76-5 The Geochemical Behavior of Manganese in Circumneutral Mine-Impacted Stream and Porewater of Fourmile Creek near Cripple Creek, Colorado

Session: Contaminants Near Groundwater-Surface Water Interfaces

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Abstract:

Precious metal mining often impacts surface and ground water quality. Despite ongoing remediation efforts, Fourmile Creek downstream of Colorado’s largest gold mine has received elevated concentrations of sulfate, manganese, and other metals since 1941. The effluent’s near-neutral pH and slightly elevated temperature provide an opportunity to study the fate and transport of metals across the stream-aquifer interface in a unique geochemical system.  Porewater samples were collected from three depths at the discharge point and at upstream and downstream locations using a specially constructed durable and corrosion-resistant drive-point sampler. Samples were obtained from depths between 20 and 50 cm below the streambed during peak snow-melt stream discharge conditions in early June and evaluated for major ions and metals using IC, ICP-MS and OES. Dissolved oxygen (DO), redox potential (Eh), pH, specific conductance, and temperature were measured in the field. Porewater manganese and sulfate concentrations were two to three orders of magnitude higher at the discharge point than upstream and remained elevated downstream. Concentrations of dissolved iron vanished from porewaters 350 meters downstream while dissolved manganese remained >1000 ppb at 50 cm.  Trace metals cobalt and arsenic followed the same trend as manganese with higher concentrations with depth as well as downstream. These observed increases in dissolved manganese and other metals with depth were nearly inversely proportional to changes in DO. Porewater pH was higher at the discharge point and downstream while DO and Eh were lower at the discharge point and higher downstream. Sulfurous odors emanating from porewater and a darker overall appearance with black staining on streambed sediments at the discharge point and downstream also qualitatively support the correlation of anoxic conditions with increased porewater manganese concentrations. XRD and XRF methods will be used to identify mineral phases present in the streambed and used in combination with porewater analyses to understand the processes that constrain the fate of redox-sensitive metals and sulfate in circumneutral conditions. To assess seasonal impacts on these processes, a second set of profiles will be obtained during late-fall low stream flow. Improving our understanding of these processes at the surface-groundwater interface will help address outstanding contamination concerns in Fourmile Creek and geochemical processes that occur in near-neutral pH mountain stream environments.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-10039

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