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Geochemistry and Isotopic Composition of H2S-Rich Water in Flooded Underground Mine Workings in Butte, Montana
Primary Issue Addressed: Miscellaneous Activities
Secondary Issues Addressed: Characterization; Modeling
Project Site: West Camp Pumping Well, Butte, Montana
Collaborating Entities: Montana Tech
Cost Share: None
Project Description
The focus of the study was to collect groundwater samples from the West Camp pumping well, which is a groundwater extraction well that drains flooded mine workings in the southwest portion of the Butte, Montana, mining district. Butte is well known for the Berkeley Pit lake, which is an open-pit copper mine flooded with over 100 billion liters of acidic and metal-rich water. The main objective of this study is to collect a suite of groundwater samples from the West Camp groundwater extraction well for comprehensive chemical and isotopic analysis to more fully understand the processes that control the geochemistry of the West Camp mine waters.
The project objective was completed by sampling the West Camp groundwater pumping well approximately six times over the course of 1 year (approximately every 2 months) and analyzing the waters for constituents such as trace metals as well as stable and radiogenic isotopes. Historic analytical results have revealed that the concentrations of many contaminants of interest are relatively low in the West Camp groundwater and typically below the previous instrument detection limits. In this investigation, however, the use of an inductively coupled plasma mass spectrometer technique lowered the detection limits. Other measurements included iron speciation (Fe+2/Fe+3), arsenic speciation (As+5/As+3), hydrogen sulfide (H2S), dissolved organic carbon, and nutrient analysis.
Collection of data on the sulfur isotopic composition (?34S) content of coexisting sulfate and sulfide tested the hypothesis that H2S in the flooded West Camp was formed by bacterial sulfate reduction (BSR). It is widely recognized that BSR produces H2S that is highly depleted in the heavy 34S isotope relative to the sulfate from which it is derived. Depletion factors of greater than 30 per mil are typical of sulfide formed by BSR. In addition, new data on the oxygen isotopic composition (?18O) content of water and coexisting dissolved sulfate may give clues as to the source of sulfate in the West Camp waters, the extent of bacterial sulfate reduction (which results in a shift in ?18O of sulfate to more positive values), and possibly the mechanism of pyrite oxidation in the flooded mine workings (e.g., aerobic versus anaerobic).
Status
All project work has been completed on this project. An in-depth data review
will be provided in the final report, which is expected to be published
in fiscal 2006.
The results suggest that there are relatively few temporal changes
in the West Camp mine water, both the physical and chemical parameters
are remarkably stable with time, as are the isotopic composition
of water and dissolved S species (H2S, sulfate). Trace metal analysis
confirms the very low concentrations of most common heavy metals,
with arsenic being the only contaminant that is above regulatory
standards. Most of the dissolved arsenic is present in the As(III)
valence state, and all of the dissolved iron (Fe) is present as
Fe(II), consistent with the strongly reducing nature of the mine
waters in the West Camp. Nutrient levels are very low (nitrate
and ammonia below detection, phosphate ~ 20 to 30 µg/L),
as are concentrations of total and heterotrophic bacteria. Isotopic
results confirm the hypothesis that H2S in the West Camp waters
was formed by bacterial sulfate reduction. Sulfate-reducing bacteria
were isolated in the West Camp waters but were near the lower
limit of quantification. Most of the bacteria responsible for
H2S generation may be present as biofilms, coating mineral or
wood material in the flooded mine workings.