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

Abstract

Unconsolidated sediment at the bottom of the Berkeley pit lake is a mixture of detrital silicate minerals derived from sloughing of the pit walls and secondary minerals precipitated out of the water column. The latter include gypsum and K-rich jarosite. The pore waters have a similar pH to the overlying lake waters (pH 3.1 to 3.4), and have similarly high concentrations of dissolved heavy metals, including Al, Cd, Cu, Mn, Ni, and Zn. Sediment cores show that the top meter of the sediment column is moderately oxidized (jarosite-stable). Petrography, chemical analysis and geochemical modelling all suggest a transformation of poorly crystalline ferric compounds such as schwertmannite and/or ferrihydrite near the sediment surface to jarosite with depth in the core. No evidence of bacterial sulfate reduction was found in this study, despite the presence of 0.3 to 0.4 wt % organic carbon in the pit lake sediment.

Distribution of Organic Carbon in the Berkeley Pit Lake, Butte, Montana

“…The TOC and TS analyses of solids showed no variation with depth, and averaged 0.23% and 3.4%, respectively. These values are similar to the results obtained in the companion paper by Twidwell et al (2006). Based on our own mineralogical work by X-ray diffraction, as well as the results of Twidwell et al (2006), it is likely that most of the TS in the sediment was present as sulfate minerals such as jarosite and gypsum, although detrital pyrite was probably also present.…”

“…Assuming an approximate decrease in DOC and Fe concentrations towards the surface of the lake of roughly 1 mg/L and 100 mg/L, respectively, this would result in a TOC:Fe ratio in the newly formed particles of roughly 1 wt%. As shown in Table 4 of the companion paper by Twidwell et al (2006), the TOC: Fe ratio of solids in the top 5 cm of sediment at the bottom of the pit lake is indeed very close to 1 wt %, which supports the hypothesis that DOC is being scavenged by ferric precipitates near the top of the water column. If so, then it is logical to presume that these precipitates would then descend by gravity to collect at the bottom of the lake.…”

“…This value is in the range of those found in the pore waters from the Berkeley Pit sediment. Mobilization of DOC from the Fe-rich sediment into the aqueous phase would be promoted by the transformation of poorly crystalline schwertmannite to K-jarosite with depth (see Twidwell et al 2006).…”

“…The earlier papers in this Special Issue (Gammons and Duaime 2006;Twidwell et al 2006) described the limnology and geochemistry of the Berkeley pit lake and the deepwater sediment and pore water. Whereas previous papers focused on the inorganic constituents, this paper focuses on the distribution of dissolved and total organic carbon (DOC and TOC) in the pit lake water, several of the influent waters, the pit sediment, and the benthic sediment pore water.…”

An Overview of the Mining History and Geology of Butte, Montana

“…Recent journal articles include an examination of the mineralogical controls on mine water chemistry in the Berkeley and Continental Pits (Newbrough and Gammons 2002), an investigation of the geochemistry of uranium and rare earth elements in the Berkeley pit lake (Gammons et al 2003a), and a geochemical and stable isotope investigation of the pit lake and surrounding mine waters (Pellicori et al 2005). In this issue of Mine Water and The Environment, papers are presented on long-term changes in the limnology and geochemistry of the pit lake (Gammons and Duaime 2006), characterization of the pit lake sediment and its pore waters (Twidwell et al 2006), and the distribution of dissolved organic carbon in the lake and its bottom sediment (Cameron et al 2006). Other scientists at Montana Tech are actively engaged in characterizing the microbiology of the Berkeley pit lake (Mitman 1999), and exploring for bioactive metabolites isolated from extremophile organisms that could have pharmaceutical benefits (Stierle and Stierle 2005).…”