GEOCHEMISTRY OF METALS AND NUTRIENTS IN FINE-SEDIMENT PORE WATER IN BLACKTAIL AND SILVER BOW CREEKS, BUTTE, MONTANA

Historical mining in Butte, Montana has impacted surface and groundwater in the area. Although most of the known sources of contaminants of concern have been removed or remediated, metal loading continues to occur in lower Blacktail Creek and upper Silver Bow Creek. Possible sources of metals include upwelling groundwater and interaction between the stream and metal-rich sediment in the stream bed. To assess the importance of fine sediment as a source (or sink) for metals, this investigation used sediment pore water diffusion samplers (“peepers”). Peepers are ideal for capturing cm-scale vertical gradients in pore water chemistry across the sediment-water interface. Eight peepers were deployed in Blacktail Creek, six in Silver Bow Creek, one in Grove Gulch, and two in shallow ponds south of lower Blacktail Creek. Four piezometers were also installed in the ponds. Pore-water samples extracted from the peepers and piezometers were analyzed for dissolved trace metals, major ions, alkalinity, and selected nutrients. Sediment samples collected at some peeper sites were analyzed by X-ray diffraction and handheld X-ray fluorescence. Dissolved Fe, Mn, As, PO43-, and HCO3- ions generally increased with depth below the sediment-water interface. Pore-water concentrations of up to 609 mg/L Fe, 55 mg/L Mn, and 1.0 mg/L As were measured at depth. These increases in concentration were attributed to reductive dissolution of Fe- and Mn-oxides, coupled to organic matter decay, in the anoxic environment of the fine-grained sediment. Dissolved Fe2+ could then reprecipitate as Fe-oxides once it reached an aerobic environment and potentially reabsorb dissolved As. PO43- and HCO3- concentrations could also be influenced by sulfate reducing bacteria and oxidation of organic matter. Copper, lead, and zinc behaved differently, and generally had very low concentrations in the deeper samples. Bacterial sulfate reduction was indicated by trace levels of H2S, enough to precipitate Cu, Pb and Zn as sulfide minerals. In some cases, there was a zone where dissolved Cu and Zn concentrations increased sharply in the top 2–6 cm of the sediment column. This could indicate oxidation of fine-grained sulfide minerals in the near-surface pore water. Fick’s first law was used to quantify the diffusive flux of dissolved arsenic from the sediment pore water into Silver Bow and Blacktail Creeks. The same approach was use to estimate downwards diffusion of dissolved Zn from Grove Gulch into its own sediment. The calculated fluxes are negligible for the streams due to the short residence time of water passing through Lower Area One. However, upwards diffusion of As into the shallow ponds could lead to a build-up in dissolved As over time. Although the stream sediments act as a temporary metal sink, the fine sediments are eventually dispersed back into the main stream during periods of high streamflow and bioturbation. Periodic removal of these fine-grained sediments from the stream channels would reduce the total load of metals flowing down Silver Bow Creek