172-10 Seasonal Variations in Manganese Concentrations and Mineral Stability in the Hyporheic Zone of a Mountain Stream: Implications for the Fate of Metals from Circumneutral Mine Drainage

Session: Environmental Geochemistry and Health (Part II)

Presenting Author:

Morgan Johannesen

Authors:

Johannesen, Morgan¹, Schulmeister, Marcia K.², Stotler, Randy L.³

(1) Geology, University of Kansas, Lawrence, KS, USA, (2) Geology, University of Kansas, Lawrence, KS, USA, (3) Earth and Environmental Sciences, University of Waterloo, Waterloo, ON, Canada,

Abstract:

Fourmile Creek in the Cripple Creek mining district of Colorado has received metals in discharge since the 1940s. High-sulfate circumneutral effluent from a legacy dewatering adit poses a threat to the surrounding ecology and is the subject of remedial actions. In an initial evaluation of the geochemical conditions that influence the fate of downstream contaminants, we identified differences between surface water and streambed porewaters during high- and low-streamflow conditions in June and October, respectively, at three locations: the circumneutral effluent-receiving reach, 200 meters upstream, and 300 meters downstream. In this study, differences in porewater geochemistry and mineral saturation were also observed at three to five depths in the hyporheic zone of each location. Upstream porewaters in October exhibited higher pH and dissolved oxygen (DO) concentrations accompanied by lower specific conductance values. Downstream DO and specific conductance values were higher in October with lower pH than in June. These parameters remained relatively constant in the effluent-receiving reach. Surface water and porewater oxidation-reduction potentials were higher in June at each location, though all samples were within a range consistent with manganese (Mn) reduction (0.19-0.27 V) in both months. Upstream and downstream porewater Mn concentrations were substantially lower in October, declining from >1000 ppb to <5 ppb, while Mn concentrations doubled from June to October at the effluent-receiving reach. The Mn oxide mineral birnessite was identified in the fine fraction (<2 µm) of bulk sediments via X-Ray Diffraction alongside muscovite, montmorillonite, kaolinite, feldspars and quartz. Initial equilibrium speciation modeling with PHREEQC revealed differences in mineral saturation at each location: porewaters and surface water were oversaturated with respect to iron oxides and oxyhydroxides, aluminum oxides, and silicate minerals in both June and October. Samples from the adit discharge and downstream locations were additionally saturated with respect to calcite, aragonite, and fluorite. Downstream porewaters were oversaturated with respect to rhodochrosite in June, while all locations remained undersaturated with respect to birnessite in both months. Additional speciation modeling will predict controls on manganese oxide and other mineral precipitation under varying conditions. As a tributary to the Arkansas River, the fate and long-term impact of this stream is an environmental health concern, and this work will aid in understanding the potential mobility and loading of metals in Fourmile Creek.

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