Groundwater-Stream Connectivity Mediates Metal(loid) Geochemistry in the Hyporheic Zone of Watersheds Impacted by Historic Mining and Acid Rock Drainage

Abstract:

The capacity of the hyporheic zone, or the subsurface region surrounding a stream where shallow groundwater and stream water mix, to attenuate or supply metal(loid)s to rivers is an area of active and environmentally relevant research. Characterization of hyporheic storage area and mass transfer rates are important for metal flux estimates; however, the influence of streamflow and the structure of the shallow subsurface on metal-sediment interactions in the hyporheic zone are not well-constrained. To understand the relationship between streamflow, hyporheic zone structure, and metal(loid) mobility, we investigated two, ~1 km stream reaches within the Bonita Peak Mining District, a U.S. Environmental Protection Agency Superfund site located near Silverton, Colorado. The hyporheic zones of the two stream reaches, Mineral Creek at Chattanooga Fen and Cement Creek near Prospect Gulch, were characterized using a combination of salt-tracer injection tests, transient storage modeling, and geochemical sampling of the shallow streambed (< 0.7 m). Based on these data, we present a conceptual model for subsurface metal(loid) behavior in the hyporheic zones of (a) well-connected systems characterized by strong hyporheic mixing of infiltrating stream water and upwelling groundwater and (b) poorly-connected systems delineated by physical barriers that limit hyporheic mixing. The large hyporheic zone and high hydraulic conductivities of Mineral Creek created a connected stream-groundwater system, where mixing of oxygen-rich stream water and metal-rich groundwater facilitated the precipitation of metal colloids in the shallow subsurface. In Cement Creek, precipitation of iron-oxides at depth (~0.4 m) created a low hydraulic conductivity barrier between surface water and groundwater. Sediments were an important regulator of metal(loid) concentrations in this disconnected stream-groundwater system due to the formation of strong redox gradients induced by a relatively small hyporheic zone and high fluid residence times. While well-connected systems such as Mineral Creek serve as a natural metal(loid) sink, disconnected systems such as Cement Creek would benefit from remediation focused on removal of physical barriers to facilitate deeper oxygen penetration and precipitation of metal-oxides in the shallow subsurface.