> 20GC, Gammons et al 2006). It is conceivable that the sediment-water interface at the bottom of the pit lake is also a type of thermocline, separating overlying cold lake water from warm pore water in the underlying sediment.…”

Analytical Framework for a Risk-based Estimation of Climate Change Effects on Mine Site Runoff Water Quality

“…For proposed and operating mines, closure options are often evaluated on the basis of predicted longterm water quality impacts. The large number of closed and/or abandoned mine sites worldwide also requires reliable water quality estimates to assess carrying costs for industry and governments (Puura and D'Alessandro 2005;Bowell 2004, 2005a, b) From a more fundamental science perspective, the long-term predictions for mine site water quality challenge our understanding of various climate, hydrology, and geochemical models and their underlying biogeochemical and physical basis (Gammons et al 2006;Morin and Hutt 1997). In order to estimate mine waste water quality into the future, the typical approach within the regulatory, industrial, and academic communities is to develop advanced probabilistic and/or deterministic water balances for a relatively stationary post-closure site layout and topography that is coupled to steady state geochemical weathering predictions under a variety of risk scenarios (Morin and Hutt 1997).…”